CBSE · Class 12 · Chemistry · Aldehydes, Ketones and Carboxylic AcidsExplain the mechanism of nucleophilic addition reactions in aldehydes and ketones. Why are aldehydes generally more reactive than ketones towards nucleophilic addition? Discuss with suitable examples.

Mechanism of Nucleophilic Addition Reactions\nAldehydes and ketones contain a carbonyl group ($>C=O$) where the carbon-oxygen double bond is polarized due to the higher electronegativity of oxygen compared to carbon. The carbon atom carries a partial positive charge, making it an electrophilic center, while the oxygen atom carries a partial negative charge.

• Step 1: Attack of Nucleophile: A nucleophile ($Nu^-$) attacks the electrophilic carbon atom of the carbonyl group from a direction perpendicular to the plane of the carbonyl carbon and the attached atoms. This attack converts the trigonal planar carbon into a tetrahedral intermediate, where oxygen acquires a negative charge (alkoxide ion).
• Step 2: Protonation: The alkoxide intermediate captures a proton ($H^+$) from the reaction medium (acid or water) to form the neutral addition product, commonly known as a cyanohydrin, bisulfite addition compound, or alcohol depending on the reagent.

Reactivity of Aldehydes vs. Ketones\nAldehydes are generally more reactive than ketones towards nucleophilic addition reactions due to two primary factors:

• Electronic Factors (Inductive Effect): Ketones possess two alkyl groups attached to the carbonyl carbon, whereas aldehydes have only one alkyl group (or a hydrogen atom in formaldehyde). Alkyl groups are electron-donating groups (+I effect) that release electrons towards the carbonyl carbon, thereby reducing its positive charge and decreasing its susceptibility to nucleophilic attack. In aldehydes, the presence of one hydrogen atom or smaller alkyl groups results in a higher positive charge density on the carbonyl carbon.
• Steric Factors: Ketones have two bulky alkyl groups attached to the carbonyl carbon, which create steric hindrance and impede the approach of the incoming nucleophile to the electrophilic carbon. In contrast, aldehydes have only one bulky group and one small hydrogen atom, offering much less steric hindrance and allowing easier access for the nucleophile.

Examples\nWhen acetaldehyde reacts with hydrogen cyanide ($HCN$), it forms acetaldehyde cyanohydrin via nucleophilic addition. Similarly, acetone reacts with $HCN$ to form acetone cyanohydrin, but at a slower rate due to increased steric and electronic hindrance.