On a possible substitution of p-xylene by toluene in p-xylene crystals. The crystal structure of p-xylene, C8H10, at 180 K

Mr=106.17, monoclinic, P21/n, a=5.806(2), b= 5.023 (1), c=11.215 (2)/~, fl=100.48(2) ° , V= 321.61 (14) A3, Z = 2, Dx = 1.096 gcm -3, A(Mo Ka)=0.71069/~,, /x =0.66cm -1, F(000) = 116, T= 180 K, R = 0.045 for 926 reflections [I > 2.5o-(I)]. The cell dimensions are confirmed by powder diffraction. No phase transition is observed between 110 and 273 K. The crystal data reported by Biswas [Indian J. Phys. (1960), 34, 263-271] are incorrect. The bond lengths and bond angles in p-xylene are equal to the corresponding values in toluene within experimental error. The aromatic ring is essentially planar and slightly deformed at the substituent side. The centre of symmetry in p-xylene coincides with the space-group centre. The p-xylene molecules are packed into chains along b by nearly equal van der Waals contacts between the six ring C atoms and one methyl H of a neighbouring molecule. The chains are held together by van der Waals forces. Because of the great resemblance of packing in p-xylene and a-toluene crystals, the possibility of the existence of solid solutions of toluene in p-xylene is proposed. This proposal is verified by measuring the p-xylenerich part of the phase diagram of the toluene/p-xylene system. Separation of pure p-xylene crystals from a mixture containing toluene seems unlikely.