Anhydride is a
compound formed by the abstraction of a molecule of water,
H2O, from a substance.
The term acid anhydride is restricted sometime to the anhydride formed
especially from an acid by dehydration or one that revert to the original
substance upon hydration. In case of bimolecular, it can be composed of two
molecules of the corresponding acid. The term mixed anhydride is an acid
anhydride composed of two different acids. Examples are adenosine triphosphate
or an aminoacyl adenylate. The anhydrides of bases are oxides.
inorganic acids are usually oxides of nonmetallic elements. Carbon dioxide
(CO2) is the anhydride of
carbonic acid, dinitrogen pentoxide (N2O5) is the anhydride of
nitric acid, sodium oxide is an anhydride of sodium hydroxide,
phosphorus pentoxide (P2O5) is the anhydride of
phosphoric acid, and sulfur trioxide (SO3) is the anhydride of
sulfuric acid. An acid anhydride forms
an acid; a base anhydride forms a base.
Sulfur trioxide (acid anhydride) reacts with water to form sulfuric acid (an
acid product). Calcium oxide
(an base anhydride) reacts with water to form calcium hydroxide (a base product).
Organic anhydrides contain the carbonyl group (CO).
Organic anhydrides are formed by the condensation of original acids. Lactone, an
internal cyclic monoester, is an anhydride derived from the hydroxyl and
carboxyl radicals. In organic chemistry, most
anhydride compounds are
derived from corresponding carboxylic acids. Carboxylic anhydrides, general formula (RCO)2O, are
the dehydration product of two carboxylic acid molecules. The name of carboxylic
given first from the original acid, followed by the separate word "anhydride".
[CH3(CH2)2CO]2O is butanoic
anhydride, CH3COOCOCH2CH3 is
ethanoic propanoic anhydride (or
acetic propionic anhydride). Anhydrides are more reactive than the parent acids. Anhydrides are typically not
target molecules, but rather they are used as intermediates for the synthesis of
other organic members such as esters and amides for the industrial applications
include dyes, pharmaceuticals, pesticides, plastics, fibers, curing agents,
plasticizers and many others. The reactivity
of carboxylic acid derivatives are in order of acyl halides > anhydrides >> esters ¡Ö acids >> amides.
Anhydrides react with alcohols to form esters; acetic anhydride [(CH3CO)2O] reacts with
to form ethyl acetate (CH3COOC2H5)
used as a common solvent. Anhydrides also react with ammonia and
primary or secondary amines to form amides. Anhydrides react with water to form their corresponding acids.