Acid anhydrides (sometimes simply called "anhydrides", but this is less specific) are compounds formed by dehydration of two acidic groups. They form the basis for the synthesis of imides or can be used to create esters (with alcohols), amides (with amines) or alcohols (via reduction). Acid anhydrides hydrolize and can therefore also be used to generate acids when needed. Note that due to their reactivity with water they should always be stored water-free. Acid anhydrides are more reactive than esters, but less reactive than acyl chlorides.
Synthesis of Acid Anhydrides
Acid anhydrides can be formed by removing one molecule of water from two acidic groups. This is done by using dehydrating compounds (such as phosphorous pentoxide or even another anhydride), but in some cases it can also be done through autocondensation by heating certain compounds. The formation of the anhydride of methanoic acid is complicated and is outside the scope of this course.
Using dicarboxylic acids, anhydrides can be created simply by heating them. See for example the reaction below of glutaric acid (pentanedioic acid) to glutaric anhydride (oxane-2,6-dione).
Synthesis of Acid Anhydrides from Acyl Chlorides
The synthesis of acid anhydrides from acids always creates symmetrical anhydrides. An asymmetrical anhydride can be formed using acyl chlorides. Asymmetrical anhydrides are not often used since they give differing reaction products, which means that the unwanted product has to be removed from the reaction mixture afterwards (which is inconvenient).
Reaction of Acid Anhydrides with Amines
Acid anhydrides react with amines (or ammonia) to create amides. See for example the reaction below of acetic anhydride and methanamine.
Note that upon formation of the amide during this reaction, an acid is formed at the same time which will immediately react in an acid/base-reaction with another amine molecule. The reaction therefore needs twice as much amine to allow all the anhydride to react.
During the reaction of an anhydride of a dicarboxylic acid with an amine, an amide is also formed, but the acidic group that is formed at the same time (in the same molecule) can usually react much faster with this amide than with other amines in the reaction mixture if the reaction conditions are suitable (i.e. heating). In these cases an imide will be created. See for example the reaction below of succinic anhydride (oxolane-2,5-dione) with ammonia.
Reaction of Acid Anhydrides with Alcohols
Acid Anhydrides react with alcohols to form esters and carboxylic acids. See for example the reaction below of acetic anhydride and phenol.
Hydrolysis of Acid Anhydrides
The hydrolysis (reaction with water) of acid anhydrides produces its original acids. See for example the reaction below of benzoic anhydride and water.
Practice