A method for preparing a phthalic anhydride, comprising: transimidation of an N-alkyl phthalimide (V) wherein R??? is selected from the group consisting of halogens, aromatic groups comprising 6-18 carbons, aliphatic groups comprising 1-18 carbons, hydrogen and nitro groups, and R is an alkyl group having from 1 to 18 carbons, with substituted tetrahydrophthalic anhydride (VI) wherein R??? is sele