The formation of mono- and di-ketones in the Friedel–Crafts acetylation of anthracene

Abstract

Aluminium chloride catalysed acetylation of anthracene leads to complex mixtures of mono- and di-acetyl-anthracenes. 9-Acetylanthracene is formed in appreciable yields in benzene or ethylene chloride as solvents only under mild conditions. At higher temperatures acetylations in these solvents or in chloroform afford 1- and 2-acetylanthracene and diacetylanthracenes in increasing amounts, at the expense of the 9-isomer. The ratios of the 1- to the 2-isomer formed are characteristic of the solvent; 3·7 for benzene, 4·4 for chloroform, and 7 for ethylene chloride. The diketones formed comprise, in the sequence of the yields generally obtained, 1,5-, 1,8-, and 1,6-diacetylanthracene. The 1,6-isomer is formed by further acetylation of both 1- and 2-acetylanthracene; its structure was proved by conversion into the known anthraquinone-1,6-dicarboxylic acid. The light-absorption of these ketones and of their 2,4-dinitrophenylhydrazones has been examined. The data provide evidence of steric hindrance in 9- and 1-acetylanthracene.