Carbonyl Compounds: Aldehydes, Ketones, and Their Reactions

Aldehydes vs. Ketones

Aldehydes (R-CHO) have the carbonyl group at the end of a carbon chain, bonded to at least one hydrogen atom. Ketones (R-CO-R) have the carbonyl between two carbon groups. This structural difference affects reactivity: aldehydes are generally more electrophilic than ketones because they have less steric hindrance (one hydrogen instead of a bulky alkyl group) and less electron donation from alkyl groups. Formaldehyde (HCHO) is the most reactive common carbonyl, and acetone (CH3COCH3) is a typical ketone of moderate reactivity.

Aldehydes can be oxidized to carboxylic acids by mild oxidizing agents (Jones reagent, PCC with excess oxidant, Tollens reagent, Fehling solution), while ketones resist oxidation under the same conditions because oxidizing a ketone would require breaking a carbon-carbon bond. This difference allows chemists to distinguish aldehydes from ketones using the Tollens test (silver mirror test) or Fehling test (brick-red precipitate). Aldehydes are also more prone to air oxidation, which is why many aldehyde reagents must be used fresh or stored under inert atmosphere.

Nucleophilic Addition

The fundamental reaction of aldehydes and ketones is nucleophilic addition. A nucleophile attacks the electrophilic carbonyl carbon, breaking the pi bond and forming a new sigma bond to the nucleophile while the oxygen accepts the pi electrons to become an alkoxide. Protonation of the alkoxide completes the reaction. The products depend entirely on the nature of the nucleophile.

Hydride reduction (NaBH4 or LiAlH4) delivers H- to the carbonyl carbon, producing an alcohol: aldehydes give primary alcohols, ketones give secondary alcohols. Grignard reagents (RMgBr) and organolithium reagents (RLi) deliver a carbon nucleophile, forming a new carbon-carbon bond and producing an alcohol with an extended carbon skeleton. Water adds to give a geminal diol (hydrate), though this equilibrium usually favors the carbonyl form. Alcohols add to give hemiacetals (one addition) and acetals (two additions, with acid catalysis and water removal).

Primary amines react with aldehydes and ketones to form imines (Schiff bases, R2C=NR), with water as a byproduct. Secondary amines form enamines (R2C=CR-NR2). Both imines and enamines are important intermediates in synthesis and in enzyme-catalyzed reactions. Hydrogen cyanide (HCN) adds to give cyanohydrins (R2C(OH)CN), which are useful synthetic intermediates because the cyano group can be converted to a carboxylic acid or amine.

Carboxylic Acid Derivatives

Carboxylic acids (R-COOH) and their derivatives (esters, amides, acid chlorides, anhydrides) undergo nucleophilic acyl substitution rather than simple addition. The mechanism involves nucleophilic attack on the carbonyl carbon to form a tetrahedral intermediate, followed by expulsion of a leaving group to regenerate the carbonyl. The reactivity order follows the quality of the leaving group: acid chlorides (Cl-) > anhydrides (RCO2-) > esters (RO-) > amides (NR2-) > carboxylate ions (O-).

Acid chlorides react with water to give carboxylic acids (hydrolysis), with alcohols to give esters (esterification), with amines to give amides (amide formation), and with Grignard reagents to give ketones or tertiary alcohols. Esters can be hydrolyzed to acids and alcohols under acidic or basic conditions. Base-promoted ester hydrolysis (saponification) is irreversible because the carboxylate product is stabilized by resonance.

The Fischer esterification reacts a carboxylic acid with an alcohol under acid catalysis to form an ester and water. Because this reaction is an equilibrium, excess alcohol or removal of water (by distillation or a drying agent) drives the reaction to completion. Amide formation from carboxylic acids and amines requires activation of the acid (converting it to an acid chloride, anhydride, or active ester) because the amine would simply form a salt with the free acid rather than undergoing substitution.

Enolate Chemistry

The alpha hydrogens of carbonyl compounds (hydrogens on the carbon adjacent to the carbonyl) are weakly acidic (pKa 16-25) because the resulting carbanion (enolate) is stabilized by resonance delocalization into the carbonyl. Strong bases (LDA, NaH, KOtBu) can fully deprotonate the alpha position to form enolate ions, which are powerful nucleophiles and bases.

The aldol reaction combines two carbonyl compounds through their enolate intermediates: the enolate of one molecule attacks the carbonyl of another, forming a beta-hydroxy carbonyl (aldol product). Heating the aldol product produces an alpha,beta-unsaturated carbonyl through dehydration (aldol condensation). The Claisen condensation is the ester equivalent: an ester enolate attacks another ester carbonyl to form a beta-keto ester.

Enolate alkylation replaces an alpha hydrogen with an alkyl group using an alkyl halide as the electrophile. The malonic ester synthesis and acetoacetic ester synthesis are classic enolate-based methods for building carbon frameworks. Michael additions involve the conjugate addition of an enolate to an alpha,beta-unsaturated carbonyl, forming a 1,5-dicarbonyl compound. These reactions are workhorses of organic synthesis.