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.

Compounds involved are often acids or bases; in such cases, the anhydrides can be called acid anhydrides or base anhydrides. This doesn't imply that the anhydrides aren't 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

Na₂O (s) + SO₃ (l) → Na₂SO₄

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

2 CH₃COOH → (CH₃CO)₂O + H₂O

Anhydrides are typically 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 do the reaction leaves water behind that 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.