The carbonyl group defines the reactivity of aldehydes and ketones. The electronegativity difference between carbon and oxygen polarizes the C=O bond, placing a partial positive charge (δ+) on carbon and a partial negative charge (δ-) on oxygen. This makes the carbonyl carbon strongly electrophilic and the oxygen weakly basic. Aldehydes (RCHO) are generally more reactive than ketones (RCOR’) toward nucleophiles due to less steric hindrance and greater stabilization of the partial positive charge on carbon.
Nomenclature and Synthesis
Aldehydes are named by replacing the -e of the parent alkane with -al; ketones replace -e with -one. Common names for simple aldehydes (formaldehyde, acetaldehyde) and ketones (acetone, acetophenone) remain widely used. Several synthetic routes provide access to these compounds. Primary alcohols oxidize to aldehydes under mild conditions using PCC (pyridinium chlorochromate) without overoxidation to carboxylic acids. Friedel-Crafts acylation installs a ketone directly onto an aromatic ring using an acyl chloride and a Lewis acid catalyst (AlCl₃). Ozonolysis of alkenes cleaves the double bond, producing aldehydes or ketones depending on the substitution pattern of the alkene.
Nucleophilic Addition
The characteristic reaction of aldehydes and ketones is nucleophilic addition to the carbonyl carbon. Hydride sources such as NaBH₄ reduce aldehydes to primary alcohols and ketones to secondary alcohols; LiAlH₄ is more powerful and reduces carboxylic acids and esters as well. Grignard reagents (RMgX) add irreversibly: formaldehyde yields primary alcohols, other aldehydes yield secondary alcohols, and ketones yield tertiary alcohols. Cyanide (HCN or NaCN with acid) adds to form cyanohydrins (RCH(OH)CN), which are versatile synthetic intermediates. Water adds reversibly to form gem-diols (hydrates), and alcohols add to form hemiacetals and acetals. Acetal formation is particularly useful as a protecting group strategy — acetals are stable under basic conditions and can be removed by mild acid hydrolysis.
Keto-Enol Tautomerism and Aldol Condensation
Aldehydes and ketones with α-hydrogens exist in equilibrium with their enol tautomers. Under basic conditions, deprotonation at the α-carbon generates an enolate, a powerful nucleophile. The aldol condensation involves an enolate attacking the carbonyl of another aldehyde or ketone to form a β-hydroxy carbonyl compound (aldol), which can dehydrate to an α,β-unsaturated system. Crossed aldol reactions (between two different carbonyl compounds) and intramolecular aldol cyclizations expand the synthetic utility.
Baeyer-Villiger Oxidation and Reduction
The Baeyer-Villiger oxidation inserts an oxygen atom adjacent to the carbonyl using a peroxyacid (e.g., mCPBA), converting ketones to esters and aldehydes to carboxylic acids or formate esters. The migration aptitude follows the trend: tertiary alkyl > secondary alkyl > benzyl > primary alkyl > methyl. For deoxygenation, the Wolff-Kishner reduction (hydrazine, strong base, heat) and the Clemmensen reduction (Zn/Hg, HCl) both convert aldehydes and ketones to alkanes via different mechanisms, useful when other functional groups in the molecule are sensitive to the conditions of the alternative method.
Biological and Industrial Importance
Aldehydes and ketones appear throughout biochemistry: retinal (ketone) is essential for vision, pyruvate (α-ketoacid) is central to metabolism, and sugars exist predominantly as cyclic hemiacetals. Industrially, acetone is a common solvent, formaldehyde is used in polymer production (Bakelite), and ketones serve as intermediates in the synthesis of pharmaceuticals, fragrances, and agrochemicals.
