Read the passage carefully and answer the Questions.

Haloalkanes undergo nucelophilic substitution reactions owing to the polarity of C-X bond. The nucleophile reacts with the haloalkane on the carbon possessing a partial positive charge holding the halogen atom. The halogen atom X is replaced by a nucleophile. Depending on the kinetics and mode of bond breaking, the mechanism can be either $S_N1$ or $S_N2$ reaction. The rate of $S_N1$ reaction is governed by the stability of carbocation and in $S_N2$ reaction, the rate of reaction is governed by steric factor. Chirality is the main factor in both $S_N1$ and $S_N2$. In $S_N1$ reaction, the chirality of alkyl halide is accompanied by racemization of the product, while in $S_N2$ reaction, the product is characterized by inversion of configuration. The structure of alkyl halide and the nature of the solvent also governs the mechanism of the substitution.

Which of the following bromo isomer of butane will react at a faster rate towards $S_N2$ reaction?

n-butyl bromide

The correct answer is Option (4) → n-butyl bromide **

SN2 reactions occur fastest on least hindered (primary) alkyl halides.

Let’s analyze the options:

• t-butyl bromide → tertiary → very slow (almost no SN2)
• sec-butyl bromide → secondary → slower
• isobutyl bromide → primary but slightly hindered (–CH₂CH(CH₃)₂)
• n-butyl bromide → primary and least sterically hindered

Therefore, the fastest SN2 reaction occurs with:

✅ n-butyl bromide