Predict the correct order of reactivity of the following four primary bromides towards S _ N 2 reaction. (I) CH ₃ CH ₂ CH ( CH ₃) CH ₂ Br (II) ( CH ₃ )₃ CCH ₂ Br (III) CH ₃ CH ₂ CH ₂ CH ₂ Br (IV) CH ₃ CH ( CH ₃ ) CH ₂ CH ₂ Br

Step-by-step solution

The rate of an S _ N 2 reaction is inversely proportional to the steric hindrance around the electrophilic carbon atom. Since all the given compounds are primary alkyl bromides, we must compare the steric hindrance caused by branching at the and positions. (III) CH ₃ CH ₂ CH ₂ CH ₂ Br is a straight-chain primary bromide with no branching, offering the least steric hindrance. (IV) CH ₃ CH ( CH ₃) CH ₂ CH ₂ Br has branching at the -carbon, which introduces slight steric hindrance compared to the unbranched chain. (I)