Any chemical compound obtained, either in practice or in principle, by eliminating water (HO) from another compound. Examples of inorganic anhydrides are sulfur trioxide, SO, which is derived from sulfuric acid, HSO, and calcium oxide, CaO, which is derived from calcium hydroxide Ca(OH). The most important organic anhydride is acetic anhydride, (CHCO)O, a raw material for making cellulose acetate (used for films, fibres, and plastic goods) and aspirin. It can be thought of as acetic acid minus water. Organic anhydrides are very important starting materials for organic synthesis, as they can give rise to carboxylic acids, esters, or amides under the proper conditions.

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