These five groups share a carbonyl carbon attached to a heteroatom and undergo the same general reaction — nucleophilic acyl substitution (Chapter 13) — but differ enormously in reactivity depending on how good a leaving group that heteroatom provides. Plain –OH is a poor leaving group: a carboxylic acid typically needs activation (protonation, or conversion to an acid chloride) before it will undergo substitution as readily as the other four members of this family.

Carboxylic Acids

Structure: R–COOH Hybridization: sp² at the carbonyl carbon Geometry: Trigonal planar, ~120° Polarity: Highly polar; O–H allows hydrogen bonding (carboxylic acids often exist as dimers) Typical reactivity: Acidic at O–H (Chapter 3); the conjugate base (carboxylate) is resonance-stabilized across both oxygens. Converted to acid chlorides, esters, and amides via nucleophilic acyl substitution.

Acid Chlorides

Structure: R–COCl Hybridization: sp² at the carbonyl carbon Geometry: Trigonal planar, ~120° Polarity: Very polar Typical reactivity: The most reactive carboxylic acid derivative — chloride is an excellent leaving group. Reacts readily with water, alcohols, and amines to generate acids, esters, and amides.

Anhydrides

Structure: R–CO–O–CO–R′ Hybridization: sp² at both carbonyl carbons Geometry: Trigonal planar at each carbonyl Polarity: Polar Typical reactivity: Second most reactive derivative, behind acid chlorides. A carboxylate is a moderate leaving group, so anhydrides readily acylate alcohols and amines.

Esters

Structure: R–CO–O–R′ Hybridization: sp² at the carbonyl carbon Geometry: Trigonal planar, ~120° Polarity: Polar Typical reactivity: Moderately reactive; undergoes hydrolysis back to the carboxylic acid or transesterification, but requires more forcing conditions than acid chlorides or anhydrides. Often recognizable by pleasant, fruity odors.

Amides

Structure: R–CO–NR′₂ Hybridization: sp² at the carbonyl carbon; nitrogen lone pair delocalizes into the carbonyl, making nitrogen effectively sp² as well Geometry: Planar across the whole C(=O)–N unit Polarity: Highly polar, but nitrogen’s lone pair is tied up in resonance Typical reactivity: The least reactive carboxylic acid derivative and the most stable — nitrogen is a poor leaving group because of resonance donation into the carbonyl. This same resonance makes amide nitrogen far less basic and less nucleophilic than amine nitrogen. Forms the peptide bond in proteins.

Acetals and Hemiacetals

Structure: Hemiacetal — a carbon bonded to one –OH and one –OR; Acetal — a carbon bonded to two –OR groups. Both form from addition of alcohol(s) to an aldehyde or ketone. Hybridization: sp³ at the central carbon (contrast with the sp² parent carbonyl) Geometry: Tetrahedral, ~109.5° Polarity: Polar C–O bonds Typical reactivity: Formed and hydrolyzed reversibly under acid catalysis (Chapter 12). Acetals are stable to base and are used as protecting groups to temporarily mask a carbonyl during a synthesis.

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