CBSE · Class 12 · Chemistry · Aldehydes, Ketones and Carboxylic AcidsExplain why aldehydes are more reactive than ketones towards nucleophilic addition reactions. Give two reasons.

Aldehydes are generally more reactive than ketones towards nucleophilic addition reactions due to two primary electronic and steric factors. First, let us consider the electronic factor: aldehydes contain one alkyl group attached to the carbonyl carbon, whereas ketones contain two alkyl groups. Alkyl groups are electron-donating groups due to the inductive effect (+I effect) and hyperconjugation. Consequently, they release electrons towards the carbonyl carbon, thereby reducing the positive charge on the carbonyl carbon atom. Since ketones have two such electron-releasing groups, the positive charge on the carbonyl carbon is reduced to a greater extent in ketones than in aldehydes, making the electrophilic character of the carbonyl carbon weaker in ketones and stronger in aldehydes. Second, let us consider the steric factor: aldehydes have one bulky alkyl group and one small hydrogen atom attached to the carbonyl carbon, whereas ketones have two bulky alkyl groups. The presence of two bulky alkyl groups in ketones causes greater steric hindrance to the approaching nucleophile compared to aldehydes, which have only one alkyl group. This steric crowding in ketones significantly hinders the attack of the nucleophile on the carbonyl carbon, thereby slowing down the reaction rate. Thus, both electronic and steric factors combine to make aldehydes significantly more reactive than ketones towards nucleophilic addition reactions.