Acid anhydrides

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General structure of carboxylic acid anhydrides. The radicals R and R 'are usually the same in carboxylic acid anhydrides. In the case of R ≠ R 'it is a question of mixed carboxylic acid anhydrides.

Acid anhydrides (mostly just called anhydrides ) is the name for a group of chemical compounds that are always derivatives of acids containing oxygen . Formally, they arise from the splitting off of water (dehydration) from an acid. Inorganic oxygen acids form oxides such as B. from phosphoric acid phosphorus pentoxide . So-called carboxylic acid anhydrides are formed from the organic carboxylic acids .


In the case of organic acids ( carboxylic acids ), two acid groups are linked via an oxygen bridge in the molecule . The two acid groups can either come from one molecule ( intramolecular ) as in phthalic anhydride or belong to two different molecules ( intermolecular ) as in acetic anhydride . In the case of organic acids, the acid anhydride is named by adding the ending -anhydride to the name of the acid.

In the case of inorganic acids, the substances usually have their own names. Phosphorus pentoxide is e.g. B. the acid anhydride of phosphoric acid . Mixed anhydrides are formed by splitting off water from two different acids.

oxidic inorg. Anhydrides
anhydride Hydrolysis product
Nitrous oxide nitric acid
Sulfur trioxide sulfuric acid
Diphosphorus pentoxide phosphoric acid
carbon dioxide carbonic acid
Carboxylic acid anhydrides
anhydride Hydrolysis product
Acetic anhydride - Acetic anhydride.svg
Acetic anhydride
acetic acid
Isobenzofuran-1,3-dione 200.svg
Phthalic anhydride
Phthalic acid
For more examples see category: Carboxylic acid anhydride


Acid anhydrides can be produced in a number of ways:

Properties and use

Acid anhydrides react exothermically with water to form the corresponding acid (s). Acid anhydrides can be used for halogen-free Friedel-Crafts acylation or for drying the corresponding acid.

Other mixed anhydrides

Mixed anhydrides result from the condensation of two different acids. Formally, carboxylic acid halides are anhydrides of a carboxylic acid with a hydrohalic acid z. B. Acetyl Chloride :

Carboxylic acid halides or other anhydrides formed formally by the reaction of an acid with a carboxylic acid, however, often form their own substance classes and are not referred to as anhydrides.