Question

Which among the following is most reactive via \(S_N2\) mechanism?

• A. 1
• B. 2
• C. 3
• D. 4

Hint:

For \(S_N2\) reactions, steric hindrance is the most important factor. \[ CH_3X \gt 1^\circ RX \gt 2^\circ RX \gg 3^\circ RX \] Less crowded carbon atoms undergo faster backside attack by the nucleophile.

Answer 1

The Correct Option is B

SN2 reactions happen in a single step where the incoming nucleophile has to attack the carbon from the side directly opposite the leaving group, so anything crowding that backside approach slows the reaction down.

That means the reactivity order simply tracks how substituted the reacting carbon is, a methyl or primary carbon, with mostly small hydrogens nearby, gives the nucleophile a clear path in, while secondary and especially tertiary carbons block that path with bulky alkyl groups.

Ranking the given structures by how substituted the reacting carbon is, the primary alkyl bromide has the least crowding around the carbon bromine bond, the two secondary bromides come next, and the most hindered structure reacts slowest of all.

So the least hindered, primary substrate is the fastest towards SN2, matching structure 2.

So the correct choice is option (2).