View source for Organic acid anhydride


An '''organic acid anhydride''' is an organic compound that has two acyl groups bonded to the same [[oxygen]] atom. A common type of organic acid anhydride is a '''carboxylic anhydride''', where the parent acid is a [[carboxylic acid]], the formula of the anhydride being (RC(O))<sub>2</sub>O. Symmetrical acid anhydrides of this type are named by replacing the word ''acid'' in the name of the parent carboxylic acid by the word ''anhydride''. One or both acyl groups of an acid anhydride may also be derived from another type of organic acid, such as sulfonic acid or a phosphonic acid.

==Properties==
Acid anhydrides react with halogen acids, reaction which yields equal amounts of the acylated product and the carboxylic acid:

: RC(O)OC(O)R  +  HY  →  RC(O)Y  +  RCO<sub>2</sub>H

Acid anhydrides will react with water to form their respective carboxylic acid, though the reaction is slow.

==Preparation==

===Industrial===
[[Acetic anhydride]] is industrially produced by the carbonylation of [[methyl acetate]], catalyzed by [[hydrogen iodide]] and dicarbonyldiiodorhodiate (I).

: CH<sub>3</sub>CO<sub>2</sub>CH<sub>3</sub> + CO → (CH<sub>3</sub>CO)<sub>2</sub>O

[[Maleic anhydride]] is produced by the oxidation of [[benzene]] or [[butane]].

===Laboratory===

Laboratory routes rely on the dehydration of the corresponding acids or their salts. A variety of methods have been developed, because anhydrides are of interest in many reactions and are often expensive or unavailable for purchase.

====With phosphorus pentoxide====

[[Phosphorus pentoxide]] is a common [[dehydration reaction|dehydrating agent]]:

:2 CH<sub>3</sub>COOH + P<sub>4</sub>O<sub>10</sub> → CH<sub>3</sub>C(O)OC(O)CH<sub>3</sub> + P<sub>4</sub>O<sub>9</sub>(OH)<sub>2</sub>

However, tar is a common byproduct.

====With acetic anhydride====

Acetic anhydride will react with many carboxylic acids to convert them to their anhydrides. This is an equilibrium reaction which is driven to completion by the relatively high volatility of AcOH:

:2 RCOOH + Ac<sub>2</sub>O + ''heat'' → (RCO)<sub>2</sub>O + 2 AcOH (g)

Alternatively, acetic anhydride can be produced by the reaction of other anhydrides with the salts of acetic acid:

:2 AcONa + (RCO)<sub>2</sub>O >> 2 RCOONa + Ac<sub>2</sub>O

====From acyl chlorides====

[[Acyl chloride]]s will react with the salts of carboxylic acids to yield acid anhydrides:

:CH<sub>3</sub>C(O)Cl + CH<sub>3</sub>COONa → (CH<sub>3</sub>CO)<sub>2</sub>O + NaCl

For this reason, preparations of acyl chlorides are also of interest in the preparation of acid anhydrides. When acyl chlorides are prepared from the salts of carboxylic acids, using an excess of the salt often produces the anhydride.

====From ethenone====

Simple and mixed anhydrides containing the acetyl group can be prepared from [[ethenone]], also known as ketene:

: CH<sub>3</sub>COOH + H<sub>2</sub>C=C=O → (CH<sub>3</sub>CO)<sub>2</sub>O

: RCOOH + H<sub>2</sub>C=C=O → RCO<sub>2</sub>C(O)CH<sub>3</sub>

However, the application of this process is limited by the high toxicity of ethenone.

====Dehydration with in situ sulfur bromide====

Acetic anhydride is produced when sodium acetate is treated with [[sulfur]] dissolved in a large quantity of [[bromine]]:

: 4 AcONa + 1/8 S<sub>8</sub> + 2 Br<sub>2</sub> → 2 Ac<sub>2</sub>O + 4 NaBr + SO<sub>2</sub>

This method is popular due to its use of cheap reagents and avoidance of complex procedures.

====Thermal dehydration of pre-cyclic diacids====

Molecules containing two carboxyl groups separated by two or three carbon atoms can be thermally dehydrated to their anhydrides with the elimination of H2O, where the anhydride forms part of a five- or six-membered ring. Diacids having longer or shorter chains, such as malonic acid and adipic acid, give poor yields of anhydrides by this method.

: COOHCH<sub>2</sub>CH<sub>2</sub>COOH + heat → C<sub>2</sub>O<sub>3</sub>C<sub>2</sub>H<sub>4</sub> + H2O

====Diacid-catalyzed thermal dehydration====

High-boiling acids such as [[benzoic acid]] are dehydrated by heating at 230-260 C for a long time with catalytic amounts of a pre-cyclic diacid such as [[phthalic acid]]. This method relies on driving the thermal equilibrium between the acid and its anhydride to the right with the loss of water according to [[Le Chatelier's principle]].

: 2 BzOH + catalytic Ph(COOH)<sub>2</sub> + heat → Bz2O + H2O (g)

This method is described in [https://patents.google.com/patent/US3513180 US patent 3513180].

==References==
<references/>
===Relevant Sciencemadness threads===
*[http://www.sciencemadness.org/talk/viewthread.php?tid=9 Acetic anhydride preparation]
*[http://www.sciencemadness.org/talk/viewthread.php?tid=6300 acid anhydrides from sulfuric acid]

[[Category:Organic compounds]]
[[Category:Carboxylic acid anhydrides]]