Question

Which compound undergoes nucleophilic substitution with NaCN at the fastest rate?

• A.
• B.
• C.
• D.

Answer 1

The correct option is A

Since NaCN is a strong nucleophile, it favours $S_{N}2$ reaction.
The rate of nucleophilic substitution depends on the halogen group attached to free end or branched end of the carbon chain.
If the halogen chain is attached to free end and is away from the branched end, it will react faster with $NaCN$
Also,
$\text{Rate of}{S}_{N}2\text{reaction}\propto \frac{1}{\text{Steric hindrance}}$
So (a) is the correct option.