Kryptoracemates

Racemic crystals normally crystallise in centrosymmetric space-groups containing equal numbers of enantiomers. More rarely, racemates may crystallise in non-centrosymmetric space-groups having glide symmetry or, even more rarely, in space-groups devoid of a centre of inversion, having no rotary-inversion axes nor glide plane. The latter class of crystals forms the subject of the present bibliographic review—a survey of kryptoracemic behaviour. The term kryptoracemic alludes to the presence of a hidden or non-crystallographic centre of inversion between two molecules that might otherwise be expected to crystallise in an achiral space-group, often about a centre of inversion. Herein, examples of molecules with stereogenic centres crystallising in one of the 65 Sohncke space-groups are described. Genuine kryptoracemates, i.e. crystals comprising only enantiomorphous pairs, are described followed by an overview of non-genuine kryptoracemates, whereby the crystal also contains other species such as solvent and/or counter-ions. A full-range, i.e. one to six, stereogenic centres are noted in genuine kryptoracemates. Examples will also be described, whereby there are more than one enantiomeric pair of molecules in the crystallographic asymmetric unit. A more diverse range of examples are available for non-genuine kryptoracemates. There are unbalanced species where in addition to the enantiomeric pair of molecules, there is another enantiomeric molecule present. There are examples of co-crystals, solvated species and salts. Finally, special examples will be highlighted where the counter-ions are chiral and where they are disparate, both circumstances promoting kryptoracemic behaviour.