alkyl halide - csci.tu.edu.iqcsci.tu.edu.iq/cd/images/organiicchmoodd133202020333.pdfalkyl iodide is...

Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh

.jebur// 2020 //class 2

Alkyl halide

CH3CH2CH2CH2ClEthyl ChlorideButyl Chloride

CH3CH2 Cl CH3CH2CH2CH2Cl

chloroethane 1-chlorobutane

CH3

CH

CH3

Cl

H2C

HC CH3

CH3

F

1-fluoro -2-methyl propane

2-chloropropane

CH3

CH

CH3

Isopropyl

CH3CH2 Cl

Physical properties:

Alkyl halides have higher boiling point than alkane of the number of

carbon because the greater M.wt., Also iodide compound have higher b.p

than Br > Cl > F

They are insoluble in water in spite of their polarity soluble in most

organic solvent.

Iodo , bromo, poly chloro compound are more dense than water .

Industrial source:-



Certain important halides are prepared industrially.

Preparation:

1- from alcohol by reaction of alcohol with hydrogen halide some

time the reaction require catalyst for least reactive hydrogen halide

with 1 and 2 alcohol

In this method rearrangement be occur, this can be minimized by using

phosphoro halide.



2- halogenation of contain hydrocarbon

3- Addition of hydrogen halide to alkenes.

C C

H X

C C HX HX= HCl, H Br, HI+

CH3 C C

H

H

H

HBr

peroxides

no peroxides

CH3CHBrCH3

CH3CH2CH2Br

anti Markonikov`s addition

Markonikov`s addition

2- bromo propaneor isopropylbromide

1- bromo propaneor n-propylbromide

4- Addition of halogens to alkenes and alkynes :

5- Halide exchange



Alkyl iodide is often prepared from the corresponding bromide or

chloride by treatment with a solution iodide in acetone.

C

H

CC C

X

CC

C

H

C

XAllylic hydrogen Allylic halide

benzylic hydrogen benzyl halide

Reactions:

1- Nucleophilic substitution

Halide ion is weak base, so the hydrogen can be readily displaced as

halide ion by other stronger bases. These bases possess an un shared pair

of electron and seeking the positive site (nucleus) . Electron rich reagents

are called nucleophilic reagents. The typical reactions of alkyl halides are

the nucleophilic substitution.

R : X + : Z R : Z + X Nucleophilic substitution

Nucleophilic reagent (strong base) leaving group ,The nucleophilic

reagents are either bearing negative charge like OH , CN , RO or neutral

bases like ammonia and water which contain unshared pair of electrons .



1- Elimination Reaction : Dehydrogenation previously discussed .



CC C

X

C

H

HX

2- Preparation of Grignard reagents

4- Reduction

Nucleophilic aliphatic substitution: Second order and first order

kinetics.

For example reaction of methyl bromide with sodium hydroxide to

produce Methanol.

The reaction is carried out in aqueous ethanol is order to soluble all

compound if the reaction result from collision between hydroxide ion and

methyl bromide molecule , then the rate will depend on the conc. of both

of those two reactant .i.e.

Rate α conc of CH3 Br x conc . of OH as the conc increase the rate will be

increase



Rate = K[CH3 Br][ OH-]

K is the rate constant . for a particular reaction K value is constant at a

given temperature and a given solvent

Therefore this reaction is second order reaction because the rate depends

on conc of two substances. Now the reaction of tetra – butyl bromide with

hydroxide ion .The rate of reaction depend only on the conc . of alkyl

halide and if we change the conc . Of OH the rate always the same.

Rate α cone of tetra butyl bromide

H3C

C Br

H3C

H3COH-

H3C

C OH

H3C

H3C Br

Rate = K [Rx] .

This reaction is first order reaction because it depend on the conc of only

one substant.

Now why reaction

1- depend on conc . of OH while reaction 2- independence on conc . of

OH

From the order of reaction we can say that the two nucleophilic

substitutions proceed by two different mechanisms.

The SN2 reaction Mechanism and kinetics:

For the reaction

CH3Br OH CH3OH Br

Rate = K[CH3 Br][ OH-]



The rate depends upon the conc of both reactions i.e. the reaction occur

by the collision between a hydroxide ion and methyl bromide molecule.

The OH attack CH3Br from the side back side for away from bromide i.e.

attacks the molecule from the rear.

C2H5

C Br

H3C

H

OH

Br

C2H5

CHO

CH3

H

C2H5

CHO

CH3

H

Br

SN2

The HO- C bond form simultaneously with breaking of C- Br bond and

the C- Br bond in the T.S is not completely and the C- OH is not

completely formed and each group have partially negative charge. the –

OH and – Br are located is for apart as possible .

1- This mechanism is called bimolecular nucleophilic substitution (SN2)

which is follows second order kinetics particles (SN2 substitution

nucleophilic bimolecular).

Bimolecular is used because the r.d.s. involves collision of two particles.

This mechanism is consistent with the kinetics of reaction.

2- Another evidence for SN2 is the stereochemistry. In which the reaction

yield a product whose configuration is opposite to that of reactant.

3- i.e. the SN2 reaction proceeds with complete inversion of

configuration. Evidence

The rate of SN2 reaction depends on size of groups attached by the

reaction center. The increase of the group’s size will decrease bulk of

substitution the rate of SN2 reaction. i.e. the differences in rate between



two SN2 reactions seem to be due chiefly to steric factors and not to

electronic factors .

In SN2 reactions the order of reactivity of Rx is CH3X > 1º > 2º > 3º

The SN1 reaction Mechanism, kinetic, relative reactivity.

The reaction between tert – butyl bromide and hydroxide ion to yield tert

– butyl alcohol follows first order kinetic , i.e. the rate depends upon the

conc.of only one reactant , tert – butyl bromide .

C2H5

C Br

H3C

H

C2H5

C

OH

H3C

H

The rate is consistent with the following mechanism:

The first step is the slow dissociation of t- butyl bromide to bromide ions

and tert– butyl alcohol. then the carbonium ions combine rapidly (syep2)

with hydroxide ions to yield tert- butyl alcohol .The rate of reaction is

determined by the slow step (breaking of C – Br bond) r.d.s : A single

step whose rate determines the overall rate of a stepwise reactions . The

energy require for breaking of C- Br bonds coming from the formation of

ion – dipole bonds between the two kinds of ions and the solvent.

This mechanism called SN1 substitutes unimoleculer nucleophilic

substitution.

Evidence

1- The mechanism is consistent with the first order kinetics. i.e. SN1

reaction follows first order kinetics. Therefore the rate of reaction

determines by the rate of ionization of alkyl halides, that is , depend only

on the conc of alkyl halide .



3-The SNI reaction Stereo chemistry:

C6H13

CH

CH3

H

C6H13

C

OH

H3C

H

H2O

a

b

x

b

a

C6H13

CHO

CH3

H

Enantiomers

SNI reaction proceeds with racemization. The carbonium ion has a flat

structure , then the nucleophilic reagent OH – attached itself to the

carbonium ion either face of this flat ion .

3- The reactivity, In SN1 mechanism is face carbonium ion 'is formed then

any group stabilize the carbonium ion will increase the rate of reaction .

therefore the order of reactivity of alkyl halides in SN1 reaction is the

same order of stability of carbonium ions .In SN1 reactions the order of

reactivity of R x is alkyl, benzyl > 3º > 2º> 1º

The rate of SN1 reaction is affected largely by electric factors, i.e. the

ability of substituent’s to release or with draw electrons.

4- Rearrangement:



If the SN mechanism involves formation of carbonium ion , then we

expect occurrence of rearrangement to more stable carbonium ion .

CH3C

CH3

CH3

CH2Br

CH3C

CH3

CH3

CH2OC2H5

CH3C

OC2H5

CH3

CH2CH3

CH3C

CH3

CHCH3

C2H5O-

SN1

C2H5OH

NO rearranagment

Rearranagment

Neo pentyl ethyl ether

Tert-pentyl ethyl ether

2-methyl -2-butene

SN1

Also SN1 reaction is accompanied by elimination reaction.

2- Elimination reaction:

Also there are two mechanisms for elimination. E2 (bimolecular

elimination) and E1 (unimoleculer elimination).

E2 involve single step , base pulls a hydrogen ion a way from carbon ,

and simultaneously a halide ion separates .

B

C

H

C H:B

X

X CC

E2 (bimolecular elimination

An this mechanism the r.d.s. involves two molecules . Some time alkyl

halide particularly tert – alkyl halide can carried out by two , dissociation

step of alkyl halides into halide ions and carbonium ions in slow step ,



than the carbonium ion loss proton to base to form alkene in the second

step .

E elimination

B

XC

H

C

H:B

X

C C

CC CC

CarbanionH

slow

H

fast

The r.d.s (slow step) involve one molecule which the first step .

The order of reactivity of alkyl halide toward E2 or E1 is the same. 3º >

2º > 1º .

Evidence of E1 mechanism

1- Follow first – order kinetics.

2- The reaction accompanied by rearrangement.

Evidence for E2 mechanism

1- Follow second – order kinetics.

2- Are not accompanied by rearrangement.



B

C

H

CH:B

X

CC

X

CC

X

X CC

Carbanion

orientation of elimination :-

The formation of more stable alkene is called saytzelf orientation

according to substituted alkenes.

Elimination as substitution

There is competition between substitution and elimination both reactions

result from attack by the same nucleophilic reagent : attack at carbon

causes substitution , attack at hydrogen causes elimination .

C

H

C

X

Z:

E2 , SN2

The speed of elimination of alkyl halides depend chiefly upon the

stability of the alkene formed . text – alkyl halides which yield highest

branched (more stable) alkenes , undergo elimination fastest and

substitution slowest primary halides , undergo substitution fastest and

elimination slowest .



Reaction of others ethers are un reaction compounds. They are star

towards bases oxidizing and reducing agents ethers undergo just one k of

reaction, which is cleavage by acids>

alkyl halide - csci.tu.edu.iqcsci.tu.edu.iq/cd/images/organiicchmoodd133202020333.pdfalkyl iodide is...

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