Aldehydes, Ketones and Carboxylic Acids - Nucleophilic Addition Reactions

Nature of Carbonyl Group: The carbonyl carbon is $sp^2$ hybridized and forms three $\sigma$ bonds. The fourth valence electron remains in a $p$-orbital, forming a $\pi$ bond with oxygen. Due to the high electronegativity of oxygen, the $C=O$ bond is highly polar: $>C^{\delta+} = O^{\delta-}$.

Mechanism of Nucleophilic Addition: A nucleophile ($Nu^-$) attacks the electrophilic carbonyl carbon from a direction perpendicular to the plane of $sp^2$ hybridized orbitals. Hybridization changes from $sp^2$ to $sp^3$, forming a tetrahedral intermediate which then captures a proton ($H^+$) to give the final product.

Reactivity Trends: Aldehydes are generally more reactive than ketones towards nucleophilic addition due to: (1) Steric factors: Two relatively large substituents in ketones hinder the approach of the nucleophile. (2) Inductive effect: Two electron-releasing alkyl groups in ketones reduce the electrophilicity of the carbonyl carbon.

Addition of $HCN$: Aldehydes and ketones react with hydrogen cyanide to yield cyanohydrins. The reaction is slow with pure $HCN$ and is catalyzed by a base to generate the stronger nucleophile $CN^-$.

Addition of Alcohols: Aldehydes react with one equivalent of monohydric alcohol in the presence of dry $HCl$ gas to form hemiacetals, and with another equivalent to form acetals. Ketones react with ethylene glycol to form cyclic ketals.

Addition of Ammonia Derivatives: Nucleophiles such as $NH_3$ and its derivatives ($H_2N-Z$) add to the carbonyl group. The reaction is followed by the elimination of a water molecule to form compounds containing the $>C=N-Z$ linkage.