Formation of crystalline complexes between polymethylated quinones and hydroquinones

The effect of increased substitution by methyl groups on the formation of solid-state complexes of quinones with hydroquinones has been investigated. Both the ease and the stoicheiometry of complex formation of di-, tri-, and tetra-methylated quinones with hydroquinones, either by ‘solid–solid’ reaction or by recrystallization from a solvent, has been found to depend in part on the number of methyl substituents on each component. When there is methyl substitution on the hydroquinone moiety, a complex with components in 1 : 1 ratio is generally formed, while with methyl groups on the quinone molecule the ratio of quinone to hydroquinone in the complex is generally 1 : 2.Furthermore the ease of complex formation can be highly structure-sensitive. Thus, although 1,4-benzoquinone reacts readily with 2,3,5,6-tetramethylhydroquinone to give a 2 : 1 complex, 2,3,5,6-tetramethylbenzoquinone and hydroquinone do not react when the two solids are ground together, crystallized from a solvent by the conventional procedure, or kept for long periods in contact with each other. Reaction of the two solids ground together can, however, be induced by heating the mixture at 80 °C; this is a convenient preparative method for the 1 : 1 product. Furthermore, seeding the solid mixture of the quinone and hydroquinone by addition of a small amount of the 1 : 1 product induces solid–solid reaction. Finally, a 1 : 2 complex of duroquinone with hydroquinone is formed when an ethyl acetate solution saturated with and containing an excess of each component suspended in the mixture is seeded with crystals of either the 1 : 1 or 1 : 2 complex. Similarly 2,5-dimethyl-1,4-benzoquinone can be induced to form both a 1 : 1 and a 2 : 1 complex with 2,5-dimethylhydroquinone.