Question

Which one of the following is not an allylic halide?
(A) 4-Bromopent-2-ene.
(B) 3-Bromo-2-methylbut-1-ene.
(C) 1-Bromobut-2-ene.
(D) 4-Bromobut-1-ene.

Answer 1

Allylic halides are compounds in which halogen atoms attached to $s{p^3}$ hybridized carbon atom next to carbon-carbon double bond (C = C).
The skeleton of an allylic is C=C-C.
Example: $C{H_3} - CH = CHC{H_2}Cl$ is an allylic halide.

Complete step by step answer:
Let us discuss given options one by one by drawing structures.
(1) 4-Bromopent-2-ene

Bromine atom is attached to $s{p^3}$ C-atom and next to $s{p^2}$ hybridized carbon atom.
This is an allylic halide.
(2) 3-Bromo-2-methylbut-1-ene

Bromine atom attached to $s{p^3}$ hybridized C-atom next to $s{p^2}$ hybridized C-atom.
This is an allylic halide.
(3) 1-Bromobut-2-ene

Bromine attached to $s{p^3}$ hybridized C-atom.
This is an allylic halide.
(4) 4-Bromobut-1-ene

Bromine is attached to $s{p^3}$ C-atom next to $s{p^3}$ hybridized C-atom.
Therefore, this is not an allylic halide.

Therefore, from the above explanation the correct option is (D) 4-Bromobut-1-ene is not an allylic halide.

Note: Alkyl halide is less stable than vinyl halide because C-X bond in vinyl halide is stronger due to more double bond character.
While identifying allylic halide, write a type of hybridization on each C-atom and find to which Br-atom is attached.