MP Board · 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 any two important name reactions involving the preparation or reaction of carbonyl compounds.

Mechanism of Nucleophilic Addition Reactions\nAldehydes and ketones contain a carbonyl group ($>C=O$). The carbon-oxygen double bond is polarized because oxygen is more electronegative than carbon, making the carbonyl carbon electrophilic and oxygen nucleophilic.

• Step 1: The nucleophile ($Nu^-$) attacks the electrophilic carbon atom of the polar carbonyl group from a plane perpendicular to the plane of the carbonyl carbon, forming a tetrahedral alkoxide intermediate.
• Step 2: The alkoxide intermediate quickly grabs a proton ($H^+$) from the reaction medium to form the neutral addition product, often called a cyanohydrin, bisulfite addition compound, etc.

Reactivity: Aldehydes vs. Ketones\nAldehydes are more reactive than ketones due to two main factors:

• Electronic Factors: Ketones have two alkyl groups attached to the carbonyl carbon, which exert a strong electron-donating inductive effect ($+I$ effect). This reduces the positive charge on the carbonyl carbon, making it less attractive to the incoming nucleophile. Aldehydes have only one alkyl group (or hydrogen atom in formaldehyde), hence they possess a higher positive charge on the carbon.
• Steric Factors: Ketones have two bulky alkyl groups that hinder the approach of the nucleophile due to steric hindrance. In contrast, aldehydes have only one alkyl group and a smaller hydrogen atom, providing easier access for the nucleophile.

Important Name Reactions

1. Aldol Condensation: Aldehydes and ketones having at least one alpha-hydrogen atom undergo a self-condensation reaction in the presence of dilute alkali ($NaOH$) to form $\beta$-hydroxy aldehydes (aldol) or $\beta$-hydroxy ketones (ketol), which upon heating lose water to yield $\alpha,\beta$-unsaturated carbonyl compounds.
2. Cannizzaro Reaction: Aldehydes that do not have an $\alpha$-hydrogen atom (such as formaldehyde and benzaldehyde) undergo self-oxidation and reduction (disproportionation) reaction on treatment with concentrated alkali. One molecule is reduced to alcohol and another is oxidized to a carboxylic acid salt.