Question

$KI$ in acetone, undergoes ${S_N}^2$ reaction with each of $P,Q,R$ and $S$. The rates of the reaction vary as _______________.

• A. $P>Q>R>S$
• B. $S>P>R>Q$
• C. $Q>R>P>S$
• D. $R>P>S>Q$

Answer 1

Answer: (B)

$S>P>R>Q$

The correct order for the rate of the reaction towards ${S_N}^2$ reaction is $S>P>R>Q$.

Methyl halide are more reactive than primary alkyl halides towards ${S_N}^2$ substitution reaction.

Similarly primary alkyl halides are more reactive towards ${S_N}^2$ substitution reaction than secondary halides.
This is due to steric hindrance and the electron donating effect of alkyl groups which increases the electron destiny on the carbon attached to halogen and makes it less electrophilic.

This explains the order $P>R>Q$

$S$ has maximum rate due to electron withdrawing nature of $PhCO$ group which decreases the electron density on carbon and increases the electrophilicity.

The relative reactivities towards ${S_N}^2$ reaction with $KI$ in acetone for $S,P,R$ and $Q$ are $1000,200,79$ and $0.02$ respectively.

Thus, the option B is correct.