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.

An allylic halide is a compound that has a halogen (e.g., bromine) atom bonded to a carbon atom that is adjacent to a carbon-carbon double bond. In other words, it is a halide that is bonded to a carbon atom that is part of an allylic system.

Let's examine each of the given compounds:

(A) 4-Bromopent-2-ene - This compound has a bromine atom bonded to a carbon atom that is adjacent to a carbon-carbon double bond, so it is an allylic halide.

(B) 3-Bromo-2-methylbut-1-ene - This compound also has a bromine atom bonded to a carbon atom that is adjacent to a carbon-carbon double bond, so it is an allylic halide.

(C) 1-Bromobut-2-ene - This compound has a bromine atom bonded to a carbon atom that is adjacent to a carbon-carbon double bond, so it is an allylic halide.

(D) 4-Bromobut-1-ene - This compound has a bromine atom bonded to a carbon atom that is not adjacent to a carbon-carbon double bond. It is bonded to the carbon at the end of the carbon chain, so it is not an allylic halide.

So, the answer is (D) 4-Bromobut-1-ene, as it is not an allylic halide.