Question

Arrange the following compounds in increasing order of their reactivity towards HCN : $\mathrm{CH_3COCH_3,\ CH_3CHO,\ (CH_3)_3C\text{-}CO\text{-}CH_3}$.

Hint:

More steric crowding → less reactive to HCN.

Answer 1

Concept: In the addition of HCN, a nucleophile attacks the carbonyl carbon. Anything that crowds that carbon or pushes electrons onto it makes the attack harder and the compound less reactive.

Step 1: Compare the crowding
$\mathrm{CH_3CHO}$ has only a small hydrogen and one methyl group near the carbonyl, so the carbon is open and easy to attack. $\mathrm{CH_3COCH_3}$ (acetone) has two methyl groups, so it is a bit more crowded and a bit less reactive. $\mathrm{(CH_3)_3C\text{-}CO\text{-}CH_3}$ has a big bulky tert-butyl group on one side, so it is the most crowded and the hardest to attack.

Step 2: Put them in order
More crowding and more electron-pushing alkyl groups mean lower reactivity. So from least reactive to most reactive:\[ (CH_3)_3C\text{-}CO\text{-}CH_3 < CH_3COCH_3 < CH_3CHO \]

Answer: $(CH_3)_3C\text{-}CO\text{-}CH_3$ < $CH_3COCH_3$ < $CH_3CHO$ (increasing reactivity towards HCN).