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 :
The chlorination of toluene proceeds by a free radical mechanism.
Mechanism:
It involves the following steps :
Step 1: Formation of chlorine free radicals :
Step II: Attacks of free radicals on toluene :
Step III: Formation of benzyl chloride :
(2) From Benzyl alcohol:
By the action of PCl5 or thionyl chloride (SOCl2) on benzyl alcohol, chloride is formed.
(3). By chloromethylation of benzene :
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 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 :
- Sulphonation :
- Chlorination :
[B] Reaction due to side chain :
2- Wurtz reaction: When heated with sodium in dry ether benzyl chloride gives 1,2 -diphenylmethane bibenzyl).
3- Formation of Grignard reagent: Benylchloride reacts with magnesium in dry ether to yield Grignard reagent.
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 :