text: Organic acid anhydrides are prepared in industry by diverse means. Acetic anhydride is mainly produced by the carbonylation of methyl acetate. Maleic anhydride is produced by the oxidation of benzene or butane. Laboratory routes emphasize the dehydration of the corresponding acids. The conditions vary from acid to acid, but phosphorus pentoxide is a common dehydrating agent:
  :2 CHCOOH + PO → CHC(O)OC(O)CH + "PO(OH)"
  Acid chlorides are also effective precursors:
  :CHC(O)Cl + HCONa → HCOCOCH + NaCl
  Mixed anhydrides containing the acetyl group are prepared from ketene:
  :RCOH + HC=C=O → RCOC(O)CH