Question

Why is \(CH_3CHO\) more reactive than acetone towards reaction with HCN ?

Hint:

Reactivity towards nucleophilic addition: \[ \text{Formaldehyde} \gt \text{Aldehydes} \gt \text{Ketones} \] Reason:

• Aldehydes have less steric hindrance.

• Ketones contain two electron-releasing alkyl groups which reduce the positive charge on the carbonyl carbon.
Therefore, \[ CH_3CHO \gt CH_3COCH_3 \] towards reaction with HCN.

Answer 1

Step 1: Mechanism of HCN addition.
HCN adds via nucleophilic addition: $CN^-$ attacks the electrophilic carbonyl carbon. Rate depends on the positive charge on $C=O$ and on steric accessibility.
Step 2: Electronic effect comparison.
Acetone ($CH_3COCH_3$) has two methyl groups donating electrons ($+I$ effect) to the carbonyl carbon, reducing its delta+ charge. Acetaldehyde ($CH_3CHO$) has only one methyl group; its carbonyl carbon carries a greater delta+ charge and is more susceptible to $CN^-$ attack.
Step 3: Steric effect and conclusion.
Two methyl groups in acetone create more steric crowding around $C=O$ than the single methyl in $CH_3CHO$, further hindering nucleophilic approach. Both effects make $CH_3CHO$ more reactive. \[ \boxed{CH_3CHO > CH_3COCH_3 \text{ in reactivity toward HCN}} \]