Aldehydes feature a carbonyl group bonded to at least one hydrogen (R-CHO) and are named with the suffix -al (methanal, ethanal). The carbonyl carbon is \(sp^2\) hybridized with trigonal planar geometry. In IR, aldehydes show a strong C=O stretch near 1720–1740 cm\(^{-1}\) plus two weak but diagnostic aldehyde C–H stretches near 2720 and 2820 cm\(^{-1}\). Because the carbonyl carbon bears an H, aldehydes oxidize easily to carboxylic acids and give positive Tollens' (silver mirror), Fehling's, and Benedict's tests—the latter two turning blue \(Cu^{2+}\) to brick-red \(Cu_2O\). Ketones do not give these tests because they have no removable H on the carbonyl carbon. Aldehydes and ketones both undergo characteristic \(^1\)H NMR signatures: aldehyde C–H protons appear far downfield at 9–10 ppm, while in \(^{13}\)C NMR the carbonyl carbon resonates in the 190–220 ppm range for aldehydes and ketones (lower for esters, amides, and acids at 160–180 ppm).
Ketones feature a carbonyl bonded to two carbon groups (R-CO-R′) and use the suffix -one (propanone, butanone). Their IR carbonyl stretch appears near 1705–1725 cm\(^{-1}\) without the aldehyde C–H peaks. Reaction of an aldehyde or ketone with a Grignard reagent followed by aqueous workup yields a secondary alcohol from an aldehyde and a tertiary alcohol from a ketone. The iodoform test specifically detects methyl ketones (R-CO-CH₃), producing a yellow precipitate of \(CHI_3\). When an alcohol adds to a carbonyl, a hemiacetal forms (containing both -OH and -OR on the same carbon); reaction with a second equivalent of alcohol under acid catalysis yields an acetal with two -OR groups on the same carbon and loss of water—acetals are useful as protecting groups for carbonyls during reactions elsewhere in the molecule.
Both aldehydes and ketones undergo keto–enol tautomerism, an equilibrium between the keto form (C=O) and the enol form (C=C with an adjacent -OH) interconverted via an alpha-hydrogen shift. The carbons directly adjacent to the carbonyl are called alpha carbons, and their hydrogens are unusually acidic because deprotonation yields an enolate whose negative charge is resonance-stabilized by the carbonyl. Two aldehyde or ketone molecules can react with base to give a beta-hydroxy carbonyl through the aldol addition reaction; subsequent heating dehydrates this intermediate in an aldol condensation to give an alpha,beta-unsaturated carbonyl. Primary amines react with carbonyls to form imines (C=N-R), while secondary amines form enamines (C=C-NR₂), both with loss of water. Other important carbonyl reactions include the Wittig reaction (forming an alkene from an aldehyde or ketone with a phosphorus ylide), the Cannizzaro reaction (base-mediated disproportionation of a non-enolizable aldehyde into a carboxylate and an alcohol), Michael additions (1,4-conjugate addition of an enolate to an alpha,beta-unsaturated carbonyl), and the Robinson annulation (Michael addition followed by intramolecular aldol condensation to build fused six-membered rings). Hard nucleophiles such as Grignard and organolithium reagents favor 1,2-addition to alpha,beta-unsaturated carbonyls, while softer cuprates (R₂CuLi) favor 1,4-(conjugate) addition at the beta carbon.