Acid Anhydrides

Acid anhydrides, a group of substances containing oxygen that form acids when combined with water. Inorganic acid anhydrides are formed by burning certain elements in an oxygen atmosphere. For example, carbon or sulfur burn to form CO₂ or SO₂, the anhydrides of carbonic or sulfuric acid, which are unstable in their free state and are known only in the form of salts. Other anhydrides, conversely, are hygroscopic, binding water into very strong acids, for example, the anhydride of sulfuric acid SO₃, the anhydride of phosphoric acid P₂O₅, etc. Anhydrides of organic monobasic acids are obtained by removing a water particle from two acid molecules: 2 CH₃COOH = (CH₃CO)₂O + H₂O. Anhydrides of polybasic acids are formed as a result of intramolecular water elimination and are compounds of cyclic structure: (CH₂COO)_n

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Anhydrides of phthalic acid. Anhydrides of organic acids are very often obtained by the action of the acid's acid chloride on the sodium salt of the same or another acid: CH₃COCl + NaOCOCH₃ = (CH₃CO)₂O + NaCl. With alcohols, acid anhydrides form complex esters-esters (see), with ammonia-amides (see) of acids, with amines-substituted amides of acids. In organic synthesis, anhydrides of organic acids are used to introduce acid residues into various compounds.