Chapter 19
Kryptoracemates
Edward R. T. Tiekink
Abstract 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.
Keywords Kryptoracemate · Molecular packing · Crystal structure ·
Crystallisation · Chirality
E. R. T. Tiekink (B)
School of Science and Technology, Research Centre for Crystalline Materials, Sunway University,
5 Jalan Universiti, Bandar Sunway, Selangor Darul Ehsan 47500, Malaysia
e-mail: edwardt@sunway.edu.my
© Springer Nature Singapore Pte Ltd. 2020
M. Sakamoto and H. Uekusa (eds.), Advances in Organic Crystal Chemistry,
https://doi.org/10.1007/978-981-15-5085-0_19
381
Kryptoracemates
Edward R. T. Tiekink
Abstract 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.
Keywords Kryptoracemate · Molecular packing · Crystal structure ·
Crystallisation · Chirality
E. R. T. Tiekink (B)
School of Science and Technology, Research Centre for Crystalline Materials, Sunway University,
5 Jalan Universiti, Bandar Sunway, Selangor Darul Ehsan 47500, Malaysia
e-mail: edwardt@sunway.edu.my
© Springer Nature Singapore Pte Ltd. 2020
M. Sakamoto and H. Uekusa (eds.), Advances in Organic Crystal Chemistry,
https://doi.org/10.1007/978-981-15-5085-0_19
381