Question

Arrange the following compounds in increasing order of their reactivity towards HCN : \[ (CH_3)_3C-CO-CH_3,\qquad CH_3COCH_3,\qquad CH_3CHO \]

Hint:

Aldehydes are generally more reactive than ketones towards nucleophilic addition because they have less steric hindrance and a more electrophilic carbonyl carbon.

Answer 1

Step 1: Factors governing reactivity towards HCN.
HCN addition to $C{=}O$ is a nucleophilic attack. Reactivity increases with (a) greater electrophilicity of the carbonyl carbon and (b) less steric hindrance around it. Electron-donating alkyl groups reduce both factors simultaneously.
Step 2: Aldehyde is more reactive than ketones.
$CH_3CHO$ (one $CH_3$ group) has the least steric shielding and the most electrophilic carbonyl among the three. $CH_3COCH_3$ (two $CH_3$ groups) is less reactive than the aldehyde but still more accessible than the bulky ketone.
Step 3: Bulky ketone is least reactive.
$(CH_3)_3C{-}CO{-}CH_3$ has a very bulky tert-butyl group that severely shields the carbonyl carbon from nucleophilic attack. Increasing order of reactivity towards HCN: \[ \boxed{(CH_3)_3C{-}CO{-}CH_3 < CH_3COCH_3 < CH_3CHO} \]