The chloro-methyl exchange rule and its violations in the packing of organic molecular solids

Abstract

Since chloro and methyl groups have nearly the same volume, they may be interchanged in a molecule without altering the crystal structure. Such isostructural behaviour is found when crystal stabilization is mainly through dispersive and repulsive interactions. Violations of this rule are, however, observed when directional forces or weak bonds are involved and the failure of complete chloro-methyl exchange in a substance like hexachlorobenzene, points to the importance of weakly attractive Cl…Cl interactions.