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The carbonyl group (C=O) is the most important functional group in organic chemistry, present in aldehydes, ketones, carboxylic acids, esters, and amides. The C=O bond is highly polarized because oxygen is electronegative, making the carbonyl carbon a potent electrophile susceptible to nucleophilic attack. Aldehydes (RCHO) are more reactive than ketones (RCOR') toward nucleophilic addition because they are less sterically shielded and less stabilized by electron-donating alkyl groups. The alpha carbon adjacent to the carbonyl is acidic (pKa ≈ 20 for ketones) because the resulting carbanion is resonance-stabilized as the enolate.
Draw resonance structures of the carbonyl group to show partial positive charge on carbon and partial negative on oxygen. Rank the reactivities of formaldehyde, acetaldehyde, acetone, and benzaldehyde toward a nucleophile, justifying each ranking with steric and electronic arguments.
The carbonyl group (C=O) is the most important functional group in organic chemistry, appearing in aldehydes, ketones, esters, amides, carboxylic acids, and many other compound classes. Its reactivity stems from one source: the electronegativity of oxygen. The shared electrons in the C=O double bond are pulled strongly toward oxygen, leaving the carbon electron-poor. This partial positive charge (δ+) on carbon makes the carbonyl carbon an electrophile — a target for electron-rich species called nucleophiles. Understanding this polarity is the key to predicting carbonyl reactivity.
A point that confuses nearly every student initially: nucleophilic attack occurs at the carbonyl carbon, not the oxygen, even though oxygen bears the δ− charge in the ground state. The δ− oxygen actually repels nucleophiles, which carry their own electron pairs. The δ+ carbon attracts them. When a nucleophile donates electrons to the carbon, the π electrons of the C=O bond shift entirely onto oxygen, generating an alkoxide (or similar) intermediate. This is the general mechanism for nucleophilic addition to carbonyls: nucleophile attacks C, π bond breaks toward O, tetrahedral intermediate forms.
Comparing aldehydes and ketones reveals how both steric bulk and electron density shape reactivity. In a ketone, two alkyl groups flank the carbonyl carbon — they donate electron density through induction and hyperconjugation, reducing the δ+ charge, and they physically block the approach of a nucleophile. In an aldehyde, only one alkyl group and one hydrogen occupy those positions. The result: aldehydes are more electrophilic and less sterically hindered, so they react faster with nucleophiles. Formaldehyde (H₂C=O), with two hydrogens and no alkyl groups, is the most reactive simple carbonyl compound.
The alpha carbon — the sp3 carbon directly adjacent to C=O — has a surprisingly acidic C–H bond, with pKa around 20 for ketones compared to roughly 50 for a typical alkane C–H. This enormous difference in acidity is due entirely to resonance stabilization of the conjugate base. When the alpha proton is removed by a base, the resulting carbanion delocalizes its negative charge through the carbonyl system onto oxygen, forming the enolate ion. Because the conjugate base (enolate) is far more stable than an unresolved carbanion, the equilibrium for proton removal lies much further toward the deprotonated side — the acid is stronger. Enolate chemistry is what makes carbonyls so versatile in synthesis.
The aldehyde/ketone reactivity you learn here is the foundation for the rest of carbonyl chemistry. Carboxylic acid derivatives (esters, amides, acid chlorides) also contain C=O, but the heteroatom attached directly to the carbonyl changes the mechanism: instead of simple addition, these compounds undergo acyl substitution, where the nucleophile adds and then a leaving group departs. The carbonyl carbon's electrophilicity, nucleophilic attack at C rather than O, and resonance-driven enolate acidity are threads that run through all of it.