Anhydride

A compound made up of two acid molecules under water removal

Anhydride from the Greek "anhidros" = without water, is the denomination for the compounds that have been formed with elimination of water. It is usually applied to two classes of substances:

• in inorganic chemistry to salts without crystallization water that are usually known hydratedly. An inorganic anhydride is a metaloid oxide that can be combined with water to form an inorganic acid.

Example: Mn2O7+H2O=2HMnO4{displaystyle {ce {Mn2O7 + H2O= 2HMnO4}}}

• in organic chemistry the anhydrore are composed of two molecules of acid underwater elimination.

Acid oxides

A acid oxide is a binary chemical compound that results from the combination of a non-metal element with oxygen, and in contact with water, usually gives an inorganic acid. They have the general formula A2Ox{displaystyle {ce {A2Ox}}} (if non-metal valence (in this case x) is odd) and AOx{displaystyle {ce {AOx}}} (if the metal valence (in this case, x would be half the valence) is par).

The most characteristic examples are SO2{displaystyle {ce {SO2}}}, which generates in contact with oxygen and atmospheric humidity H2SO4{displaystyle {ce {H2SO4}}}nitrogen oxides (NOx{displaystyle {ce {NOx}}}) produce HNO3{displaystyle {ce {HNO3}}, the CO2{displaystyle {ce {CO2}}}da H2CO3{displaystyle {ce {H2CO3}}}, etc, which are the main polluting gases responsible for the acid rain due to the transformation of the anhydrous form in the acid form (when they react with the atmospheric water vapor).

Acid anhydrides

Carboxylic acid anhydrates are composed of the generic formula R− − C(=O)− − O− − C(=O)− − R♫{displaystyle {ce {R-C(=O)-O-C(=O)-R'}}}}} with R{displaystyle {ce {R}}} and R♫{displaystyle {ce {R}}} = organic remains. In the presence of water, two carboxylic acid molecules are usually hydrolyzed:

R− − C(=O)− − O− − C(=O)− − R♫+H2O=R− − CO2H+R♫− − CO2H{displaystyle {ce {R-C(=O)-O-C(=O)-R' + H2O = R-CO2H + R'-CO2H}}}}

They are less miscible with water than the corresponding acids since they are not donors of hydrogen bonds.

They are also more easily attacked by nucleophilic agents. For this reason they are often used as activated derivatives of acids, eg in esterification. This is how acetylsalicylic acid is obtained by the action of acetic acid anhydride on salicylic acid.

Summary

Symmetrical anhydrides are conveniently generated from acids in the presence of a strong dissicant such as phosphorus pentoxide.

Asymmetric anhydrides (with R different from R') can be obtained by reacting the salt of a carboxylic acid with the chloride of the other corresponding acid.

Biochemistry

In cells, anhydrides are also found as activated derivatives of carboxylic acid. Usually they are non-symmetric derivatives.

The triphosphate adenosin (ATP{displaystyle {ce {ATP}}}) is also an anhydride - in this case derived from phosphoric acid instead of carboxylic acids. The energy released in its hydrolysis is the basis of most biochemical processes.