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

Concept: The reaction of aldehydes and ketones with HCN is a nucleophilic addition reaction. In this reaction, the cyanide ion \((CN^-)\) acts as a nucleophile and attacks the electrophilic carbon atom of the carbonyl group. The ease of nucleophilic addition depends mainly upon:

• The magnitude of positive charge on the carbonyl carbon.

• Steric hindrance around the carbonyl carbon.
Greater positive charge and lower steric hindrance increase the rate of nucleophilic addition.

Step 1: Writing the structures of acetaldehyde and acetone. Acetaldehyde: \[ CH_3CHO \] \[ CH_3-\overset{O}{\underset{\|}{C}}-H \] Acetone: \[ CH_3COCH_3 \] \[ CH_3-\overset{O}{\underset{\|}{C}}-CH_3 \] Both compounds contain the carbonyl group, but the groups attached to the carbonyl carbon are different.

Step 2: Comparing the electronic effects. Methyl groups exhibit a \(+I\) (electron-releasing) effect. In acetone, there are two methyl groups attached to the carbonyl carbon. \[ CH_3COCH_3 \] Both methyl groups donate electron density towards the carbonyl carbon. As a result, the positive charge on the carbonyl carbon decreases. Therefore, the carbonyl carbon becomes less electrophilic and less susceptible to nucleophilic attack. In acetaldehyde, \[ CH_3CHO \] only one methyl group is present, while the other substituent is hydrogen. Hence, the electron-releasing effect is smaller and the carbonyl carbon carries a greater positive charge. Thus, nucleophilic attack occurs more readily.

Step 3: Comparing steric hindrance. For nucleophilic addition to occur, the nucleophile must approach the carbonyl carbon. In acetaldehyde: \[ CH_3CHO \] only one methyl group is present near the carbonyl carbon. Therefore, steric hindrance is comparatively low. In acetone: \[ CH_3COCH_3 \] two methyl groups surround the carbonyl carbon. These bulky groups obstruct the approach of the nucleophile. Hence, steric hindrance is greater in acetone. As a result, nucleophilic addition becomes more difficult.

Step 4: Applying these effects to the HCN reaction. The cyanide ion \[ CN^- \] attacks the carbonyl carbon. Because acetaldehyde has:

• Greater electrophilic character of the carbonyl carbon.

• Lower steric hindrance.
it reacts faster with HCN. Acetone has:

• Lower positive charge on the carbonyl carbon.

• Greater steric hindrance.
therefore it reacts more slowly.

Step 5: Conclusion. Thus, acetaldehyde undergoes nucleophilic addition more readily than acetone. Hence, \[ \boxed{ CH_3CHO \gt CH_3COCH_3 } \] in reactivity towards HCN.

Final Answer: \[ \boxed{ CH_3CHO \text{ is more reactive than acetone towards HCN because it has less steric hindrance and a more positively polarized carbonyl carbon.} } \]