Carbonyl chemistry and nucleophilic addition

The carbonyl group, C=O, found in ketones, aldehydes, carboxylic acids, and their derivatives, is one of the most reactive functional groups in organic chemistry because of a single structural fact: oxygen is considerably more electronegative than carbon, so the shared electron density of the double bond sits unevenly, pulled toward oxygen. This leaves the carbonyl carbon bearing a partial positive charge and the oxygen a partial negative charge — the bond is strongly polarised rather than the roughly nonpolar carbon-carbon double bond found in alkenes.

This polarisation directly sets up the carbonyl group's dominant reaction mode: nucleophilic addition. A nucleophile is drawn to the electron-poor carbonyl carbon (not the oxygen, which is already electron-rich) and forms a new bond there, using the carbon-oxygen pi bond's electron pair, which moves entirely onto the oxygen. The result is a tetrahedral intermediate — the carbon, previously flat and sp2-hybridised as part of the planar C=O group, becomes sp3-hybridised and bonded to four different groups, with the oxygen now bearing a full negative charge (an alkoxide) that is typically protonated in a subsequent step. This mechanism, nucleophile attacks carbon, pi electrons move to oxygen, underlies reactions ranging from simple addition of water or alcohols across the carbonyl to more elaborate reactions with organometallic reagents used to build new carbon-carbon bonds in synthesis.

The same carbonyl polarisation opens a second, complementary reactivity mode centred not on the carbonyl carbon itself but on the carbon immediately adjacent to it, the alpha carbon. A C-H bond on the alpha carbon is unusually acidic — far more acidic than an ordinary C-H bond elsewhere in the molecule — because removing that proton produces a carbanion whose negative charge is not stuck on one carbon but can delocalise into the adjacent carbonyl group, spreading onto the more electronegative oxygen and gaining substantial stabilisation in the process; this delocalised, resonance-stabilised species is called an enolate. The neutral, protonated form of this same delocalised structure is the enol, existing in a dynamic equilibrium with the ordinary "keto" form — keto-enol tautomerism — and this alpha-carbon acidity and enol/enolate chemistry underlies a large family of carbon-carbon bond-forming reactions in organic synthesis, including aldol condensation.