Acidic oxide in the context of Organic acid anhydride

Dichlorine hexoxide is the chemical compound with the molecular formula Cl₂O₆ or O₂Cl−O−ClO₃, which is correct for its gaseous state. However, in liquid or solid form, this chlorine oxide ionizes into the dark red ionic compound chloryl perchlorate or dioxochloronium(V) perchlorate [ClO₂][ClO₄], which may be thought of as the mixed anhydride of chloric and perchloric acids. This compound is a notable perchlorating agent.

A base anhydride is an oxide of a chemical element from group 1 or 2 (the alkali metals and alkaline earth metals, respectively). They are obtained by removing water from the corresponding hydroxide base. If water is added to a base anhydride, a corresponding hydroxide salt can be [re]-formed.

Base anhydrides are Brønsted–Lowry bases because they are proton acceptors. In addition, they are Lewis bases, because they will share an electron pair with some Lewis acids, most notably acidic oxides. They are potent alkalis and will produce alkali burns on skin, because their affinity for water (that is, their affinity for being slaked) makes them react with body water.

An organic acid anhydride is an acid anhydride that is also an organic compound. An acid anhydride is a compound that has two acyl groups bonded to the same oxygen atom. A common type of organic acid anhydride is a carboxylic anhydride, where the parent acid is a carboxylic acid, the formula of the anhydride being (RC(O))₂O. Symmetrical acid anhydrides of this type are named by replacing the word acid in the name of the parent carboxylic acid by the word anhydride. Thus, (CH₃CO)₂O is called acetic anhydride. Mixed (or unsymmetrical) acid anhydrides, such as acetic formic anhydride (see below), are known, whereby reaction occurs between two different carboxylic acids. Nomenclature of unsymmetrical acid anhydrides list the names of both of the reacted carboxylic acids before the word "anhydride" (for example, the dehydration reaction between benzoic acid and propanoic acid would yield "benzoic propanoic anhydride").

One or both acyl groups of an acid anhydride may also be derived from another type of organic acid, such as sulfonic acid or a phosphonic acid. One of the acyl groups of an acid anhydride can be derived from an inorganic acid such as phosphoric acid. The mixed anhydride 1,3-bisphosphoglyceric acid, an intermediate in the formation of ATP via glycolysis, is the mixed anhydride of 3-phosphoglyceric acid and phosphoric acid. Acidic oxides are also classified as acid anhydrides.