Compounds consisting of two acyl groups bonded to the same oxygen atom acyl-O-acyl. Symmetric and mixed anhydrides have identical and different acyl groups, respectively. E.g.
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acetic anhydride,
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acetic benzenesulfonic anhydride,
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(thiobenzoic) anhydride.
Source:
PAC, 1995, 67, 1307. 'Glossary of class names of organic compounds and reactivity intermediates based on structure (IUPAC Recommendations 1995)' on page 1311 (https://doi.org/10.1351/pac199567081307)