10: and haloalkanes and haloarenes - target...

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Chapter 10: Haloalkanes and Haloarenes

; Halogen derivatives of alkanes or of arenes: When one or more hydrogen atoms of alkanes or arenes are replaced by corresponding number of halogen

atoms, the resulting compounds are called halogen derivatives of alkanes (haloalkanes) or halogen derivatives of arenes (haloarenes) respectively.

Haloalkanes: The halogen derivatives of saturated aliphatic hydrocarbons are called as haloalkanes or alkyl halides.

OR Haloalkanes are obtained by replacing one or more hydrogen atom(s) of an alkane with the corresponding

number of halogen atom(s). eg. H3C Cl (Chloromethane) In haloalkanes, halogen atom(s) is/are bonded to sp3 hybridised carbon atom(s) of an alkyl group. Haloarenes: The halogen derivatives of aromatic hydrocarbons are called as haloarenes or aryl halides.

OR Haloarenes are obtained by replacing one or more hydrogen atom(s) of an arene with corresponding

number of halogen atom(s). In haloarenes, halogen atom(s) is/are bonded to sp2 hybridised carbon atom(s) of an aryl group. Note: Several organic compounds containing halogen exist in nature and some of them are clinically useful.

Substance Contains halogen atom Description i. Chloramphenicol (Antibiotic) Chlorine a. Produced by soil micro-organisms.

b. Used in treatment of typhoid fever. ii. Thyroxine (Hormone) Iodine a. Produced inside our body.

b. Deficiency causes goiter. iii. Chloroquine (Synthetic halogen

compound) Chlorine Used in treatment of malaria.

iv. Halothane Used as an anaesthetic during surgery.

v. Certain fully fluorinated compounds

Fluorine Being considered as potential blood substitutes in surgery.

Introduction10.0

10.0 Introduction 10.1 Classification 10.2 Monohalogen derivatives of alkanes 10.3 Nomenclature of haloalkanes 10.4 Nature of C X bond in haloalkanes 10.5 Preparation of haloalkanes 10.6 Physical and chemical properties of

haloalkanes

**10.7 Stereochemistry 10.8 Nucleophilic substitution mechanism 10.9 Haloarenes 10.10 Nature of C X bond in haloarenes *10.11 Preparation of haloarenes 10.12 Physical and chemical properties of

haloarenes 10.13 Uses and environmental effects of some

haloalkanes and haloarenes * marked section is only for JEE (Main) ** marked section is for NEET UG

10 Haloalkanes and Haloarenes

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Chemistry Vol ‐ 2.2 (Med. and Engg.) Haloalkanes and haloarenes are classified as monohalogen derivatives or polyhalogen (di-, tri-, etc.) derivatives of alkanes and arenes respectively, based on the number of halogen atoms in their structure. Classification of haloalkanes on the basis of the number of halogen atoms:

Classification 10.1

Geminal dihalides Both the halogen atoms are attached to same C-atom. eg.

Vicinal dihalides Both the halogen atoms are attached to adjacent (vicinal) C-atom. eg.

Monohaloalkanes (Monohalogen derivatives of alkanes)

One hydrogen atom of an alkane is substituted by one halogen atom. General formula: CnH2n+1X [n is an integer] R X [X = F, Cl, Br, I and R = alkyl group] eg. CH3 CH2 Br Ethyl bromide (Bromoethane)

Polyhaloalkanes (Polyhalogen derivatives of alkanes)

More than one hydrogen atom of alkanes are substituted by corresponding number of halogen atoms.

Trihalogen derivatives Three hydrogen atoms ofan alkane are substitutedby three halogen atoms. General formula:

CnH2n1X3

[X = F, Cl, Br, I and n is aninteger] eg. CHI3 Iodoform

Dihalogen derivatives Two hydrogen atoms of analkane are substituted bytwo halogen atoms. General formula: CnH2nX2 [X = F, Cl, Br, I and ‘n’ isan integer]

H

H3C C Br Br

Ethylidene bromide (1,1-Dibromoethane)

Br

H2C CH2

Br

Ethylene dibromide (1,2-Dibromoethane)

Haloalkanes

Tetrahalogen derivatives Four hydrogen atoms of analkane are substituted byfour halogen atoms. General formula:

CnH2n2X4 [X = F, Cl, Br, I and n is aninteger] eg. CCl4 Carbon tetrachloride

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Classification of monohalocompounds on the basis of nature of CX bond: Monohalogen derivatives of alkanes (alkyl halides) are obtained by substituting one hydrogen atom of an alkane by one halogen atom and are further classified as follows: Common and IUPAC names of some monohalogen derivatives:

No. Formula Common name IUPAC name i. CH3Br Methyl bromide Bromomethane ii. CH3CH2Cl Ethyl chloride Chloroethane iii. CH3CH2CH2Br n-Propyl bromide 1-Bromopropane iv. Br

| CH3 CH CH3

Isopropyl bromide (sec-Propyl bromide)

2-Bromopropane

v. CH3CH2CH2CH2Cl n-Butyl chloride 1-Chlorobutane vi. CH3CH CH2CH3

| Br

sec-Butyl bromide 2-Bromobutane

vii. CH3 CHCH2Cl CH3

Isobutyl chloride 1-Chloro-2-methylpropane

Monohalogen derivatives of alkanes10.2

Compounds containing sp3CX bond

Compounds containingsp2CX bond

Alkyl halides (Haloalkanes)

Halogen atom is bonded to an alkyl group. General formula: CnH2n+1X. X

Vinylic halides Halogen atom isbonded to sp2-hybridised carbonatom of C=C. eg.

Monohalocompounds

Allylic halides Halogen atom isbonded to sp3-hybridised carbonatom next to C=Ci.e., to an allyliccarbon. eg.

CH2X

Aryl halides (Haloarenes)

Halogen atom is bonded to sp2-hybridised carbon atom of an aro-matic ring. eg. X

Benzylic halidesHalogen atom isbonded to sp3-hybridised carbon atom next to anaromatic ring. eg.

CH2X

Primary alkyl halide (1) Halide group is attached toprimary carbon atom of analkyl group. eg. CH3CH2CH2Br n- Propyl bromide (1-Bromopropane)

Secondary alkyl halide (2) Halide group is attached to secondarycarbon atom of an alkyl group. eg.

Tertiary alkyl halide (3) Halide group is attached to tertiarycarbon atom of an alkyl group. eg.

CH3

H3C C Br

H

Isopropyl bromide(2-Bromopropane)

CH3

H3C C Br

CH3

tert-Butyl bromide (2-Bromo-2-methylpropane)

Alkyl halides

Nomenclature of haloalkanes 10.3

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Chemistry Vol ‐ 2.2 (Med. and Engg.)

viii. Br | CH3 C CH3 | CH3

tert-Butyl bromide

2-Bromo-2-methylpropane

ix. Br | CH3 C CH2 CH3 | CH3

tert-Pentyl bromide 2-Bromo-2-methylbutane

x. CH3 CHCH2Br CH3

Isobutyl bromide 1-Bromo-2-methylpropane

xi. CH3 | H3C C CH2I | CH3

Neopentyl iodide 1-Iodo-2,2-dimethylpropane

xii. CH2 = CHCl Vinyl chloride Chloroethene

xiii. CH2 = CH CH2 Br Allyl bromide 3-Bromopropene

xiv. CH2Cl2 Methylene chloride Dichloromethane

xv. CHCl3 Chloroform Trichloromethane

xvi. CCl4 Carbon tetrachloride Tetrachloromethane

xvii.

Benzyl chloride Chlorophenylmethane

i. In an alkyl halide, highly electronegative halogen atom is bonded to less electronegative carbon atom.

Therefore, C X bond in alkyl halide is polar in nature. ii. The carbon atom carries partial positive charge (+) as it is less electronegative than halogen and halogen

atom carries a partial negative charge (). iii. In the formation of CX bond, sp3 hybrid orbital of carbon atom overlaps with half filled p-orbital of

halogen atom. iv. C X bond strength decreases down the group 17 of the periodic table because orbital overlap is most

efficient between orbital of same principle quantum number (i.e., in the same row of periodic table) and efficiency decreases as difference in principle quantum number increases. Halogen atom Its overlapping orbital in C–X bond

F 2pz Cl 3pz Br 4pz I 5pz

v. The size of the halogen atom increases from F to I, as a result of which, the bond length also increases and the bond formed is weaker. Hence, C X bond strength in CH3 X decreases in the order: CH3F > CH3Cl > CH3Br > CH3I as the 2sp3 orbital of carbon cannot penetrate into the larger p-orbitals sufficiently to form strong bonds.

C X

+

CH2Cl

Nature of C X bond in haloalkanes10.4

5


Bond enthalpy, bond length and dipole moment of CX bond in CH3X: Bond Bond Enthalpy (kJ/mol) Bond Length (Å) Dipole moment (Debye)

CH3 F 452 1.42 1.847

CH3 Cl 351 1.77 1.860

CH3 Br 293 1.91 1.830

CH3 I 234 2.12 1.636 Monohalogen derivatives of alkanes (haloalkanes) can be prepared by the following methods: i. From halogenation of alkanes: a. Direct halogenation of alkanes in the presence of UV light, heat or suitable catalyst gives the

corresponding alkyl halides. b. The displacement of H-atom from hydrocarbon during halogenation follows the order: Benzylic allylic > 3 H-atom > 2 H-atom > 1 H-atom > H-atom of methane > vinylic arylic c. The reactivity of halogens decreases in the order: F2 > Cl2 > Br2 > I2

d. Fluorination of alkanes is highly exothermic and violent, resulting in the cleavage of CC bonds. Chlorination is fast and exothermic while bromination is slow, as bromination of alkanes is less exothermic than chlorination. Direct iodination is not possible as reaction is reversible and highly endothermic.

1. Chlorination: Alkanes react with chlorine in the presence of UV light or diffused sunlight or at high temperature to yield the corresponding alkyl chlorides.

R H + Cl2 h , UV light

or Δ R Cl + HCl

Alkane Alkyl chloride eg. CH3 H + Cl2

h , UV lightor

CH3 Cl + HCl

Methane Methyl chloride 2. Bromination: Alkanes are heated with bromine in the presence of anhydrous aluminium tribromide

to give the corresponding alkyl bromides. R H + Br2

Anhydrous AlBr3 R Br + HBr Alkane Alkyl bromide

eg. CH3 CH2 H + Br2

Anhydrous AlBr3 CH3 CH2 Br + HBr Ethane Ethyl bromide Note: i. Direct halogenation of an alkane is a chain reaction and follows free radical mechanism. ii. This method of preparation gives the mixture of mono, di, tri and tetra halogen derivatives of an

alkane and it is difficult to separate each component in pure form. eg. Preparation of methyl chloride by direct halogenation method results in the formation of mono,

di, tri and tetra chloromethane derivatives. ̀ Therefore, halogenation (chlorination and bromination) of an alkane is not useful for laboratory preparation

of alkyl halide, because it gives mixture of different alkyl halides which are difficult to separate. Consequently, the yield of any one component is less due to the formation of other component.

Preparation of haloalkanes 10.5

CH4 + Cl2 hHCl

CH3 Cl Cl2HCl CH2Cl2

Cl2HCl CHCl3

Cl2HCl CCl4

Methane Methylchloride

Dichloromethane Trichloromethane Tetrachloromethane

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Chemistry Vol ‐ 2.2 (Med. and Engg.) 3. Iodination: i. Alkanes react with iodine to form the corresponding alkyl iodides. ii. This reaction is reversible and endothermic because hydroiodic acid (HI) during the course of

reaction acts as strong reducing agent and reduces alkyl iodide back to alkane. eg. C2H5 H + I2 C2H5 I + HI iii. So, this reaction is carried out in the presence of oxidising agent like mercuric oxide (HgO),

iodic acid (HIO3), dilute nitric acid (HNO3), etc., which reacts with HI and prevents backward reaction.

iv. In the presence of mercuric oxide (HgO): eg. 2CH4 + 2I2 + HgO 2CH3 I + HgI2 + H2O

v. In the presence of iodic acid (HIO3): eg. 5C2H5 H + 2I2 + HIO3 5C2H5 I + 3H2O

vi. In the presence of dilute nitric acid (HNO3):

eg. 8C2H5 H + 4I2 + dil.HNO3 8C2H5I + 3H2O + NH3

Note: Iodination stops at monoiodo stage. 4. Fluorination: Alkanes react with fluorine in an explosive manner. Halogenation of alkanes is not a

suitable method for preparing alkyl fluorides as the byproduct formed (hydrofluoric acid) is poisonous and corrosive.

ii. From halogenation of alkenes: a. When alkenes are treated with Br2 or Cl2 in the presence of solvent like CCl4, the addition reaction

takes place across the double bond to give vic-dihalides.

eg. H2C = CH2 + Br2 4CCl Br CH2 CH2 Br Ethene 1,2-Dibromoethane b. This reaction is used for detection of unsaturation (multiple bond) in an organic compound. The

disappearance of reddish brown colour of bromine due to formation of colourless vic-dibromide indicates the presence of a multiple bond.

Note: i. The reaction of alkenes (except ethylene) with Cl2 or Br2 at higher temperature (about 773 K) gives

substitution reaction product instead of addition reaction product. This is because at higher temperature, the addition reaction is reversible and the substitution reaction is irreversible. The hydrogen atom of allylic carbon is replaced with the halogen atom to form allylic halides and the reaction is called as allylic halogenation.

eg. H3C CH = CH2 + Cl2 773K Cl CH2 CH = CH2 + HCl

Propene 3-Chloropropene (Allyl chloride)

Methane Mercuricoxide

Methyliodide

Ethane Ethyl iodide Hydroiodic acid

Ethane Iodic acid

Ethyl iodide

Ethane Ethyliodide

Nitricacid

Alkane Alkyl iodide Hydroiodic acid

R H + I2 R I + HI

C = C + X2 4CCl X C C X

Vic-dihalideAlkene

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ii. Allylic halogenation is also carried out by using specific reagents like N-bromosuccinimide (NBS) and sulphuryl chloride at 473 K in the presence of light and peroxide as initiator.

eg. a.

b. H3C CH = CH2 + SO2Cl2 473K

h ,Peroxide Cl CH2 CH = CH2 + SO2 + HCl

Propene Sulphuryl 3-Chloropropene chloride iii. By addition of hydrogen halides to alkenes:

a. Alkyl halides can be obtained by the electrophilic addition of hydrogen halides like HCl, HBr, HI across the double bond of alkene.

Order of reactivity of hydrogen halides is: HI > HBr > HCl > HF

b. In case of symmetrical alkenes, alkyl group or number of hydrogen atoms present on either side of the doubly bonded carbon atoms is same, therefore during addition of HX, only one type of product is formed.

eg.

c. In the case of unsymmetrical alkenes, carbon atoms involved in double bond are non-equivalent, so the addition of HX in unsymmetrical alkene takes place according to Markownikoff’s rule.

d. According to Markownikoff’s rule, “during addition of an unsymmetrical reagent across the double bond of an unsymmetrical alkene, the negative part of reagent attacks on the carbon atom with less number of hydrogen atom(s) (more substituted carbon) and positive part of the reagent attacks on carbon atom with more number of hydrogen atom(s) (less substituted carbon)”. eg.

1.

2. e. But the addition of HBr to an unsymmetrical alkene in the presence of peroxide like Na2O2, H2O2,

benzoyl peroxide (C6H5CO)2O2 follows Anti-Markownikoff’s rule.

H3C CH = CH CH3 + HCl H3C CH2 CH CH3

But-2-ene

2-Chlorobutane

Cl

C = C + HX C C

XH

Alkyl halide

Hydrogenhalide

Alkene

I

H3C C = C CH3 HI

Markownikoff ’s rule H3C C C CH3 + H3C C C CH3

1 2 3 4 1 3

H

H

I

H 4 4 3 2 1

CH3

H

CH3

H

2-Methylbut-2-ene

CH3

2-Iodo-2-methylbutane (Major product)

2-Iodo-3-methylbutane(Minor product)

2

H3C CH = CH2 HCl

Markownikoff ’s rule H3C CH CH3 + CH3 CH2 CH2

Propene2-Chloropropane(Major product)

Cl

Cl

1-Chloropropane (Minor product)

Cyclohexene

+ hperoxide

Br

+

NBS 3-Bromocyclohexene Succinimide

O

O

N Br

O

O

N H

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f. According to Anti-Markownikoff’s rule, “during addition of HBr in the presence of peroxide, the negative part of reagent attacks on C-atom with more number of hydrogen atom(s) while positive part of reagent attacks on C-atom with less number of hydrogen atom(s)”.

This rule is also known as Peroxide effect or Kharasch effect or Kharasch-Mayo effect. eg. 1. 2. Note: Peroxide effect is observed only in case of HBr. HI and HCl follow Markownikoff’s rule even

in the presence of peroxide. iv. From Alcohols: Alkyl halides can be prepared from alcohols by substituting the hydroxy group of alcohols with halogen

atom. Following three types of reagents can be used to carry out this reaction. a. halogen acids b. phosphorus halides or c. thionyl chloride a. Reaction with halogen acids: 1. Chloroalkanes: i. Alcohols react with Lucas reagent (solution of concentrated HCl and anhydrous zinc chloride)

to form the corresponding alkyl chlorides. R OH + HCl Anhydrous ZnCl2

R Cl + H2O

ii. Primary and secondary alcohols react with concentrated HCl and anhydrous ZnCl2 to give the

corresponding alkyl chlorides. This process is called “Groove’s process”. eg. a. CH3 OH + HCl Anhydrous ZnCl2

CH3 Cl + H2O

b. iii. Tertiary alcohols readily react (simply by shaking) with concentrated HCl even in the absence

of anhydrous ZnCl2. eg.

CH3 C OH + HCl Anhydrous ZnCl2Room temperature

CH3 C Cl + H2O

CH3

H

CH3

H

Propan-2-ol 2-Chloropropane

(conc.)

CH3 C = CH2 + HBr (C H CO) O6 5 2 2Anti-Markownikoff ’s rule

H3C C CH2 Br + H3C C CH3

CH3

H

11 23 2 3

2-Methylpropene

1-Bromo-2-methylpropane(Major product)

CH3

CH3

Br

3 2 1

2-Bromo-2-methylpropane(Minor product)

H3C C OH + HCl Room temperature H3C C Cl + H2O

CH3

CH3

CH3

CH3

tert-Butyl alcohol tert-Butyl chloride

(conc.)

H3C CH = CH2 + HBr PeroxideAnti-Markownikoff ’s rule

H3C CH2 CH2 Br + H3C CH CH3

Br

Propene 1-Bromopropane(Major product)

123 3 2 1

2-Bromopropane(Minor product)

Alcohol (conc.) Alkyl chloride

Methanol (conc.) Methylchloride

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Note: Anhydrous ZnCl2 acts as a catalyst by helping in cleavage of C O bond. It is Lewis acid so it easily abstracts hydroxyl group of an alcohol by coordinating with oxygen of OH group. Due to this, weakening of C O bond takes place and it finally breaks to form carbocation. Chloride ion then readily reacts with carbocation to form chloroalkanes.

2. Bromoalkanes: i. Alkyl bromides are prepared by heating alcohol with hydrobromic acid (generated in situ by

treating sodium bromide or potassium bromide with conc. H2SO4). R CH2 OH NaBr Conc.H SO2 4

Reflux R CH2 Br + H2O + NaHSO4

eg. C2H5OH NaBr Conc.H SO2 4

Reflux C2H5Br + H2O + NaHSO4

ii. In the preparation of secondary and tertiary bromides from respective alcohols, conc. H2SO4 is

not used (as it may result in the dehydration of secondary and tertiary alcohols to form alkenes); instead dil. H2SO4 is used.

where R, R and R can be same or different alkyl groups. eg. a.

b. 3. Iodoalkanes: i. Alkyl iodides are prepared by heating respective alcohols with conc. hydroiodic acid (57 %). R OH + HI R I + H2O ii. Hydroiodic acid can be prepared in situ by reacting potassium iodide with 95% phosphoric

acid.

Alcohol (conc.) (57%)

Iodoalkane

Alcohol Alkyl bromide

Ethyl alcohol Ethyl bromide

H3C CH CH3 KBr dil .H SO2 4 H3C CH CH3 + H2O + KHSO4

Isopropyl alcohol

OH

Br

Isopropyl bromide

H3C C OH KBr dil. H SO2 4 H3C C Br + H2O + KHSO4

CH3

CH3

CH3

CH3

tert-Butyl alcohol tert-Butyl bromide

R CH R KBr dil.H SO2 4 R CH R + H2O + KHSO4

Secondary alcohol

OH

Br

Secondary alkyl bromide

R C OH KBr dil.H SO2 4 R C Br + H2O + KHSO4

Tertiary alcohol

R

R

Tertiary alkylbromide

R

R

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eg. a. CH3CH2CH2OH + KI + H3PO4

CH3CH2CH2I + KH2PO4 + H2O b. CH3 CH CH2 CH3 + KI + H3PO4

CH3 CH CH2 CH3 + KH2PO4 + H2O

4. Fluoroalkanes: Fluoroalkanes are not practically prepared by this method as hydrogen fluoride is least reactive. Note: i. The order of reactivity of alcohols in this reaction: Allyl alcohol > tertiary > secondary > primary. This is because of +I effect of alkyl group(s) attached to the - carbon atom of an alcohol, which

facilitates the cleavage of C O bond of an alcohol and increases the reactivity of alcohol. ii. The order of reactivity of halogen acids with alcohols is: HI > HBr > HCl > HF. This order is in accordance with bond dissociation energies. (Bond dissociation energy of HI is less

than that of HBr which is in turn less than that of HCl). b. Reactions with phosphorus halides: Haloalkanes are prepared by heating alcohols with phosphorus trihalides or phosphorus pentahalides. 1. Chloroalkanes: Alkyl chlorides are prepared by treatment of phosphorus pentachloride (PCl5) or phosphorus

trichloride (PCl3) on respective alcohols. R OH + PCl5

R Cl + POCl3 + HCl eg. CH3 OH + PCl5

CH3 Cl + POCl3 + HCl 3R OH + PCl3

3R Cl + H3PO3 eg. 3C2H5OH + PCl3

3C2H5 Cl + H3PO3

2. Bromoalkanes and iodoalkanes: i. Alkyl bromides and iodides are prepared by action of phosphorus tribromide (PBr3) or

phosphorus triiodide (PI3) on alcohols. ii. PBr3 is unstable and can be generated in situ by action of red phosphorus on Br2. 2P + 3Br2 2PBr3 3R OH + PBr3

red P Br2

3R Br + H3PO3

eg. 3CH3 CH2 CH2 OH + PBr3

red P Br2 3CH3 CH2 CH2 Br + H3PO3

Methanol Methylchloride

Phosphorusoxychloride

Phosphorustribromide

Propan-1-ol (n-Propyl alcohol)

1-Bromopropane (n-Propyl bromide)

Phosphorusacid

Phosphorus acidAlkyl bromideAlcohol

Alcohol Alkyl chloride

Phosphorusacid

Propan-1-ol Phosphoricacid (95%)

1-Iodopropane

Butan-2-ol

OH I

Phosphoric acid (95%)

2-Iodobutane

Ethanol Ethyl chloride Phosphorus acid

Red phosphorus

Phosphorustribromide

Bromine

Alcohol Alkyl chloride

Phosphorusoxychloride

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iii. PI3 is also unstable and it can be generated in situ as follows: 2P + 3I2 2PI3

3R OH + PI3

red P I2 3R I + H3PO3 eg. 3CH3 (CH2)3 CH2 OH + PI3 3CH3(CH2)3 CH2I + H3PO3 Note: i. Good yield of primary alkyl halides is obtained by this method. ii. Secondary and tertiary alcohols undergo dehydration to form alkenes and hence good yield of

secondary and tertiary alkyl halides is not obtained by this method. iii. In laboratory, lower alkyl bromides and iodides are prepared by this method. iv. PBr5 and PI5 does not exist. c. Reactions with thionyl chloride (sulphonyl chloride): 1. This reaction is used for the preparation of alkyl chlorides. When alcohols are refluxed with thionyl

chloride in the presence of pyridine base, corresponding alkyl chlorides are formed. R OH + SOCl2

PyridineReflux

R Cl + SO2 + HCl

eg. CH3 OH + SOCl2

PyridineReflux

CH3 Cl + SO2 + HCl

2. Chloro compounds obtained by this method can be easily isolated as both the byproducts of reaction

(SO2 and HCl) are gases and escape easily leaving behind pure alkyl chloride. Note: i. This process is also known as “Darzen’s procedure”. ii. Thionyl bromide is unstable and thionyl iodide does not exist, thus alkyl bromides and alkyl iodides

cannot be prepared by this method. v. By Halogen Exchange: a. This method is used for the preparation of alkyl iodides. Alkyl chlorides or bromides are heated with

solution of sodium iodide in dry acetone to give corresponding alkyl iodide. This reaction is known as “Finkelstein reaction”.

b. Sodium bromide and sodium chloride are less soluble in dry acetone and thus they get precipitated. c. These precipitates are removed by filtration and thus backward reaction is also prevented. d. Primary alkyl bromides and chlorides give best results by this reaction. R X + NaI Dryacetone R I + NaX (X = Cl, Br)

eg. CH3 CH2 Br + NaI Dryacetone CH3 CH2 I + NaBr

Thionyl chloride

Methanol Methyl chloride

Pentan-1-ol (n-Pentyl alcohol)

1-Iodopentane (n-Pentyl iodide)

Phosphorusacid

Phosphorustriiodide

Ethyl bromide

Ethyl iodideSodiumiodide

Alcohol Alkyl iodide Phosphorusacid

Phosphorus triiodide

Alkyl halide

Sodium iodide

Alkyliodide

Sodiumhalide

Red phosphorus

Phosphorustriiodide

Alcohol Thionyl chloride

Alkyl chloride

12


e. Alkyl fluorides can also be prepared by this method; by the action of mercurous fluoride (Hg2F2), silver fluoride (AgF), cobalt fluoride (CoF2) or antimony trifluoride (SbF3) on alkyl chloride or bromide.

2R X + Hg2F2 2R F + Hg2X2

(X = Cl, Br)

This reaction is known as “Swarts Reaction”. eg. 2CH3 Cl + Hg2F2 CH3 F + Hg2Cl2

Note: When the organic halides contain two or three halogen atoms on the same carbon atom, SbF3 or

CoF2 are used. eg. Physical properties: i. Physical state: a. Lower members of haloalkanes (when pure) are colourless gases at room temperature while higher

members are liquids or solids. b. Bromides and iodides develop colour when exposed to light. ii. Smell: Lower members of haloalkane series are sweet smelling liquids. iii. Solubility: Alkyl halides are very slightly soluble in water but readily soluble in organic solvents like

methanol, acetone, etc. iv. Density of haloalkanes: a. Bromoalkanes, iodoalkanes and polychloro derivatives of alkanes are heavier than water whereas

chloroalkanes and fluoroalkanes are lighter than water.

b. Density of haloalkane size of halogen atom and density of haloalkane 1

size of the alkylgroup.

c. Densities increase in the order: Fluoride < Chloride < Bromide < Iodide. v. Melting and boiling points of haloalkanes: a. Melting and boiling points of alkyl halides are greater than corresponding hydrocarbons. b. Due to the polarity of C X bond and high molecular mass, intermolecular forces of attraction

(dipole-dipole-London force and van der Waal’s force) between molecules of haloalkanes are stronger and results in increase in melting and boiling point.

c. Hence, boiling points of haloalkanes having same alkyl group increase in the order: H3C F < H3C Cl < H3C Br < H3C I d. In case of isomeric haloalkanes, branching results in decrease in boiling point. vi. Inflammable nature: Haloalkanes are less inflammable than hydrocarbons. They give green edged flame

with copper wire on heating (Beilstein test). Chemical properties: i. Reactivity of an alkyl halide (for the same alkyl group) decreases in the order given below: R I > R Br > R Cl > R F

Methyl chloride Methyl fluorideMercurousfluoride

Cl

Cl

F

F

2,2-Dichloropropane 2,2-Difluoropropane

3CH3 C CH3 + 2SbF3 3CH3 C CH3 + 2SbCl3

Alkyl halide

Mercurousfluoride

Alkyl fluoride(Fluoroalkane)

Physical and chemical properties of haloalkanes10.6

13


ii. Reactivity of alkyl halide depend on the polarity of C X bond as electronegativity of halogen atoms decreases in the order of F > Cl > Br > I; so strength of C F bond is more due to large difference in electronegativities (therefore it is more stable) whereas C I bond is less stable and shows high reactivity compared to other halogens.

iii. The order of reactivity among 1, 2 and 3 alkyl halide is: 3 alkyl halide > 2 alkyl halide > 1 alkyl halide. This is due to +I effect of an alkyl group which increases bond polarity of C X bond. Substitution reactions: Reactions in which an atom or a group of atoms is substituted by another atom or a group of atoms

respectively are known as substitution reactions. An alkyl halide shows nucleophilic substitution reaction due to polarity of C X bond. R X + Y R Y + X Alkyl Nucleophile Substituted halide alkane i. Hydrolysis: a. Alkyl halides on boiling with aqueous alkali hydroxide (KOH/NaOH) undergo hydrolysis to form the

corresponding alcohols. R X + KOH Boil R OH + KX b. During the course of reaction, X group of an alkyl halide gets substituted by OH to form an alcohol. eg. CH3 Cl + KOH Boil CH3 OH + KCl c. Alkyl halides on boiling with moist silver oxide undergo hydrolysis to form the corresponding alcohols. R X + AgOH moist Ag O2

Boil R OH + AgX

eg. CH3 (CH2)2 I + AgOH moist Ag O2

Boil CH3 (CH2)2 OH + AgI

Note: i. Silver hydroxide does not exist. ii. Silver oxide suspended in water behaves as silver hydroxide. ii. Formation of alkyl cyanides (alkane nitriles): a. Alkyl halides on boiling with alcoholic potassium cyanide form corresponding alkyl cyanides or

alkane nitriles. R X + KC N boil R C N + KX b. Halogen atom is substituted by nucleophile cyanide (CN) to form product, because of strong basic

nature of KCN, cyanide attacks through C-atom. eg. CH3 CH CH3 + K C N boil H3C C CH3 + KCl

nPropyl iodide nPropyl alcohol

Methyl chloride (aq.) Methyl alcohol

Alkyl halide (alc.) Alkyl cyanide

Alkyl halide (aq.) Alcohol Potassium halide

Alkyl halide Alcohol

2-Chloropropane (alc.)

2-Methylpropanenitrile

Cl

C N

|H

14


c. The product formed in the above reaction has one more carbon atom than the haloalkanes. Thus, the reaction is a good method for increasing the length of carbon chain.

iii. Formation of alkyl isocyanides (R N C):

a. Alkyl halide reacts with alcoholic silver cyanide (AgCN) to form corresponding alkyl isocyanide. R X + Ag C N R N C + AgX eg. C2H5 Br + Ag C N C2H5 NC + AgBr

b. In this reaction halide group is substituted by nucleophilic cyanide group to form product. c. In the presence of silver salt, nucleophilic attack takes place through N-atom of cyanide. iv. Formation of amines (ammonolysis): a. Alkyl halide on heating with alcoholic ammonia under pressure undergoes substitution reaction to

give corresponding primary amine. b. In this reaction, halide group is substituted by an amino (–NH2) group. R X + H NH2 under pressure

R NH2 + HX

eg. CH3 Cl + H NH2 under pressure

CH3 NH2 + HCl

Note: ‘R’ group in alkyl halide can be primary, secondary or tertiary. c. Order of reactivity of haloalkanes (for the same alkyl group) with NH3 is RI > RBr > RCl. d. When an alkyl halide is in excess, mixture of primary amine, secondary amine, tertiary amine and

quaternary ammonium salt is obtained.

R X + NH3

,underpressure

HX

R NH2R X

,under pressure,HX

R2NH R X,under pressure,HX

R3NR X

,under pressure

[R4N]+X

eg.

C2H5 Cl + NH3

,underpressure

HCl

C2H5 NH2C H Cl2 5

,under pressureHCl

(C2H5)2NH C H Cl2 5,under pressure

HCl

(C2H5)3N

e. This reaction is known as “Hoffmann’s ammonolysis reaction” or alkylation of ammonia. f. When excess of ammonia is used, primary amine is obtained as a major product. v. Formation of ethers (Williamson’s synthesis): a. Alkyl halide is heated with alkali alkoxide (KOR/NaOR) to give corresponding ether. This reaction is

known as “Williamson’s synthesis”.

Ethyl bromide (alc.) Ethyl isocyanide (Carbylaminoethane)

Methyl chloride

(alc.) Methylamine (Primary amine)

Alkyl halide (alc.) Alkyl isocyanide

Alkyl halide (alc.) Primaryamine

2 amine1 amineAlkyl halide

3 amineQuaternary ammonium

salt

(1 Alkyl halide)

Tetraethyl ammonium chloride(Quaternary salt)

C2H5 Cl

[(C2H5)4N]+Cl

, under pressure

Ethyl chloride

Ethylamine(1 amine)

Diethylamine(2 amine)

Triethylamine (3 amine)

15


R X + Na OR R O R + NaX b. In this reaction, halide group undergoes substitution with alkoxy OR group. c. Sodium alkoxide can be prepared by action of sodium metal on alcohol. 2R OH + 2Na 2R ONa + H2 eg. CH3 I + NaOCH3

CH3 O CH3 + NaI d. When haloalkanes are heated with dry silver oxide, symmetrical ethers are obtained. 2R X + Ag2O R O R + 2AgX eg. 2C2H5 CH2Cl + Ag2O C2H5 CH2 O CH2 C2H5 + 2AgCl Note: Silver oxide used should be completely dry, as traces of moisture may result in alcohol formation. vi. Formation of esters: a. Ethanolic solution of silver salt of a fatty/carboxylic acids on heating with haloalkanes give corresponding esters. b. In this reaction, halogen group is substituted by carboxylate (R COO ) group. c. During this reaction, carboxylate ion (R COO) acts as a nucleophile. eg. CH3 I + Ag O C CH3

C H OH2 5 CH3 O C CH3 + AgI

vii. Formation of alkyl nitrite and nitroalkanes: a. Alkyl halide (R X) on treatment with KNO2 forms alkyl nitrite (R O N = O) whereas on

treatment with AgNO2 forms nitroalkane (R NO2). b. The nitrite ion possesses two nucleophilic centres (i.e., it is an ambident nucleophile). c. The linkage through oxygen results in the formation of alkyl nitrites whereas the linkage through

nitrogen results in the formation of nitroalkanes.

Methyliodide

Sodium methoxide

Dimethyl ether

Alcohol Sodium alkoxide

n-Propyl chloride (1-Chloropropane)

Dry silver oxide

Dipropyl ether (1-Propoxypropane)

Alkyl halide Sodium alkoxide

Ether

Alkyl halide Dry silver oxide

Ether

Methyl iodideSilver acetate

Methyl acetateO

O

O Alkyl halide Ester

Silver salt of carboxylic acid

O

R X + Ag O C R C H OH2 5 R O C R + AgX

R X + K+O – N = O Heat R O N = O + KXAlkyl halide

Alkyl nitritePotassium nitrite

R X + AgNO2 R NO2 + AgXAlkyl halide

NitroalkaneSilver nitrite

16


Note: Nucleophilic substitution of alkyl halides (R – X): Reagent Product Reagent Product

i. KOH / NaOH / moist Ag2O

R – OH viii. RCOOAg RCOOR

ii. Alcoholic KCN R – CN ix. NaI R – I iii. Alcoholic AgCN R – NC x. KNO2 R – O – N = O iv. Alcoholic NH3 R – NH2 xi. AgNO2 R – NO2

v. NaOR R – O – R xii. LiAlH4 R – H vi. H2O R – OH xiii. R – M+ R – R vii. Dry Ag2O R – O – R

Elimination Reactions: Elimination reactions are those reactions in which a molecule loses two atoms or groups attached to

neighbouring carbon atoms with formation of double bond between carbon atoms. OR The reaction in which two atoms or groups are removed from adjacent carbon atoms in a molecule to form

an unsaturated compound is called an elimination reaction. Dehydrohalogenation (formation of alkenes): i. When alkyl halides are heated with alcoholic solution of alkali hydroxide (KOH/NaOH), halogen atom

from -carbon atom and a hydrogen atom from adjacent -carbon atom gets eliminated to form corresponding alkenes.

ii. This reaction is also called as “dehydrohalogenation of an alkyl halide”. iii. As hydrogen atom is eliminated from -carbon atom, it is also known as “-elimination reaction”. eg. CH3 CH2 CH2 I + K+OH CH3 CH = CH2 + H2O + KI iv. In dehydrohalogenation of secondary and tertiary alkyl halides there is possibility of formation of two

isomers of alkene, in such a case elimination takes place according to Saytzeff’s rule. v. According to Saytzeff’s rule, “when there is a possibility of formation of two types of alkenes by

dehydrohalogenation of alkyl halide, then H-atom is eliminated preferentially from C-atom having least number of H-atom(s)”. In other words, in dehydrohalogenation reaction, more substituted double bond formation is always preferred.

vi. In dehydrohalogenation, reactivity of alkyl halide is in the following order: RI > RBr > RCl > RF (when same alkyl group is present). Ease of dehydrohalogenation in case of haloalkanes follows the order: Tertiary > secondary > primary (when same halogen group is present). eg. H3C CH2 CH CH3

alc.KOHHBr H3C CH = CH CH3 + CH3 CH2 CH = CH2

Reaction with metals: Alkyl halides react with metals such as sodium to form corresponding higher saturated hydrocarbon and

with magnesium to form organometallic compounds. i. Reaction with sodium or Wurtz synthesis: a. Haloalkanes when treated with metallic sodium in the presence of dry ether form corresponding

symmetrical higher alkanes.

n-Propyl iodide (alc.) Propylene

Br 2-Bromobutane

But-2-ene(80%)

But-1-ene (20%)

R C C X + K+OH R C = C + H2O + KX

H

H

H

H

Alkyl halide Alkene

H

H

H(alc.)

17


2R X + 2Na dryether R R + 2NaX b. This reaction is called as “Wurtz synthesis”. c. The product formed contains more number of carbon atoms than reactants; thus, this method is

preferably used for the preparation of higher alkanes. eg. 2CH3 CH Br + 2Na dry ether H3C CH CH CH3 + 2NaBr d. Tertiary halides do not undergo this reaction. ii. Reaction with magnesium or formation of Grignard’s reagent: a. Grignard’s reagent can be prepared by reaction of alkyl halide with pure and dry magnesium in the

presence of dry ether. R X + Mg dry ether R Mg X Alkyl halide (Dry) Alkyl magnesium halide (Grignard’s reagent) b. Grignard’s reagent is chemically known as alkyl magnesium halide and represented by general

formula R Mg X. eg. CH3 I + Mg dryether CH3 Mg I Methyl (Dry) Methyl magnesium iodide iodide

c. The Grignard reagents are very reactive compounds and react with any source of proton to form corresponding hydrocarbons.

R Mg X + ZH R H + ZMgX Grignard reagent Alkane

where, Z = OH, OR, NH2,etc. Note: i. Compound in which less electropositive carbon atom is directly attached to highly electropositive metal

atom is called “Organometallic compound”. ii. In this compound, C-atom has partial negative charge and metal atom has partial positive charge. iii. In Grignard’s reagent, C Mg bond is highly polar and Mg X bond is ionic in nature. Hence, Grignard’s reagent are highly reactive towards organic as well as inorganic reagents. R Mg X iv. a. Traces of moisture (if remained during preparation of Grignard’s reagent) readily react with Grignard

reagent to form corresponding alkane; also Grignard’s reagent in free state is explosive in nature. b. Hence, Grignard’s reagent is never stored and always prepared at the time of requirement. c. It is used in the absence of air, under inert atmosphere like dry ether (as a solvent). Stereochemistry plays an important role in deciding the product of any reaction specially nucleophilic substitution reaction. Some basic stereochemical notations and concepts are given below: Ordinary Light: Ordinary light consists of electromagnetic radiations of different wavelengths, vibrating in all possible directions in space and perpendicular to direction of propagation of light. Monochromatic Light: i. Ordinary light after passing through monochromator (prism or grating monochromator) emerges out as a

ray of single wavelength and is called as “Monochromatic ray of light”. ii. Monochromatic ray of light vibrates in different planes, perpendicular to the direction of propogation of light.

+

Alkyl halide Higher alkane

CH3

CH3

CH3

2-Bromopropane 2,3-Dimethylbutane

Stereochemistry 10.7

18


Plane Polarized Light: i. A beam of light vibrating in only one plane in space is called “Plane Polarized Light”. ii. Ordinary beam of light after passing through Nicol’s prism (crystalline calcium carbonate) emerges out as

plane polarized light. iii. Nicol’s prism is called as polarizer in which vibrations in all other planes are cut off except one plane. iv. Nicol’s prism is combination of two prisms made of calcite crystals and fused base to base by Canada balsam. Optical Activity: i. When solution of certain organic compounds come in contact with plane polarized light, they rotate the

plane of plane polarized light by some angle either in clockwise or anticlockwise direction. This property of organic substance is called as “Optical activity”.

ii. Polarimeter is the instrument used to measure optical activity (i.e., to measure the magnitude and the

direction of the rotation of plane of plane polarized light) of an optically active compound. iii. The polarimeter consists of a light source, two nicol prisms and the sample tube to hold the substance. The

prism placed near the source of light is called polariser while the other placed near the eye is called analyser. Optically Active Molecule: If a molecule is capable of rotating plane of plane polarized light in either clockwise or anticlockwise

direction, it is called as “Optically Active Molecule”. eg. Lactic acid, 2-Iodobutane, glucose, fructose, etc. d - l configuration: Depending upon the behaviour of molecules of the compound towards plane polarized

light; they can be differentiated as follows: i. Dextro Rotatory Molecules: a. If a molecule is capable of rotating plane of plane polarized light to the right i.e., in the clockwise

direction then it is called as “Dextro Rotatory Molecule”. (Latin, dexter = right) b. These are designated as (+) or (d). eg. ii. Laevo Rotatory Molecules: a. If a molecule is capable of rotating plane of plane polarized light to the left i.e., in the anticlockwise

direction then it is called as “Laevo Rotatory Molecule”. (Latin, Laevus = left)

1.

H3C C* C2H5

H

(+)/(d)–2-Iodobutane

I 2.

H C* OH

COOH

CH3

(+)/(d)-Lactic acid

Ordinary light Nicol’s prism(Polarizer)

Plane Polarized light

Plane Polarized Light

Rotation of Plane Polarized Light due to an Optically Active Substance

Sample containing optically

active substance Plane polarized light

or

Clockwise rotation by

Anticlockwiserotation by

Analyser

19


b. These are designated as () or (l). eg. 1. 2. iii. Racemic Mixture: a. Equimolar mixture of dextro and laevo form of the same compound is known as “Racemic Mixture”

or “Racemic modification” or “Racemate”. b. The process of conversion of enantiomer into a racemate is called as racemisation. c. Racemic mixture is optically inactive and does not rotate the plane of plane polarized light. d. When dextro and laevo forms of molecule cancel each other’s rotation (which is equal but in opposite

direction), it is known as “External Compensation”. e. It is designated as (dl) or (). eg. dl -Lactic acid, 2-Iodobutane, etc. Optically Inactive Molecules: Optically inactive molecules are those which do not rotate the plane of plane polarized light. eg. Ethyl chloride, water, etc. Chirality: i. Four valencies of carbon atom are arranged along the four corners of regular tetrahedron. If all 4 atoms or

groups attached to such carbon atom are different, then it is called as “Asymmetric” or “Chiral carbon atom” or “Stereocentre” and it is denoted by star or asterisk (*) on it .

ii. When molecule contains asymmetric carbon atom, the symmetry of molecule is lost, i.e., its mirror image is non-superimposable with itself, such molecule is known as “Asymmetric molecule”.

eg. 2-Iodobutane

a. 2-Iodobutane contains asymmetric carbon atom, its mirror image is non-superimposable on each other. b. B is the mirror image of A. Position of CH3 group in A does not coincide with the position in mirror

image B. c. Same is the case for ethyl group also. Under such condition, mirror image is non-superimposable on

each other. iii. Therefore, it can be said that molecule on whole must be non-superimposable on its mirror image, such a

molecule is called as “Chiral Molecule” and the property of non-superimposability is called “Chirality”. iv. Chiral molecule exists as d and l form and is an optically active molecule whereas molecule which is

superimposable on its mirror image is called “Achiral molecule” and it does not exist as d and l form, therefore are optically inactive.

Vant Hoff Le Bel Theory: i. Phenomenon of optical activity was discovered by French physicist Biot in 1815. ii. Though optical activity was discovered, for many years direct correlation between optical activity and the

structure of molecule was not known. iii. First convincing explanation for this corelation was given by Dutch scientist J. Vant Hoff and French

scientist Le Bel in 1874. iv. They independently put forward the theory of optical activity.

A B

I

CCH3

H

H5C2

*

I

CC2H5

H

H3C

*

HO C* H

COOH

CH3

()/(l)-Lactic acid

H5C2 C* CH3

I

H

()/(l)-2-Iodobutane

20


v. Almost all the scientists until 1874 believed that all the molecules are always two dimensional i.e., they are flat entities.

vi. Van’t Hoff and Le Bel for the first time proposed the three dimensional structure of molecules. vii. According to this theory:

a. In sp3 hybridized carbon atom, all four valencies are pointed towards four corners of regular tetrahedron. b. If all the four valencies of carbon atom are satisfied by different atoms or groups of atoms, then C-

atom is known as asymmetric carbon atom. c. 1. In an asymmetric carbon atom, two bonds are on plane of the paper shown by ordinary line.

2. One bond is below the plane of paper (i.e., away from observer) and is shown by dotted line. 3. The other bond is above the plane of paper (i.e., towards the observer) and is shown by thick wedge. eg. 2-Iodobutane

d. Asymmetric centre is denoted by asterisk (*) on it. e. Molecules containing asymmetric carbon atom always exists as a pair of isomers which are non-

superimposable mirror images of each other. Optical Activity of Lactic Acid: i. Number of possible stereoisomers shown by a molecule is dependent upon the number of asymmetric

carbon atoms present. ii. Number of possible stereoisomer is given by a formula = 2n

where, n = number of asymmetric carbon atom(s) present in that molecule. iii. Lactic acid contains one asymmetric carbon atom which is attached to COOH, OH, CH3 groups and H

atom. Therefore, Number of stereoisomers = 2n = 21 iv. Lactic acid shows 2 stereoisomers i.e., d and l form which are non-superimposable mirror images of each other. Note: i. Stereoisomers which are non-superimposable mirror images of each other and rotate the plane of the plane

polarized light through the same angle but in opposite directions are known as enantiomers or enantiomorphs. eg. d and l forms of lactic acid are called enantiomorphs of each other. ii. Enantiomers have identical physical properties (except the direction of rotation of plane polarized light,

though the amount of rotation is same) and chemical properties (except towards optically active reagents.) R, S Configuration: i. R, S nomenclature system was devised by Cahn, Ingold and Prelog which indicates configuration i.e.,

arrangements of atoms or groups around chirality centre. ii. Rules for R, S nomenclature to determine the priority of groups attached to chiral centre are given as follows: a. Groups arranged around chiral centres are given a priority order. Higher the atomic number of an atom directly attached to chiral centre, higher its priority. b. If two groups have identical atom directly attached to the chiral centre then the next atom in group is

considered to determine the priority.

I

CCH3

H

H5C2

*

l-lactic acid

C*

COOH

H

CH3

HO

d-lactic acid

C*

COOH

OH

CH3

H

21


eg. This is because carbon atom in methyl group is attached to 3 hydrogen atoms, whereas in ethyl group it is attached to 2 hydrogen and 1 carbon. Hence ethyl group is given higher priority.

c. When group(s) having multiple bonds (CHO, C N, >C = C<) is/are attached to chiral centre, then atoms attached to double or triple bond are considered as duplicate or triplicate. Priority is given by considering triplicate and duplicate structure as shown below:

eg. is given higher priority over C = C because in , carbon is attached to 2 oxygen atoms (one oxygen + one phantom oxygen) and in carbon atom is attached to

other 2 C atoms (1 C-atom and other phantom C-atom). d. In the case of benzene ring, it is considered as one of the resonating structure. e. The order of priority is as follows:

I > Br > Cl > SO3H > F > OCOCH3 > OH > NO2 > NH2 > COOCH3 > COOH > CONH2 > COCH3

> CHO > CH2OH > CN > C6H5 > C2H5 > CH3 > D > H iii. Tetrahedral structure of molecules can be drawn as follows: Two bonds in the plane of paper (indicated by line), one bond above the plane of paper (indicated by thick

wedge) and one bond below the plane of paper (indicated by dotted line). iv. The group attached to the dotted line should have the least priority. eg. 2-Chlorobutane (order of priority Cl > C2H5 > CH3 > H)

H C CH3

Cl

C2H5

2-Chlorobutane

Order of priority: Cl > C2H5 > CH3 > H

C = O

C

C = O

C

CHHC

C

(C)

(C) (C)

(a)

CH3

Cl

C

HH5C2

*4

3

1

2

(b)

CH3

Cl

CC2H5

*

H4

3

1

2

C = C

H H

C = O C O

(N)

C N C N (C)

(N) (C)

C C

(O) (C)

;

C = C C C

(C)

C = O C O

H H

(C) (O) (C)

HHH H

;

22


v. It is like holding dotted line in a hand and looking at the structure from opposite side (as if viewing a bouquet of flowers) then with this view, order of priority is 1 2 3 4.

vi. In structure (b), order of priority is in clockwise direction i.e., from right therefore it is R-configuration (Latin word rectus meaning right).

vii. In structure (a), order of priority is in anticlockwise direction i.e., from left therefore it is S-configuration (Latin word sinister meaning left).

viii. Two more examples: a. b. Nucleophilic substitution reaction: i. A reaction in which one nucleophile is substituted by other nucleophile is known as “Nucleophilic

Substitution Reaction”. ii. Nucleophilic substitution reaction can proceed by two different paths depending on the nature of substrate,

the nucleophile, the leaving group and solvent. The two paths are: a. SN1 b. SN2 mechanisms. Note: i. Mechanism of reaction: Mechanism of reaction is a step by step description of exactly how the reactants are transformed into

product in as much details as possible. ii. Transition state: During the course of reaction, reactants change from one form to other through certain state. This

state is known as Transition state. The minimum energy necessary to fulfill all the conditions for the formation of transition state is

called as the energy of activation of the reaction (Eact). iii. Energy Profile Diagram: The energy changes of chemical reaction are depicted by energy profile diagram which shows the

progress of the reaction along a path from reactants to the product. iv. Rate Determining Step (R. D. S): a. Slowest step in the reaction mechanism which determines the rate of reaction is known as Rate

Determining Step. b. The rate determining step involves breaking of bond which requires input of energy and hence

it is the slowest step in the course of the reaction.

1I

C

H

*

R-1-Iodo-2-methyl-1-phenylpropaneS –1-Iodo-2-methyl-1-phenylpropane

I

C*

C6H5

C(CH3)2

1

2 H5C6 4 4

H 33 C(CH3)2

2

R –Lactic acid

CH3

OH

CCOOH

*

H4

3

1

2

S –Lactic acid

CH3

OH

C

H HOOC

* 4

3

1

2

Nucleophilic substitution mechanism10.8

23


SN2 Mechanism: i. When primary alkyl halide reacts with aqueous alkali, corresponding primary alcohol is formed. The

reaction is called as hydrolysis of an alkyl halide. R CH2 X + OH R CH2 OH + X

1 Alkyl halide (aq) 1 Alcohol eg. H3C Br + NaOH(aq)

CH3 OH + NaBr Methyl bromide Methyl alcohol

In this reaction, OH nucleophile substitutes halide ion of 1 alkyl halide to form 1 alcohol.

H3C Br + OH H3C OH + Br Methyl bromide Methyl alcohol

ii. In this reaction, the rate of formation of 1 alcohol is found to be proportional to the concentration of an alkyl halide and also to that of base used. Rate [H3C Br] [OH] Rate = k [H3C Br] [OH] (where k = proportionality constant)

The rate of reaction is dependent on the concentration of both the reactants. Therefore, it is second order i.e., bimolecular reaction. Hence this reaction is known as “ Nucleophilic Substitution Bimolecular Reaction” and denoted as “SN2”.

iii. Mechanism: a. It is one step concerted mechanism in which the formation of C OH bond and breaking of C Br

bond takes place simultaneously. b. The formation of transition state is slow step i.e., rate determining step. c. Transition state (T. S.) is the highest energy state in the course of reaction. d. It may change into product or may go back to reactants. e. In the transition state, both incoming nucleophile (OH) and outgoing halide group (X) share the

negative charge and C-atom carry partial positive charge. f. When the C OH bond forms completely, at the same instant C Br bond breaks completely and the

reaction is completed. iv. Energy Profile Diagram:

In SN2 mechanism, heat of reaction (ΔH) is negative, hence it is exothermic reaction. The product formed is

of much lower energy than reactant, therefore product is more stable. v. Stereochemistry: a. In SN2 mechanism, nucleophile (OH) attacks the C-atom of 1 alkyl halide from backside, this is due

to the following reasons: 1. It is least hindered (crowded) site for attack of OH. 2. Electrostatic attraction between carbon atom (with + charge) and OH (with charge). 3. Electrostatic repulsion between OH and Br is minimum.

R = reactants: CH3 Br + OH

T. S. = transition state Eact = Activation energy ΔH = Heat of reaction P = Products: H3C OH + Br

Energy profile diagram for SN2 mechanism

H

Eact

T.S.

P

R

Pot

enti

al e

nerg

y

Reaction co-ordinate

24


b. SN2 mechanism results in the inversion of configuration i.e., in the product, OH occupies position exactly

opposite to that of Br and positions of H2 and H3 atoms are exactly opposite in product to that in reactant. Note: i. Order of reactivity for halide atom is I > Br > Cl > F; because as size of atom increases, the bond

dissociation energy decreases. ii. Reactivity of an alkyl halide in SN2 mechanism is in the following order: CH3X > 1 alkyl halide > 2 alkyl halide > 3 alkyl halide. iii. In 1896, Paul Walden theoretically anticipated inversion of configuration. iv. In 1935, Ingold and Hughes gave experimental evidences for inversion of configuration. v. The inversion is known as ‘Walden’ inversion. SN1 Mechanism: i. When tertiary alkyl halide reacts with aqueous alkali, tertiary alcohol is formed.

eg.

R C X + NaOH R2 C OH + NaX

R1

R3

3 Alkyl halide

(aq.)

3 Alcohol

R1

R3

H3C C Br + NaOH H3C C OH + NaBr

CH3

CH3

CH3

CH3

tert-Butyl bromide (3 Alkyl halide)

(aq.)

tert-Butyl alcohol(3 Alcohol)

C *

H1

H2

H3

HO

HO + slow stepR.D.S.

Fast + BrBr HO

C +

H1

H3H2

(Inversion of configuration) Backside attack of nucleophile

C*

H1

Br

H3

H2

1alkyl halide 1 alcohol Transition state

NuNu Nu

C

HH

H

Isopropylhalide (2)

Methylhalide

Ethyl halide (1)

HH

HC

HH

H CH

H H

X C

H H

XC

H

XC

HH

H

XNu

CH

H H

HH

H C

C

tert-Butyl halide (3) (Maximum steric

hindrance)

C

Steric hindrance increases

25


ii. Study of reaction kinetics shows that, the rate of formation of 3 alcohol is proportional to the concentration of only 3 alkyl halide.

Rate [(H3C)3C X] Rate = k [(H3C)3C X] It is the first order reaction i.e., unimolecular reaction. Hence this reaction is known as “Nucleophilic

Substitution Unimolecular Reaction” and denoted as “SN1”. iii. Mechanism: A two step mechanism has been proposed for this type of substitution.

a. The first step is a slow (rate determining) step, which involves heterolytic cleavage of C X bond to form a carbocation as an intermediate.

b. Second step is fast, in which nucleophile OH attacks highly reactive carbocation, to form a product.

iv. Energy Profile Diagram: From energy profile diagram it is clear that, in SN1 mechanism ΔH is negative. Hence, it is an exothermic

reaction. v. Stereochemistry: a. In this reaction, carbocation formed has planar structure (C-atom is sp2 hybridized) therefore

nucleophile OH can attack from both front and back side of carbocation. b. Back side attack of OH results in the inversion of configuration i.e., OH occupies position opposite

to halide ion and position of remaining group is opposite to that of reactant.

C*

CH3

OH

CH3

CH3

CH3

C+

CH3CH3

+ OH Fast

3 AlcoholCarbocation

C *

CH3

X

CH3

CH3

Carbocation

Slow stepR.D.S.

CH3

C+

CH3CH3

+ X

3 Alkyl halide

Energy profile diagram for SN1 mechanism

R

Pot

enti

al e

nerg

y

P

ΔH

T.S.2

T.S.1

act2E

Reaction co-ordinates

act1E

C +

R

R R

R = reactant i.e., T.S.1 = Transition state of first step

T.S.2 = Transition state of second step Eact1 = Activation energy of first step Eact2 = Activation energy of second stepΔH = heat of reaction P = product i.e.,

= Carbonium ion/carbocation

(where R, R and R may be same or different.)

C

R R

+

R

R C X

R

R

R C OH

R

R

26


c. Front side attack of OH results in the retention of configuration i.e., position of X is taken up by OH and remaining groups are exactly at the same position as that of reactant.

d. In SN1 mechanism of optically active reactants, the two configurations formed are non-superimposable mirror images of each other i.e., enantiomers and they are formed in nearly equal proportions. Therefore product formed is a racemic mixture () which is optically inactive.

Note:

i. In SN1 mechanism, the reactivity of the halide, R-X, follows order: R–I > R–Br > R – Cl > R–F; because as size of atom increases, the bond dissociation energy decreases.

ii. Stability order for carbocation is 3 > 2 > 1.

eg. a. 3H C

, H3C 2CH

b. H3C CH

c. iii. 3 alkyl halides prefer SN1 mechanism; 2 alkyl halides show mixed mechanism whereas 1 alkyl halides

prefer SN2 mechanism.

iv. Halides in which halogen atom is bonded to a sp3 hybridized carbon atom next to an aromatic ring are called benzylic halides.

eg. a. b.

1 Carbocation(Least stable)

Carbocation

Slow stepR.D.S.

3 Alkyl halide

C *

R1

X

R3

R2

+ X

R1

C+

R3R2

C*

R1

R2

R3

HO

R1

C+

R3R2

Back sideattack

Front sideattack

Inversion of configuration (50%)

HO

C*

R1

OH

R3

R2

Retention of configuration (50%)

CH2X

(where X = F, Cl, Br, I)

Benzylic halide

CH3H3C

C+

CH3

3 Carbocation(Most stable)

CH3

2 Carbocation

CH2Br

Bromophenylmethane(1 benzylic halide)

H3C C Br

CH3

2-Bromo-2-phenylpropane(3 benzylic halide)

27


v. Benzylic halides form carbocation which undergoes stabilization through resonance as follows: vi. Halides in which halogen atom is bonded to a sp3

hybridized carbon atom next to a carbon carbon double bond are called as allylic halides.

eg. a. H2C = CH CH2 b. H3C CH = CH CH I 3-Haloprop-1-ene forms carbocation which undergoes stabilization through resonance as follows: CH2 = CH CH2 H2C CH = CH2 vii. Benzylic and allylic halides may be primary, secondary or tertiary in nature; but they undergo SN1 mechanism. Comparison between SN2 and SN1:

No. Factor SN2 SN1 i. Kinetics 2nd order 1st order ii. Molecularity Bimolecular Unimolecular iii. Number of steps One step Two steps iv. Bond making and

bond breaking Simultaneous First the bond in the reactant breaks

and then a new bond in product is formed

v. Transition state One step, one transition state Two steps, two transition state vi. Direction of attack

of nucleophile Only back side attack Back side attack and front side attack

vii. Stereochemistry Inversion of configuration (If substrate is optically active)

Racemisation (If substrate is optically active)

viii. Type of substrate Mainly 1 substrates Mainly 3 substrates ix. Polarity of solvent Non-polar solvent favourable Polar solvent favourable x. Nucleophile Strong Nucleophile favourable Weak Nucleophile favourable xi. Intermediate No intermediate Intermediate involved

Haloarenes: i. The halogen derivatives of aromatic hydrocarbons are called as haloarenes or aryl halides.

OR Haloarenes are obtained by replacing one or more hydrogen atom(s) of an arene with corresponding

number of halogen atom(s). ii. They are obtained by substituting H-atom of aromatic ring with halogen atom. Ar H X2

Lewis acid310K 320K

Ar X + HX (where X = F, Cl, Br, I)

Benzene Haloarene

+ +

CH2+

CH2

(I) (II)

+CH2

(III)

+ +(IV)

CH2

Cl

CH3

3-Chloroprop-1-ene (1 allylic halide)

4-Iodopent-2-ene(2 allylic halide)

Haloarenes 10.9

3-Haloprop-1-ene (Allyl halide)

CH2X

28


Classification: Depending upon the number of halogen atom(s) attached to an aromatic ring, they are classified as follows: Nomenclature: Common and IUPAC names of some of the haloarenes:

Sr. No. Structure Common Name IUPAC Name

i.

Chlorobenzene Chlorobenzene

ii.

o-Chlorotoluene 1-Chloro-2-methylbenzene or 2-Chlorotoluene

iii.

m-Chlorotoluene 1-Chloro-3-methylbenzene or 3- Chlorotoluene

iv.

p-Chlorotoluene 1-Chloro-4-methylbenzene or 4-Chlorotoluene

v.

Bromobenzene Bromobenzene

vi.

o-Dibromobenzene 1,2-Dibromobenzene

vii.

m-Dibromobenzene 1,3-Dibromobenzene

viii.

p-Dibromobenzene 1,4-Dibromobenzene

Cl

Cl

CH3

1 3

Cl

CH3

1

4

Cl CH3

1 2

Br

Br

3

1

Br

Br

Br 1

4

Br Br

1 2

Haloarenes

Monohaloarenes Dihaloarenes Trihaloarenes Polyhaloarenes

One halogen atom isattached to benzene.

Two halogen atomsare attached tobenzene.

Three halogen atomsare attached tobenzene.

More than three halogenatoms are attached tobenzene.

Chlorobenzene

eg.Cl

eg.

1,2-Dichlorobenzene

ClCl

eg.

1,2,3-Trichlorobenzene

ClCl

Cl

1,2,3,5-Tetrachlorobenzene

eg. Cl

Cl

Cl

Cl

29


ix.

m-Bromochlorobenzene 1-Bromo-3-chlorobenzene

x.

sym-Tribromobenzene 1,3,5-Tribromobenzene

xi.

— 1,2,3,5-Tetrabromobenzene

i. In haloarenes, halogen atom having p-orbital with unpaired electron overlaps with sp2 hybrid orbital of C-

atom of benzene ring to form C X bond. ii. Lone pair of electrons from halogen atom is involved in electron system of aromatic ring, showing

extended conjugation. As result of resonance, C X bond shows following structures: iii. Thus, in aryl halides, C X bond acquires partial double bond character making itself stronger and shorter

in length than in alkyl halides. Haloarenes can be prepared by the following methods: i. By Electrophilic Substitution: a. Chloroarenes and bromoarenes can be prepared from benzene or aromatic hydrocarbons by treatment

with Cl2 or Br2 in the presence of Lewis acid like iron, FeCl3, FeBr3, BCl3, AlCl3, etc. at ordinary temperatures (310 K 320 K).

b. This method is called as direct halogenation of aromatic compounds. c. Lewis acid acts as catalyst and halogen carrier for electrophilic substitution. d. If excess of reagent is used, then second halogen atom is introduced at ortho or para position with

respect to the first halogen. This is because halogens are o-, p-directing groups. The ortho and para isomers can be easily separated due to large difference in their melting points.

X X X X+++

(where X = F, Cl, Br, I)

H+ Br2

FeBr3310K 320K

dark

Br+ HBr

Benzene Bromobenzene

H+ X2

Lewis acid310K 320K

dark

X+ HX

Benzene Halobenzene

Br

+ Br2 FeBr3

310K 320Kdark

Bromobenzene excess

BrBr

+

Br

+ HBr

o-Dibromobenzene

p-DibromobenzeneBr

Cl

Br

3

1

Br

Br

3

1

Br 5

Br Br

Br

Br

Nature of C X bond in haloarenes 10.10

Preparation of haloarenes 10.11

30


e. Direct iodination of benzene ring is a reversible reaction due to the HI (strong reducing agent) which is formed as a byproduct, hence reaction is carried out in the presence of strong oxidising agent (like nitric acid or iodic acid or mercuric oxide).

The above reversible reaction can proceed in forward direction in presence of an oxidising agent.

eg. 1. 5HI + HIO3 3H2O + 3I2

2. 2HI + HgO HgI2 + H2O f. Fluorine reacts violently and uncontrollably with benzene or other aromatic hydrocarbons. Thus,

fluoroarene compounds cannot be prepared by direct fluorination method.

ii. Sandmeyer’s reaction:

a. When primary aromatic amine (like aniline) is treated with sodium nitrite and dilute HCl at 273 K – 278 K, it results in the formation of benzene diazonium salt.

b. Reaction of freshly prepared diazonium salt solution with cuprous (I) salt (cuprous chloride or cuprous bromide dissolved in corresponding halogen acids) results in the formation of chloro or bromobenzene respectively. This reaction is known as “Sandmeyer’s reaction”.

eg. 1. 2. c. Diazonium salt on treatment with KI gives iodobenzene.

eg.

Physical properties:

i. Density of haloarenes:

a. Bromo, iodo and polychloro derivatives of arenes are heavier than water.

b. The density of haloarenes increases with increase in number of carbon atoms, halogen atoms and atomic mass of halogen atoms.

ii. Melting and boiling points of haloarenes:

Boiling points of isomeric dihalobenzenes are nearly the same. However, the melting point of para-isomer is higher as compared to ortho- and meta-isomers. It is because of the symmetry of para-isomers which fit in the crystal lattice better as compared to ortho- and meta isomers.

Iodic acid

Mercuric oxide

+ I2 + HI

Benzene Iodobenzene

I

I

Benzene diazonium chloride

+ KI

Iodobenzene

+ N2 + KClN

NCl

NH2 NaNO /dil.HCl2

273K 278K,H O2

Cu Br /HBr2 2HCl

Aniline Benzene diazonium chloride

Br + N2

Bromobenzene

N

NCl

Physical and chemical properties of haloarenes 10.12

NaNO /dil.HCl2273K 278K,

H O2

NH2

Cu Cl /HCl2 2HCl

Aniline Benzene diazonium chloride

Cl + N2

Chlorobenzene

N

NCl

31


eg. Chemical properties: The reactions of haloarenes include: i. Substitution reactions ii. Reactions with metals Substitution reactions: i. Haloarenes undergo substitution reactions which can be either nucleophilic substitution or electrophilic

substitution. ii. Aryl halides are less reactive than alkyl halides and do not undergo nucleophilic substitution reactions

easily due to the following reasons: a. Resonance effect: 1. In aryl halides, the C X bond acquires partial double bond character due to resonating structures. 2. This makes the C X bond cleavage in aryl halide more difficult than the C X bond cleavage in

akyl halide. b. Different hybridization states of C-atom in C X bond:

Type of compound

State of hybridization of C-atom in a CX bond

% s-character Bond length of C X bond

Strength of CX bond

Alkyl halide sp3 Less (25%) Longer Weaker Aryl halide sp2 More (33.33%) Shorter Stronger

eg. 1. X-ray analysis confirms that CCl bond length in chlorobenzene is 169 pm while that in methyl

chloride is 177 pm. 2. Reduction in bond length imparts stability, making bond cleavage difficult in aryl halides. c. Polarity of CX bond: 1. sp2 hybridised C-atom; Less tendency to release e s towards X-atom. 2. sp3 hybridised C-atom; More tendency to release e s towards X-atom. 3. Thus sp2 hybridized C-atom is more electronegative than sp3 hybridized C-atom. eg.

Dipole moment Compound

1.73 D Chlorobenzene (Aryl halide)

2.05 D Chloroethane (Alkyl halide) 4. Polarity reactivity Lesser polarity of aryl halides, lesser is their reactivity compared to alkyl halides. d. Repulsion: Electron rich arenes repel the attacking nucleophile (which is also electron rich); resulting in the

lesser reactivity towards substitution reactions. e. Instability of phenyl cation: 1. Phenyl cation formed due to self ionization of aryl halide will not be stabilized by resonance.

H3C Cl

177 pm

Cl

169 pm

Boiling point (K)Melting point (K)

o-Dichlorobenzene

ClCl

m-DichlorobenzeneCl

Cl

p-DichlorobenzeneCl

Cl

453 256

446249

448 323

32


2. Unstable phenyl cation (carbocation) cannot undergo SN1 reaction, thus ruling out the possibility of SN1 mechanism.

3. Pi () electrons of aromatic ring blocks the backside attack of nucleophile, thus ruling out the possibility of SN2 mechanism.

iii. However, under drastic conditions, aryl halides undergo nucleophilic substitution reactions. Nucleophilic Substitution Reactions:

Under drastic conditions like high temperature and under pressure, halide group attached to arenes can be replaced by OH, CN or NH2 group.

i. Dow’s Process: When aryl halide reacts with NaOH at 623 K under pressure of 200 atm300 atm, forms sodium phenoxide which on acidification gives phenol. This process is known as “Dow’s process”.

ii. Chlorobenzene on heating with anhydrous copper cyanide and sodium cyanide at 473 K under pressure

gives cyanobenzene. iii. Chlorobenzene on heating with aqueous NH3 in the presence of catalyst cuprous oxide, at 473 K under

pressure gives aniline. iv. Effect of substituents on the reactivity of haloarenes: a. It has been found that presence of electron withdrawing groups like NO2, COOH, CN at o –

and/or p – position with respect to halogen atom greatly activates haloarenes to undergo nucleophilic displacement reactions.

eg. o and p – Nitrochlorobenzene easily undergo nucleophilic attack of OH to give o and p –Nitrophenol.

1. NO2 group is at ortho position with respect to halogen atom:

HO Cl

NO

O+

fast step

OH

N+ O

O+ Cl

Resonance hybrid o-Nitrophenol

slow step

N+HO Cl

O

O

HO ClN

OO+

HOCl

N+ O

O

Resonating structures

Cl

NO

O

+HO

o-Nitrochlorobenzene

+ NaOH 623K, HCl200atm 300atm

dil.HCl

NaCl

Chlorobenzene

Cl ONa

Sodium phenoxide

OH

Phenol

+ CN anhydrousCuCN NaCN473 K,pressure

CN

+ Cl

Chlorobenzene Cyanobenzene

Cl

+ 2NH3 + Cu2O 473Kunder pressure

NH2

+ 2CuCl + H2O

Chlorobenzene Aniline

Cl

2 2

33


2. NO2 group is at para position with respect to halogen atom: b. From the above mechanism it is clear that, carbanion formed by the attack of OH gets stabilized

because of electrons of benzene ring as well as negative charge on C-atom attached to electron withdrawing NO2 group.

c. Hence, o- and p-substituted aryl halides show greater reactivity towards nucleophilic attack. d. But in the case of m-substituted aryl halide, there is no negative charge at m-position in the

resonating structures; due to this the presence of electron withdrawing group at m-position has no effect on reactivity.

e. It is observed that as number of electron withdrawing groups at ortho - and para-position (with respect to halogen atom) increases, the reactivity of haloarenes also increases.

eg. 1. 2. Electrophilic Substitution Reactions: i. In the case of chlorobenzene, following resonating structures are obtained. ii. In chlorobenzene, electron density is more at o and p – position (since chlorine is o and p directing).

Cl Cl+ Cl+Cl+

HO ClOH

N N++

O OOO

fast step + Cl

Resonance hybrid p-Nitrophenol

HO

ClClHO

N OO

slow step

+N

O O

+

HO Cl

N+

OO

N+

O O

HO Cl

Resonating structuresp-Nitrochlorobenzene

+ Cl+ OH (i) NaOH

(ii) H ,368K

1-Chloro-2,4-dinitrobenzene

Cl

NO2

NO2

OH

2,4-Dinitrophenol(55% yield)

NO2

NO2

+ Cl

OH

2,4,6-Trinitrophenol(Picric acid) (93% yield)

O2N NO2

NO2

+ OH warmH O2

1-Chloro-2,4,6-trinitrobenzene

Cl

O2N NO2

NO2

34


iii. It is observed that, halogen atoms are highly electronegative, they pull electrons of benzene ring towards themselves due to – I effect and hence aryl halides show reactivity towards electrophilic attack.

iv. Hence, weaker resonating structures control o , p orientations and stronger inductive effect controls reactivity of aryl halides.

v. When an electrophile (E) attacks on ortho and/or para positions of aryl halide; more stable chloronium ion is formed as follows:

vi. Attack of an electrophile at meta position forms comparitively less stable chloronium ion. vii. Thus, electrophilic substitution reaction in aryl halide (i.e., chlorobenzene) occurs slowly and under drastic

conditions compared to benzene. viii. Halogenation: Chlorobenzene reacts with Cl2 in the presence of anhydrous FeCl3 or sunlight to give

o-dichlorobenzene or p-dichlorobenzene. eg. Note: a. Benzene when treated with chlorine in the presence of bright sunlight or ultraviolet light, adds up

three molecules of chlorine to give benzene hexachloride/BHC (C6H6Cl6). b. This is an addition type of reaction. c. Benzene hexachloride is commercially known as BHC. d. It exists in eight isomeric forms.

e. The gamma () isomer is called “Gammexane” or “Lindane” which is used as an insecticide. ix. Nitration: Chlorobenzene reacts with nitrating mixture i.e., conc.HNO3 and conc. H2SO4 to give 1-Chloro-

4-nitrobenzene (major product) and 1-Chloro-2-nitrobenzene (minor product). This reaction is known as “Nitration”.

Cl+ Cl+

H

E

EH

Carbocation at o-position/ chloronium ion

Carbocation at p-position/ chloronium ion

Benzene hexachloride (BHC)

C

C

C

C

C

Cl

Cl

H

Cl

H

H C

Cl H

H

Cl H

Cl

+ 3Cl2 bright sunlight

or UV light

H CC |

H

H C C H

C H

Benzene

H | C

+ HNO3 conc.H SO2 4

+

Chlorobenzene1-Chloro-2-nitrobenzene

(Minor product)

+ H2O

1-Chloro-4-nitrobenzene(Major product)

(conc.)

Cl Cl

NO2

Cl

NO2

+ Cl2 anhydrous FeCl3

or sunlight +

Chlorobenzene

Cl

+ HCl

p-Dichlorobenzene(Major product)

o-Dichlorobenzene(Minor product)

Cl

Cl

Cl

Cl

35


x. Sulphonation: Chlorobenzene on heating with conc. H2SO4 yields 4-chlorobenzene sulphonic acid (major product) and 2-chlorobenzene sulphonic acid (minor product).

xi. Friedel-Craft’s Reaction: a. Introduction of an alkyl or acyl group in the haloarene ring or in the substituted benzene ring in the

presence of anhydrous aluminium trichloride is known as “Friedel-Craft’s Reaction”. b. The reaction can be carried out by reacting aryl halide (i) with alkyl chloride (Friedel-Craft’s

alkylation reaction) or (ii) with acyl chloride (Friedel-Craft’s acylation reaction). eg. Reaction with Sodium Metal (Wurtz Fittig Reaction): i. When an aryl halide is heated with alkyl halide, it undergoes coupling reaction in the presence of sodium

metal and dry ether to give alkyl benzene. This reaction is known as “Wurtz Fittig Reaction”. eg. ii. In the above reaction, along with toluene, ethane (obtained by coupling of two methyl groups) and diphenyl

(obtained by coupling of two phenyl groups) are also produced as byproducts. a. 2H3C Cl + 2Na dryether CH3 CH3 + 2NaCl

b. Note: Reaction of haloarenes with sodium metal is called as “Fittig reaction”.

Methyl chloride

Ethane

Cl + 2Na + Cl CH3

ChlorobenzeneMethyl benzene

(Toluene)Methyl chloride

dryether CH3 + 2NaCl

anhydrous AlCl3

ChloromethaneCH3Cl

Ethanoyl chloride (Acetyl chloride)

H3C C Cl

O

+ + HCl

CH3

Cl

1-Chloro-4-methylbenzene (Major product)

1-Chloro-2-methylbenzene (Minor product)

Chlorobenzene

Cl

ClCH3

+ + HCl

C CH3

Cl

4-Chloroacetophenone(Major product)

O

Cl

2-Chloroacetophenone (Minor product)

C CH3

O

+ 2Na

Chlorobenzene Diphenyl

dryether + 2NaCl

Cl

2

+ H2SO4 + H2O

(conc.) Chlorobenzene

Cl

+

4-Chlorobenzenesulphonic acid

(Major product)

SO3H

Cl

2-Chlorobenzene sulphonic acid

(Minor product)

SO3H

Cl

10: and haloalkanes and haloarenes - target...

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