Acetic anhydride is so commonly used that it is rarely, if ever, produced in the lab.

Industrially, acetic anhydride is produced on a large scale by heating of acetic acid to high temperatures (800 to 1000 degrees Celsius). The water produced can be removed in its vapor form, pushing the reaction to completion.

Acetic anhydride, as stated above, is derived from acetic acid, which gives vinegar its potent odor. Anhydrides, including acetic, are often readily hydrolyzed¹ to the starting acids. For this reason, acetic anhydride has approximately the same smell as vinegar.

It is easy and inexpensive to purchase acetic anhydride commercially. It is available from all major chemical companies and has the general appearance of water (see Figure 3). As the chemical bottle states, it is somewhat combustible and, due to its reactivity (see below), is rather corrosive.

Acylating agents, which include acid anhydrides and acid chlorides (where chlorine replaces the -OH group in a carboxylic acid), are chemicals that can easily transfer an acyl group (a molecular fragment that forms a bond from a carbonyl³ group to the larger molecule) to other molecules. Acetic anhydride is used in this capacity to acetylate (i.e. acylate with a CH₃CO group) molecules. A significant use of acetic anhydride is in the production of aspirin (chemically, acetylsalicyclic acid⁴ or, more properly, o-acetoxybenzoic acid), in which the hydroxyl (-OH) group of salicyclic acid is acetylated (hence the chemical name). This reaction is summarized in Figure 4.

Another use of acetic anhydride's acylation capability is in protection of functional groups in organic synthesis. Protection refers to the selective inactivation of certain chemical groups that would otherwise cause unwanted side-reactions during a multi-step synthesis. In many cases protection is only temporary and the protecting group will be removed after the potentially problematic reaction step is completed. In other cases the protecting group remains in the final product. An example of the latter can be found in Figure 5, which shows a hypothetical synthesis tangentially related to an actual synthesis once performed by the author. See its caption for discussion and details. Note that this discussion is rather technical and assumes you have a basic knowledge of organic chemistry.

One final important use of acetic anhydride is in the preparation of other anhydrides. Treating more complex acids (for which anhydrides are not commercially available) with acetic anhydride can produce the anhydride of the other acid (with acetic acid produced as a by-product which can often be distilled out). This property is frequently used by organic chemists who often find themselves needing complex anhydrides for use in more elaborate acylation reactions.