CHAPTER 6: Aryl Halides and Aralkyl Halides

B.sc 1st year Book
Organic Chemistry
(Page 2)

Benzyl chloride

IUPAC Name : chloromethylbenzene Structural formula: C6H5CH2Cl

Methods of preparation :

(1) From direct chlorination of toluene:

When toluene is chlorinated at its boiling point in the presence of light, the hydrogen atoms of the methyl group are replaced one by one by chlorine atoms, and benzyl chlorides: is formed first.
Example :
toluene to chlorobenzene
The chlorination of toluene proceeds by a free radical mechanism.

Mechanism:

It involves the following steps :
Step 1: Formation of chlorine free radicals :
toluene to chlorobenzene free radical
Step II: Attacks of free radicals on toluene :
Formation of chlorine free radicals
Step III: Formation of benzyl chloride :
 benzyl chloride to free radical

(2) From Benzyl alcohol:

By the action of PCl5 or thionyl chloride (SOCl2) on benzyl alcohol, chloride is formed.
benzyl alcohol to benzyl chloride

(3). By chloromethylation of benzene :

benzene to benzyl chloride

Chemical properties:

Benzyl chlorides are many times more reactive than aryl chlorides. The increased activity of the chlorine atom is due to the formation of a stable carbocation as shown below :
The stability of benzyl carbocation arises from the distribution of the positive charge over the benzene ring to resonance.
benzyl carbocation to resonance hybrid
Benzyl chlorides give the following reactions:

[A] Reaction due to benzene nucleus :

Electrophilic Substitution reactions:

It gives the usual electrophilic substitution reactions of the benzene ring in 0 – and p-positions. For example :
  • Nitration :
benzyl chloride -di-nitrobenzene
  • Sulphonation :
sulphonation
  • Chlorination :
chlorination

[B] Reaction due to side chain :

oxidation
2- Wurtz reaction: When heated with sodium in dry ether benzyl chloride gives 1,2 -diphenylmethane bibenzyl).
wurdz reaction
3- Formation of Grignard reagent: Benylchloride reacts with magnesium in dry ether to yield Grignard reagent.
Grignard reagents
4- Reduction: When reduced with the zinc-copper couple, benzyl chloride forms toluene.
5- Nucleophilic Substitution reactions: The chlorine atom in benzyl chloride is much more reactive t methyl-chlorides. Therefore, benzyl-chlorides undergoes almost all nucleophilic substitution reactions wh are characteristic of alkyl halides.
For example :
benzyl chloride to toluene
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