Consider the following four alkyl bromides: (A) CH ₃ Br (B) CH ₃ CH ₂ Br (C) ( CH ₃ )₂ CHCH ₂ Br (D) ( CH ₃ )₃ CCH ₂ Br Predict the correct decreasing order of their reactivity towards S _ N 2 reaction.

Step-by-step solution

In an S _ N 2 reaction, the nucleophile attacks the -carbon from the side opposite to the leaving group. The rate of reaction is inversely proportional to the steric hindrance around the -carbon. All the given compounds are methyl or primary bromides. However, as the branching at the -carbon increases, steric crowding in the transition state increases, which significantly lowers the rate of the S _ N 2 reaction. (A) CH ₃ Br has no alkyl groups and is the least hindered. (B) CH ₃ CH ₂ Br has one methyl group at the