| abstract |
A process for preparing substituted phthalic anhydrides, example 4-methylphthalic, in which the Diels-Alder addition product of a conjugated diene, example isoprene, and maleic anhydride is reacted with sulfur in the presence of a catalyst. The catalyst is zinc oxide and 2-mercaptobenzothiazole. This reaction is carried out at elevated temperatures. A resulting thio derivative by-product can further be reacted with water at elevated temperatures in the presence of catalytic amounts of a base to form 4-methylphthalic anhydride. This base can be sodium hydroxide or N,N,N', N'-tetramethylguanidine. o-Dichlorobenzene can be added to prevent solids distillation during the process. |