Question

Which of the following is allylic halide?
(a) (1-Bromoethyl) benzene
(b) Benzyl chloride
(c) 1-Bromo benzene
(d) 3-Chloro cyclohex-1-ene

Answer 1

Hint: An allylic carbon is a carbon atom bonded to a carbon atom that in turn is doubly bonded to another carbon atom.

Step-by-step answer:
> In (1-Bromoethyl) benzene or \[{{C}_{8}}{{H}_{9}}Br\], we can see that on the allylic carbon of the molecule, the halogen atom is present with a methyl group. Hence, it cannot be termed as an allylic halide.

(1-Bromoethyl) benzene

> In Benzyl chloride or (Chloromethyl) Benzene with a molecular formula of \[{{C}_{7}}{{H}_{7}}Cl\], we can see that on the allylic carbon of the molecule, the halogen atom is present along with a methyl group. This is why Benzyl chloride does not qualify as an allylic halide.

Benzyl chloride

> 1-Bromo benzene or \[{{C}_{6}}{{H}_{5}}Br\] as we can see does not have the halogen atom Br bonded to \[s{{p}^{3}}-hybridised\] carbon atom next to the Carbon-Carbon double bond \[\left( C=C \right)\]. This denotes that 1-Bromo benzene is not an allylic halide.


 1-Bromo benzene

> An allylic halide is an alkyl halide which has one or more halogen atoms on the allylic carbons of the molecule. In the below given figure of 3-Chloro cyclohex-1-ene, we can see a halogen atom bonded to \[s{{p}^{3}}-hybridised\] carbon atom next to the Carbon- Carbon double bond \[\left( C=C \right)\]. This denotes that 3-Chloro cyclohex-1-ene or \[{{C}_{7}}{{H}_{11}}Cl\] is allylic halide.
  
  3-Chloro cyclohex-1-ene

So, we can figure out that Option D is the correct answer.

Note: Allylic halide is the one in which the carbon atom next to the double bonded carbon atom carries one or more halogen atoms. Allylic halides are reactive in both \[{{S}_{N}}1\] and \[{{S}_{N}}2\] mechanisms.