[an-hahy-drahyd, -drid]
anhydride [Gr.,=without water], chemical compound formed by removing water, H2O, from another compound; the anhydride can also react with water to form the original compound. An acid anhydride reacts with water to form an acid; e.g., sulfur trioxide, SO3, reacts with water to form sulfuric acid, H2SO4. A basic anhydride reacts with water to form a base; e.g., calcium oxide, CaO, reacts with water to form calcium hydroxide, Ca(OH)2. Anhydrides of organic acids have many uses. They react with alcohols to form esters; e.g., acetic anhydride, (CH3CO)2O, reacts with ethanol, C2H5OH, to form ethyl acetate, CH3COOC2H5, a useful solvent. They also react with ammonia and primary or secondary amines to form amides. Other important acid anhydrides include maleic anhydride and phthalic anhydride.
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.

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