Acetic Anhydride
Properties
| State | Liquid at room temperature |
| Color | Colorless |
| Solubility | Reacts slowly with water; miscible with common organic solvents |
| Melting Point | -73 °C |
| Boiling Point | 140 °C |
About Acetic Anhydride
Acetic anhydride is the default acetylating reagent in synthetic organic chemistry, and the practical reasons are mundane: it acetylates alcohols and amines cleanly, the only byproduct is acetic acid (easy to wash away), it's cheaper per mole of acetyl delivered than acetyl chloride, and it doesn't fume aggressively in moist air. Structurally it's two acetyl groups bridged by a single oxygen, formed conceptually by dehydrating two molecules of acetic acid. The classic teaching example is the aspirin synthesis: salicylic acid plus acetic anhydride gives aspirin and a stoichiometric equivalent of acetic acid, with a drop of phosphoric or sulfuric acid as catalyst. That reaction runs at the bench in undergraduate labs everywhere, partly because it's robust and partly because you can crystallize and titrate the product within a single afternoon. Industrially, the largest demand is cellulose acetate — about half of all acetic anhydride produced ends up there, feeding into cigarette-filter tow, photographic film, and certain textile fibers. Production runs around 2.5 million tonnes annually, with most of it made via the ketene route (thermal cracking of acetic acid to ketene, then trapping with another acetic acid molecule). It is a controlled chemical precursor under international drug-control conventions because it acetylates morphine to heroin as cleanly as it acetylates anything else.
Where you'll encounter it
If you've taken organic chemistry, you've almost certainly used acetic anhydride at the bench — usually in the aspirin lab, sometimes for protecting an amine as an acetamide before doing chemistry elsewhere on the molecule. It also shows up in the Beckmann rearrangement of oximes, in the preparation of mixed anhydrides for peptide coupling, and as the standard acetylating agent for sugars where chloride byproducts would interfere. Outside of synthesis, the cellulose acetate it produces is what cigarette filters and old-style camera film were made from — and the fact that those filters are still ubiquitous is part of why production volumes haven't fallen even as photographic film has.
Common Uses
- Standard reagent for acetylating alcohols and amines
- Aspirin synthesis from salicylic acid
- Cellulose acetate manufacturing for filters and films
- Acetyl-protection step in carbohydrate and peptide chemistry
- Beckmann rearrangement and mixed-anhydride coupling
Safety Information
Corrosive and lachrymatory — exposure to vapor causes immediate eye irritation and tearing, and skin contact produces severe burns. Reacts with water, releasing heat and acetic acid; storage requires an inert dry atmosphere. Flammable, with a flash point around 49 °C. Listed as a Table I precursor under the UN Convention Against Illicit Traffic, so purchase and handling are tracked in most jurisdictions.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.