downloaded from · 2018-11-23 · 223 • ethyl chloride and ethyl bromide • chlorobenzene and...

Halo alkanes and Haloarene

• One Mark Questions:

1. Give IUPAC name of the following organic compound:

CH3-CH=C- CH-CH3

CH3 Br

2. Write the structural formulae of 4-Chloropent-2-ene.

Q3.Arrange the following halides in order of increasing SN2 reactivity

CH3Cl,CH3Br,CH3CH2Cl, (CH3)2CHCl

Q4.What is the order of reactivity of different alkyl halides in nucleophilic substitution reaction?

Q5.An alkyl halide C4H9Cl is optically active. What is its structure?

Q7.Which type of solvents are generally used to carry out SN1 reaction?

Q8.Identify the chiral and achiral molecules in following pair of compounds?

A:CH3CHCH2CH3 B :CH3CH2CH2CH2Br

Br

Q9.Give two uses of iodoform (CHI3)?

Q10.Write a chemical reaction in which iodide ion displaces diazonium group from a diazoniumsalt?

Q11.How you will convert 2-Bromopropane to 1-bromopropane?

Q13.How you will convert aniline into cholobenzene?

Q14.Name the iodine containing hormone, the deficiency of which causes goiter?

Q15.Name the synthetic halogen compound which is used in treatment of malaria?

Downloaded from www.studiestoday.com


221

Q16.Which isomer of C4H9Br will have lowest boiling point?

Q17.Write the IUPAC name of DDT?

Q18.Why sulphuric acid is not used during reaction of alcohol with KI?

Q19.Out of CH3Br and CH3Cl which will have higher boiling point and why?

Q20.Which one of following has highest dipole moment?

• i) CH2Cl2 (ii) CHCl3 (iii) CCl4

• Two marks questions:

Q1.Define the following terms:

• i) Ambidient nucleophile (ii) Chirality

Q2.Write short note on sandmeyer reaction?

Q3.Write the structures of main products:

(i).Chlorination of benzene in presence of UV light.

(ii).Propene is treated with HBr in presence of benzoyl peroxide.

Q4.Complete the following reactions:

(i).C6H5N2Cl +KI -----> ?

(ii) CH2=CH2 +Br2 ----- CCl4 -------->?

Q5.Explain why haloarenes are much less reactive than haloalkanes towards nucleophilicsubstitution reaction?

Q6.Write short note on:

(i).Wurtz reaction

(ii).Wurtz-Fittig reaction.

Q7.How you will convert:

(i).Ethyl chloride into ethyl alcohol.



222

(ii).Ethyl chloride to ethane

Q8.Alkyl halides are insoluble in water though they contain polar C-X bond?

Q9.Give one test to distinguish between:

(i).Chloroform and carbontetrachloride

(ii).Methanol and ethanol.

Q10.Write short note on :

(i).Finkelstein reaction

(ii).Swarts reaction

• Three Marks Questions:

1.Explain why:

(a).Dipole moment of cholorobenzene is lower than cyclohexyl chloride.

(b).Grignard reagent should be prepared under anhydrous conditions?

(c).Chloroform is stored in dark Brown bottles?

Q2.What happens when:

(a).Ethyl chloride treated with alcoholic KOH

(b).Alcohol reacts with thionyl chloride?

Q3.How you will conert:

(a).Chlorobenzene into toluene

(b).Chlorobenzene to phenol

(c).Ethyl bromide to diethyl ether.

Q5.Give the chemical test to distinguish between following pair of compounds:



223

• Ethyl chloride and ethyl bromide

• Chlorobenzene and benyl chloride

Q6.Give reasons:

(i).Boiling point of alkyl bromide is higher than alkyl chloride.

(ii).Alkyl halides are better solvents than aryl halides.

(iii).Haloalkanesare used as solvent in industry are choloro compounds rather than bromocompounds.

Q7.Answer the following:

(i).What effect should the following resonance of vinyl chloride has on its dipole moment.

CH2=CH-Cl -CH2—CH=Cl+

(ii).Iodoform is obtained by the reactions of acetone with hypoiodite but not with iodide ion.

(iii).Vinyl chloride is hydrolysed more slowly than ethyl chloride.

Q8.Write the structure of major organic product in each of following reactions:

(i).CH3-CH2-CH2-Cl +NaI Acetone

(ii).CH3-CH2-CH2OH +SOCl2

(iii).CH3CH2CH=CH2 +HBr Peroxide

Q9.Give uses of following:

• CCl4

• DDT

• Choloroform

Q10.Distinguish between SN1 and SN2 reactions?



224

• Five Marks Question

1.What happens when:

(a).n-butyl chloride is treated with alcoholic KOH

(b).Bromonenzene is teated with Mg in presence of dry ether

(c).ethyl chloride is treated with aquous KOH

(d).Ethyl bromide is Na in presence of dry ether.

(e).Methyl chloride is treated with KCN

Q2.Primary alkyl halide A C4H9Br reacted with alcoholic KOH give compound B.Compound B isreacted with HBr to give C which ia an isomer of A.When A was reacted with Na metal it give acompound (D) C8H18 that was different than the compound when n-butyl bromide reacted withsodium .Give the strural formulae for A and write the equations for all the reactions?

Q3.Write short note on:

(i).Fittig reaction

(ii).Friedal Craft Alkylation

(iii).Friedal Craft Acylation

(iv).Gatterman reaction

(v).Carbylaaminereation

Q4.Give reasons:

(i).Benzyl chloride undergoes SN1 reactions faster than cyclohexy methyl chloride.

(ii).p-Dichlorobenzene has higher melting point than ortho-dichlorobenzene.

(iii).Out of chlorobenzene and choloromethane ,which is more reactive towards nucleophilicsubstitution reaction?

(iv). Thionyl chloride is preffered for preparing alkyl chlorides from alcohols.

(V).Iodide ion is a better nucleophile than bromide ion?



225

Answer key

Answer key

• One Mark Question:

1.Ans:4-Bromo-3-methylpent-2-ene.

2.Ans:CH3-CH=CH-CH2-CH3

Cl

3.Ans:( CH3)2CHCl<CH3CH2Cl,CH3Cl<CH3Br

4.Ans.RI>RBr>RCl

5. Ans: H

CH3- C*-CH2-CH3

Cl

7.Ans: Polar protic solvents.

8.Ans: A: CH3CHCH2CH3 is chiral and B is achiral.

Br

9.Ans: 1.As antiseptic 2.As photosensitizer in emulsion of AgBr to make photographic film.

10.Ans: C6H5-N+2Cl- +KI -----------> C6H5I +KCl +N2

11.Ans: Br

CH3-CH-CH3 ------ (Alc.KOH)----> CH3-CH=CH2----------HBr/Peroxide----> CH3-CH2-CH2Br

13.Ans:C6H5NH2 + NaNO2 +HCl----->C6H5N2+Cl- -----CuCl/HCl-----> C6H5Cl +N2

14.Ans.Thyroxine

15.Ans:Chloroquine.

16.Ans: CH3

CH3-C-Br :tert-Butyl bromide

CH3



226

17.Ans:2,2-bis(1,1-Dicholoro diphenyl)-1,1,1-tricholoroethane.

18.Ans:Because it first converts KI to HI and then oxidises it to I2.

19.Ans:CH3Br.More the molecular mass, greater the vander waals force of attraction, more theboiling point.

20.Ans:CH2Cl2

• Two marks questions:

1.(i)Ans:The nucleophiles having two nucleophilic centres.For example Cyanide group.

-C N :C=N-

ii).An object which is not superimposable on its mirror image is said to be chiral.The property ofbeing chiral is known as chirality.

2.Ans:NaNO2 +HCl 273-278K NaCl +HNO2

C6H5 -NH2 +HNO2 +HCl -----273-278K-------> C6H5N2+Cl- +2H2O

C6H5N2+Cl- + CuCl/HCl ---------->C6H5Cl+N2

3.Ans: i) C6H5 Cl+3Cl2------U.V.Light------> C6H6Cl6

ii).CH3-CH=CH2 +HBr -----Peroxide ----------> CH3-CH2-CH2-Br

4.Ans : i) C6H5N2Cl +KI -------->C6H5I +KCl +N2(g)

(ii).CH2=CH2 +Br2 ------CCl4 ---------> CH2BrCH2Br

5.Ans:i.Due to resonace C-Cl bond aquires double bond charcter .

ii.In haloarenes Carbon bearing halogen is sp2 hybirdised.So C-X bond is shorter and stronger.

6.Ans.(i) RX +2Na +X-R ----Dry Ether----> R-R +2NaX

(ii).R-X +2Na + XC6H5 ------- Dry ether ----> R -C6H5 + 2NaX.

7.Ans (i).C2H5Cl +KOH (aq)------------> C2H5OH +KCl



227

(ii).C2H5Cl +2[H] ---- LiAlH4 ----> C2H6 +HCl

8.Ans:Because haloalkanes cannot form hydrogen bond with water molecules and at the sametime they cannot break the hydrogen bonds present in water molecule.

9.Ans: (a). Chloroform when heated with aniline and alc.KOH offensive smell of isocyanide isproduced.

C6H5NH2 +CHCl3 +3KOH (alc.)----- heat---->C6H5NC +3KCl +3H2O

(b).By iodoform test

In case of methanol no yellow ppt.But in case of ethanol yellow ppt are formed.

CH3CH2OH +4NaOI ---- NaOH +I2 -------> CHI3+HCOONa +NaI+H2O

CH3OH +NaOI ---- NaOH+I2 ----> No yellow ppt

10.Ans.

(a). CH3-CH2-Cl +NaI -----Acetone ----> CH3-CH2-I +NaCl

(b). CH3-CH2-Cl +AgF ---------> CH3-CH2-F +AgCl

• Three Marks question:

1.Ans. (a).SP2 carbon of chlorobenzene is more electronegative than the SP3 carbon ofcyclohexyl chloride.

(b).Grignard reagent are very reactive.These are readily reacts with water

RMgX + H2O ----> RH +Mg(OH)X

(c).Because it reacts with oxygen in presence of sunlight to form phosgene gas.

CHCl3 + ½ O2 ---Sunlight ---> COCl2 +HCl

2Ans.(a).C2H5Cl +KOH (alc) ---->CH2=CH2 +H2O +KCl

(b).R-OH +SOCl2 ----->R-Cl+HCl+SO2

3Ans.How you will conert:



228

(a).C6H5Cl +2Na+CH3Cl----- dry ether----> C6H5CH3 +2NaCl

(b). C6H5Cl + NaOH----623K----> C6H5ONa ------H2O/H+ --->C6H5OH

(c).C2H5Br+ NaOC2H5 ----> C2H5-O-C2H5 +NaBr

Ans5.Give the chemical test to distinguish between following pair of compounds:

i) C2H5Cl+KOH(aq) ---> C2H5OH +KCl

KCl+AgNO3 AgCl (White ppt)+KNO3 (soluble in NH4OH)

C2H5Br+KOH(aq) -----> C2H5OH +KBr

KBr+AgNO3 AgBr (pale yellow ppt)+KNO3 ( partially soluble in NH4OH)

• C6H5CH2Cl +KOH(aq) ----> C6H5CH2OH +KCl

KCl+AgNO 3 AgCl (White ppt)+KNO3 (soluble in NH4OH)

C6H5Cl+KOH(aq) No reaction

Ans6.Give reasons:

(i).Beacuse of higher magnitude of Vander Waal’s forces in alkyl bromide than alkyl halide.

(ii).Due to greater polarity of alkyl halides.

(iii).C-Cl bond is more polar than C-Br bond. So a better solvent than alkyl bromide.

Ans7.Answer the following:

• CH2=CH-Cl- CH2—CH=Cl+

It will decrease the dipole moment of vinyl chloride relative to ethyl chloride.

(ii).To prepare Iodoform from acetone I+ is required. As I+ can only be supplied IO- not by I-

,therefore hypoiodite is used to convert acetone into iodoform.

(iii).Due to resonace there is double bond character between Carbon and chlorine.

Ans8.



229

(i).CH3-CH2-CH2-Cl +NaI Acetone CH3-CH2-CH2-I +NaCl

(ii).CH3-CH2-CH2OH +SOCl2 CH3-CH2-CH2Cl +SO2 +HCl

(iii).CH3CH2CH=CH2 +HBr Peroxide CH3-CH2-CH2-CH2Br

Ans.9.Give uses of following:

• CCl4:Used as solvent.

• DDT:Used as insecticide

• Chloroform:Used as anesthesia in surgery.

Ans10.Distinguish between SN1 and SN2 reactions?

• Five Marks Question

Ans1.What happens when:

(a).CH3-CH2-CH2-CH2-Cl +KOH (alc) CH3-CH2-CH=CH2 +KCl +H2O

(b).C6H5Br +Mg dry ether C6H5MgBr

(c).C2H5Cl+KOH(aq) C2H5OH+KCl

(d).2C2H5Br+2Na dry ether CH3-CH2-CH2-CH3 +2NaBr

(e).CH3Cl +KCN(alc) CH3CN +HCl

Ans 2.A) CH3-CH(CH3)-CH2Br 1-Bromo-2-methylpropane.

B) CH3-C(CH3)=CH2 2-Methylprop-1-ene.

C) CH3-C(CH3)(Br)-CH3 2-Bromo-2-methylpropane

D) CH3-CH(CH3)-CH2-CH2-CH(CH3)-CH3 2,5-Dimethylhexane

Equations for reaction:

A alc.KOH B

B+HBr C

2A+2Na D +2NaBr



230

3.Ans.

i).C6H5Cl +2Na +C6H5Cl C6H5-C6H5 + 2Nacl

(ii).C6H6 +CH3Cl (Anhyd)AlCl3 C6H5CH3 +HCl

iii).C6H5Cl +CH3COCl Anhyd AlCl3 C6H5 COCH3

• C6H5N2Cl Cu/HCl C6H5Cl +N2

• CH3CH2NH2 +CHCl3 +KOH(alc.) CH3CH2NC +3KCl+3H2O

A ns: 4.

(i).Because in case of benzyl chloride the carbocation is formed after the loss of Cl- stabilized byresonance.

(ii).It is due to symmetry of p-Dichlorobenzene which fits in crystal lattice better than ortho-dichlorobenzene.

(iii).Chloromethane is more reactive being an alkyl halide

(iv). Thebyproducts of the reaction i.e.SO2 and HCl being gases escape into the atmosphereleaving behind the alkyl chloride in almost pure state.

(V).Because of bigger size and lower electronegativity.

1 MARK QUESTIONS(1-20)• 1.What happens when tert-butyl alcohol is treated with Cu / at 573 K.?• 2.Arrange the following halides in order of increasing SN² reactivity :CH3 — Cl, CH3 — Br, CH3CH2Cl, (CH3)2 CHCl• 3.Alkyl halides, though polar, are immiscible with water. Why?• 4.Grignard reagents should be prepared under anhydrous conditions. Why?• 5.Which of the following two compounds would react faster by SN² pathway: 1-bromobutane (OR) 2-bromobutane.• 6.Allyl Chloride is more reactive than n-propyl Chloride towards nucleophilicsubstitution reactions. Explain.• 7.Explain why is Chlorobenzene difficult to hydrolyse than ethyl chloride ?



231

• 8. R—Cl is hydrolysed to R—OH slowly but the reaction is rapid if a catalytic amountof KI is added to the reaction mixture.Why?• 9. Why haloalkanes are more reactive than haloarenes.• 10. Why do haloalkenes undergo nucleophillic substitution whereas haloarenes under goelectophillic substitution?•• 11.Why is Sulphuric acid not used during the reaction of alcohols with KI ?• 12.p- dichlorobenzene has highest m.p. than those of ortho and m-isomers.?• 13.Although chlorine is an electron- withdrawing group, yet it is ortho and para directing inelectrophillic aromatic substitution reactions.Why?• 14.Explain why vinyl chloride is unreactive in nucleophillic substitution reaction?• 15.Arrange the following compounds according to reactivity towards nucleophillic substitutionreaction with reagents mentioned :-• 4- nitro chloro benzene, 2,4 di nitro chloro bemzene, 2,4,6, trinitrochlorobenzenewith CH3ONa• 16.Arrange in order of boiling points.• Bromobenzene, Bromoform, chloromethane,Dibromo-methane• 1-chloropropane, Isopropyle chloride, 1-Chlorobutane.• 17.Predict the reactivity in SN1• C6H5CH2Br, C6H5CH (C6H5)Br, C6H5CH(CH3)Br, C6H5C(CH3)( C6H5)Br• 18.Why is vinyl chloride less reactive than ethyl chloride?• 19. Chloroform is stored in dark coloured & sealed bottle. Why?2 MARK QUESTIONS(21-30)• 20.The treatment of alkyl chlorides with aqueous KOH lead to the formation of alcohols but inpresence of alcoholic KOH alkenes are major products. Explain?• 21.p-Dichlorbenzene has higher melting point and lower solubility than those of o andm- isomers. Discuss.• 22.Haloalkanes react with KCN to form alkyl cyanides as major product while AgCNform isocyanide as the chief product. Explain• 23. Explain why is Chlorobenzene difficult to hydrolyse than ethyl chloride ?• 24.Which compound will react faster in SN2 reaction with OH---?• CH3Br and CH3I (SN2)• (CH3)3C-Cl or CH3Cl (SN2)•• 25.How the following conversions can be carried out?



232

• Propene to propan-1-ol• 1-Bromopropane to 2-bromopropane• 26.The treatment of alkyl chlorides with aq KOH leads to the formation of alcohols but inpresence of alcoholic KOH, alkenes are the major products. Explain.• 27.Tert-butyl chloride reacts with aq. NaOH by SN¹ mechanism while n-butyl chloridereacts by SN² mechanism. Why ?• 28.Why alkyl halides are generally not prepared in laboratory by free radicalhalogenation of alkanes ?3MARK QUESTIONS29. Why preparation of aryl iodide by electrophilic substitution requires presence of anoxidising agent? Why can aryl flouride not be prepared by this method?30. Why aryl halides are extremely less reactive towards nucleophilic substitution?

31. Complete the reaction :(a) CH3OCH3 + PCl5 ———(b) C2H5OCH3 + HCl ———(c) (C2H5)2 O + HCl ———32 Distinguish between the following pair of organic compoundsi) CCl4 and CHCI3ii) Chlorobenzene and Benzyl chloride33. How are the following conversions carried out?(i) Benzyl chloride→ Benzyl alcohol. (ii) Ethyl magnesium chloride→ Propan-1-ol.Cu /HCl(iii) C6H5N2Cl ───────→34. Write the formula of main product formed in the following chemical reactions.Na• (CH3)2 CH-C1 ──────→Dry ether

∆



233

• CH3Br + AgF──────→Dry acetone• CH3CH2Br + Nal ───────→

5MARKS QUESTIONS(41-45)35.a) Give the IUPAC name of

CH3 CH3CH3 CH2 C C CH2 CH 2 OHBr Brb)Complete the following reactions:(a) C6H5ONa + C2H5Cl -------->

(b) CH3CH2CH2OH + SOCl2 ------->(c ) Preparation of haloalkane with Alkane and halogen using U.V light is leastPreferred method.Explain?36.a).p-dichlorobenzene has higher melting point and lower solubility than o- and m-isomer. Explain?b) Which will have a higher boiling point?1 - Chloro penthane or - 2 methyl -2- chlorobutaneGive reasons?c) Chloroform is not used as anesthetic nowadays. Why?37.(a) p - nitro chlorobenzene undergoes nucleophilic substitution faster thanchlorobenzene. Explain giving the resonating structures as well.(b)Allyl chloride is more reactive than n - propyl chloride towards nucleophilicsubstitution reaction. Explain why?



234

(c) Give IUPAC name of the following organic compoundC6H5CH2Cl38.a) Why do alcohols have higher boiling points than the halo alkanes of the samemolecular mass?b) Convert the following:i) Benzene to aniline.ii) Benzene to diphenyliii) Benzene to p-Chloro toluene.ANSWERS1 MARK ANSWERS(1-20)1.Dehydration reaction will take place. Isobutene will be formed.CH3 CH2| CU ||CH3 — C — OH ——————® CH3 — C| 573 K |CH3 – H2O CH32.(CH3)2 CHCl < CH3CH2Cl < CH3Cl < CH3Br.(Hint : As the size of the alkyl group increases SN² reactivity decreases.)3. Alkyl halides cannot form H-bonds with water molecules and hence are insoluble inwater.4. Because Grignard reagents have a very strong affinity for H+ ions. In presence of water,they abstract H+ ions from water and form alkanes. To prevent this, they should beprepared under anhydrous conditions.5. The reactivity in SN² reaction depends upon the extent of steric hindrance.ibromobutane is a 1° alkyl halide and 2-bromobutane is a 2° alkyl halide. Since there will be



235

some steric hindrance in 2° alkyl halide than in 1° alkylhalide, therefore 1°-bromobutanewill react faster than 2-bromobutane in SN² reaction.6. Allyl Chloride readily undergoes ionization to produce resonance stabilized allylcarbocation. Since carbocations are reactive species they readily combinewith OH– ions to form allyl alcohol.7. The lone pair of electrons of Chlorine is Chlorbenzene participates into resonance withthe benzene ring.As a result C — Cl bond acquires a partial double bond character.Therefore, this C — Cl bond is stronger than C — Cl bond in ethyl chloride which is apure single bond. As such the Chlorobenzene is difficult to hydrolyse than ethylchloride.8. Iodide ion is a powerful nucleophile and hence reacts rapidly with RCl to form RI.KI ———® K+ + I–; R — Cl + I– ———® R — I + Cl–9.In haloarenes, there is partial double bond character b/w carbon and halogen due toresonance effect which makes him less reactive.10. Due to more electro negative nature of halide atom in haloalkanes carbon atombecomes slightly positive and is easily attacked by nucleophillic reagents.While in haloarenes due to resonance, carbon atom becomes slightly negative and attackedby electrophillic reagents.11.It is because HI formed will get oxidized to I2 by concentrated Sulphuric acid which is anoxidizing agent.12. p- dichlorobenzene is symmetrical, fits into crystal lattice more readily and has highermelting point.13. Ans. Chlorobenzene is resonance hybrid, there is –ve charge at 0 and para positions,electrophillic substitution reaction will take place at 0 and para position due to +R effect.+R effect is dominating over – I effect. .14. Vinyl chloride is unreactive in nucleophillic substitution reaction because of doublebond character between C=CL bond which is difficult to break.15. 2,4,6, trinitrochlorobenzene > 2,4 dinitrochlorobemzene > 4- nitrochlorobenzene16. Ans. (a) chloromethane < Bromobenzene < Dibromo-methane < , Bromoform(b) , Isopropyle chloride <1-chloropropane <1-Chlorobutane30 > 20> 10 (SN1)17. C6H5C(CH3)(C6H5)Br > C6H5(C6H5)Br > C6H5CH(CH3)Br > C6H5CH2Br(30) (20) (20) (10)



236

18.Due to the sp2 hybridization and resonance in vinyl chloride.19.Because it undergoes oxidation and converting to poisonous gas phosgene.2 mark questions(21-30)20. In aqueous KOH,OH- is nucleophile which replaces another nucleophile.R-X +KOH-----> R-OH +KXWhere as in alcoholic KOHC2H5OH +KOH ---->C2H5O- + K+CH3CH2-Cl + alcoholic KOH-------- CH2 =CH2 + C2H5OH(C2H5O-)21.The p-isomer being more symmetrical fits closely in the crystal lattice and thus hasstronger intermolecular forces of attraction than those of o- and m- isomers. Sinceduring melting or dissolution, the crystal lattice breaks, therefore a larger amount ofenergy is needed to melt or dissolve the p- isomer than the corresponding o- andmeta isomers.22. KCN is a ionic compound and provides cyanide ions in solution. Although both carbonand nitrogen atoms are in a position to donate electron pairs, the attack takes placemainly through Carbon atom and not through nitrogen atom since C — C bond ismore stable than C — N bond. However AgCN is mainly covalent in nature andnitrogen is free to donate electron pair forming isocyanide as the main product.23.The lone pair of electrons of Chlorine is Chlorbenzene participates into resonance withthe benzene ring. As a result C — Cl bond acquires a partial double bond character.Therefore, this C — Cl bond is stronger than C — Cl bond in ethyl chloride which is apure single bond. As such the Chlorobenzene is difficult to hydrolyse than ethylchloride.24.a) CH3I will react faster than CH3Br b) CH3Cl will react faster than 30 halide. 25.a)



237

b)26.In aq. solution, KOH is almost completely ionised to give OH– ions which being a strongnucleophile brings about a substitution reaction to form alcohols. Further in aq.solution, OH– ions are highly solvated (hydrated).This solution reduces the basic character of OH– ions which fail to abstract ahydrogen from the b-carbon of the alkyl halide to form an alkene.However an alcoholic solution of KOH contains alkoxide (RO–) ions which being amuch stronger base than OH– ions preferentially abstracts a hydrogen from the b-carbon of the alkyl halide to form alkene.27.Tert-butyl chloride reacts via SN¹ mechanism because the heterolytic cleavage of C — Clbond in tert-butyl chloride gives 3 carbocation which is highly stable and favourablefor SN¹ mechanism. Moreover, tert-butyl chloride (3°) bring a bulky molecule hassteric hindrance which will not allow SN² mechanism to take place. Hence only SN1mechanism can occur in tert-butyl chloride. However n-butyl chloride (1°) reacts via



238

SN² because ‘C’ of C — Cl bond is less crowded and favourable for nucleophile toattack from back side results in the formation of transition state. It has less sterichindrance which is a favourable factor for SN² mechanism.28.It is because :(i) It gives a mixture of isomeric monohalogenated products whose boilingpoints are so close that they cannot be separated easily.(ii) Polyhalogenation may also take place, thereby making the mixture morecomplex and hence difficult to separate.3 MARK QUESTIONS29.Reactions with I2 are reversible in nature and require presence of oxidising agent(HNO3, etc.) to oxidise HI formed during iodination and promote forward reaction.Fluoro compounds cannot be prepared due to high reactivity of flourine.30.(i) Resonance effect :Due to resonance C — Cl bond acquires partial double bond character.(ii) Difference in hybridisation of Carbon in C —X bond :in haloarene C-atom attached to halogen in sp² hybrid while sp³ inhaloalkane.C — X bond length in sp² hybrid is shorter and hence stronger and difficult tobreak.(iii) Instability of phenyl cation(iv) Possible repulsion of nucleophile to approach e– rich arenes.31. (a) 2 CH3Cl



239

(b) CH3Cl + C2H5OH(c) C2H5Cl + C2H5OH32. i)Carblamine reaction : chloroform gives offensive smell due to formation isocyanidebut CCl4 don’t.ii) Diazotisation test will be given by benzyl chloride.33.i)Hydrolysis with water.ii) Nucleophillic addition with HCHO followed by hydrolysis.iii) Cu /HClC6H5N2Cl ───────→C6H5Cl34 (a) (CH3)2 CH-C1 ──────→ (CH3)2 CH-CH(CH3)2Dry ether∆(b) CH3Br + AgF──────→CH3F

Dry acetone(c) CH3CH2Br + Nal ───────→ CH3CH2I5 MARK ANSWERS(41-45)

35. a)3,4-dibromo-3,4-dimethylhexan-1-ol.b) (a) C6H5ONa + C2H5Cl C6H5-O-C2H5 + NaCl(b) CH3CH2CH2OH + SOCl2 CH3CH2CH2Cl + HCl + SO2c) Due to formation of mixed halides primary, secondary tertiary.36.a) The p-isomer being more symmetrical fits directly in the crystal lattice and thus hasstronger inter molecular forces of attraction than o- and m- isomers.During melting ordissolution, the crystal lattice breaks. Therefore, a large amount of energy is needed to meltor dissolve the p-isomer than the resultant o- and m- isomers.



240

b) 1-chloro pentaneSurface area and hence Van der Waal’s forces of attraction decreases on branching.c)Due to the formation of poisonous gas during oxidation.37(a) In this reaction a carbanion intermediate is formed. This is stabilized by Resonanceinp-nitrochloro benzene Resonance Structure(b) In allyl chloride, the carbocation formed is stabilised due to resonancewhile the carbocation formed form n - propyl chloride i.e. is less stable, so allyl chloride ismore reactive towards nucleophilic substitution reaction.(c) Chlorophenylmethane38 a) Alcohols are capable of forming intermolecular H-bonds .b) i) C6H6 + Cl2 ----------- C6H5Cl + NH3 ---------------C6H5NH2 + HClFeCl3 Cu2O,Δii) C6H6 + Cl2 -----------C6H5Cl + 2Na + C6H5Cl ------------ C6H5-C6H5 + 2NaClFeCl3 Dry etheriii)C6H6 + CH3Cl ------------ C6H5CH3 + Cl2 ------------------ C6H4CH3Cl + HCl



downloaded from · 2018-11-23 · 223 • ethyl chloride and ethyl bromide • chlorobenzene and...

Documents