Question

Which compound in the given pair would undergo S_N2 reaction at a faster rate and why?

\[\ce{CH3 - CH2 - I and CH3 - CH2 - Br}\]

Answer 1

In general, for S_N2 reactions:

The stronger the nucleophile, the quicker the reaction. Reduced steric hindrance around the electrophilic carbon leads to quicker reactions.
When we look at the given pairs \[\ce{CH3CH2I and CH3CH2Br}\].
Iodide \[\ce{(I-)}\] is a more effective nucleophile than bromide \[\ce{(Br-)}\] due to its bigger size and higher electron density, making it more active in nucleophilic substitution processes.

Under steric hindrance:

Since both molecules share the same alkyl group (ethyl), their steric hindrance is the same.
Therefore, \[\ce{CH3CH2I}\] undergoes the S_N2 reaction faster than \[\ce{CH3CH2Br}\] due to the stronger nucleophilicity of iodide compared to bromide.