Anhydride
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In chemistry, an anhydride is a compound that can be considered as derived from another compound by subtracting the molecules of water.

For example:

2 NaOH - H₂O → Na₂O

H₂SO₄ - H₂O → SO₃

Thus sodium oxide is an anhydride of sodium hydroxide, and sulfur trioxide is an anhydride of sulfuric acid.

In organic chemistry, the compounds most commonly involved are carboxylic acids.

2 CH₃COOH - H₂O → (CH₃CO)₂O

Compounds involved are often acids or bases; in such cases, the anhydrides can be called acid anhydrides or base anhydrides. This does not imply that the anhydrides are not themselves acids or bases; they are. But the more hydrated forms may be more familiar or convenient.

Anhydride is sometimes used as a synonym for acid anhydride.

Following are two reactions, one with a typical acid and base, one with their anhydrides. The first one is more practical.

2 NaOH (aq) + H₂SO₄ (aq) → Na₂SO₄ + 2H₂O (l)

Na₂O (s) + SO₃ (g) → Na₂SO₄ (aq)

In general, anhydrides are more reactive than their corresponding acids, as they can react with water to form their corresponding acid. They often are good dehydrating agents. Acetic anhydride is useful in the acetylation of salicylic acid, as using acetic acid to perform the reaction leaves water behind, which can destroy the product, acetylsalicylic acid, or aspirin.

In biology, most of the high-energy phosphate compounds are formed from the condensation of the phosphate ion with a phosphorylated sugar. The resulting pyrophosphate bond is a classic anhydride bond.

See also

• Acid anhydride