Anhydrides - MCAT Organic Chemistry

Anhydrides

Updated Apr 17, 2026

An anhydride (literally “without water”) has the structure R-CO-O-CO-R’ - two acyl groups sharing a central oxygen. It is the product of removing water from two carboxylic acids, either explicitly or implicitly.

Anhydrides are second only to acyl halides in reactivity. Their leaving group (a carboxylate, RCOO⁻) is not as good as chloride but is much better than the alkoxide (ester) or amide nitrogen (amide).

Structure and Symmetry

Symmetric anhydrides: both acyl groups identical. Example: acetic anhydride (CH₃CO-O-COCH₃).
Mixed anhydrides: acyl groups different. Example: acetic formic anhydride (HCO-O-COCH₃).
Cyclic anhydrides: both acyl groups on the same carbon chain, forming a ring. Example: succinic anhydride (5-membered ring), glutaric anhydride (6-membered ring), phthalic anhydride (aromatic).

Formation

From carboxylic acid + heat (especially for cyclic anhydrides):

Succinic acid → succinic anhydride + H₂O. Heat drives off water, forming the 5-membered ring. Easy for cyclic products; hard for acyclic because entropy favors two separate COOH molecules.

From acyl halide + carboxylate:

R-COCl + R’-COO⁻ → R-CO-O-CO-R’ + Cl⁻. Reacts at room temperature. This is how mixed anhydrides are prepared in the lab.

From acyl halide + carboxylic acid:

R-COCl + R’-COOH → R-CO-O-CO-R’ + HCl. Similar approach; HCl leaves as a gas.

Reactivity Relative to Acid Chlorides

Anhydrides are slower than acid chlorides but still fast. Carboxylate (the leaving group) is a better LG than alkoxide (ester) but not as good as chloride. Resonance donation from the second acyl group’s lone pairs is weaker than from a single OR, so the carbonyl remains highly electrophilic.

Typical Reactions

The same as acid chlorides, just slower:

Water → two carboxylic acids (hydrolysis, reversible, essentially complete).
Alcohol → ester + carboxylic acid (one of the two COOHs becomes the ester; the other becomes a free acid byproduct).
Amine → amide + carboxylic acid.

The “half-the-acyl-wasted” pattern is why acid chlorides are usually preferred over anhydrides in synthesis: acid chlorides install 1 acyl per equivalent, while anhydrides install 1 acyl but waste the other acyl as a COOH byproduct.

Aspirin Synthesis (Classic Example)

Aspirin (acetylsalicylic acid) is made by treating salicylic acid with acetic anhydride:

Salicylic acid (HO-C₆H₄-COOH) + acetic anhydride (CH₃CO-O-COCH₃) → aspirin (CH₃CO-O-C₆H₄-COOH) + acetic acid.

The aromatic -OH of salicylic acid attacks one of the two carbonyls of acetic anhydride, installing an acetyl ester group on the phenolic oxygen. The other acyl group leaves as acetic acid. Aspirin is an ester at the phenolic position; the carboxylic acid remains intact.

Cyclic Anhydrides in Biology

Succinic anhydride and related cyclic anhydrides appear in specialized biochemistry contexts but are less common than esters or amides. Maleic anhydride (cyclic diene-dione-like structure) is a reactive Diels-Alder dienophile in industrial chemistry.

Explain why using acetic anhydride to acetylate an amine is wasteful compared to using acetyl chloride.

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Acetyl chloride installs the acetyl group with only HCl as a byproduct - every molecule of reagent becomes useful product. Acetic anhydride has two acetyl groups, but only ONE becomes the amide; the other leaves as acetate (or acetic acid), effectively wasted. Despite this, acetic anhydride is often used in practice because it is safer to handle (less corrosive and volatile than acetyl chloride) and cheap enough that wasting an acetyl group is acceptable.