Solid-state reactivity of crystalline hydroquinones with quinone vapour; crystal structures of 2,5-dimethylhydroquinone and 2,3,5,6-tetramethylhydroquinone

Use of single crystals of 2,5-dimethylhydroquinone and 2,3,5,6-tetramethylhydroquinone in studies of their reactions with 1,4-benzoquinone vapour has been found to give information not obtainable from previous studies of reactions of powders. Structures of the two crystalline hydroquinones were determined by X-ray analysis. Crystals of the dimethylhydroquinone are orthorhombic, a= 13.256(2), b= 4.5609(8), c= 11.977(2)Å, Z= 4, space group Pca21; and the structure has been refined to an R factor of 0.0335 on 704 non-zero reflections. Molecules have an anti-arrangement of hydroxy groups and form hydrogen-bonded chains with a motif like that of γ-hydroquinone. Slight differences in the geometry of the hydrogen bonding at the two ends of the molecule are responsible for the polar nature of the structure. Results of both the test for second harmonic generation and a pyroelectric test for non-centrosymmetry are positive with these crystals. The pyroelectric test applied to crystals in which twinning is known to occur shows clear evidence of such twinning and might be useful in certain other cases as a test for twinning.Crystals of the tetramethylhydroquinone are monoclinic, a= 8.317(3), b= 4.728(2), c= 13.514(5)Å, β= 125.85(2)°, Z= 2, space group P21/c; the structure has been refined to an R factor of 0.0367 on 813 non-zero reflections. Microscopic studies show evidence that the reaction is initiated at nucleation sites on reactive faces and that both quinone and hydroquinone molecules migrate during reaction. The faster reaction occurs at those faces where hydroxy groups emerge.It has been found that the shapes of the OH stretching regions of the Fourier transform i.r. spectra of these and other hydroquinones as well as the positions of the absorption maxima provide structural evidence useful for distinguishing among these compounds, and can be correlated with their crystal structures.