Chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, compounds made of atoms, molecules and ions ...

, a kryptoracemic compound or kryptoracemate (sometimes false conglomerate) is a racemic compound crystallizing in a Sohncke space group. In most of the cases, racemic compounds crystallize in

centrosymmetric In crystallography, a centrosymmetric point group contains an inversion center as one of its symmetry elements. In such a point group, for every point (x, y, z) in the unit cell there is an indistinguishable point (-x, -y, -z). Such point groups ...

crystal structures. In a kryptoracemic compound the chemical composition of the crystal is

racemic In chemistry, a racemic mixture, or racemate (), is one that has equal amounts of left- and right-handed enantiomers of a chiral molecule or salt. Racemic mixtures are rare in nature, but many compounds are produced industrially as racemates. ...

although the crystal belongs to space groups in which all ''enantiomerically pure'' molecules have to crystallize. Crystallographically, in kryptoracemic compounds, the number of independent molecules in the asymmetric unit (Z′) is necessarily greater than 1 and should take an even value (to respect the racemic composition). By extension, the scalemic compounds (or unbalanced compounds), ''i.e.'' crystal with non''-''

stoichiometric Stoichiometry refers to the relationship between the quantities of reactants and products before, during, and following chemical reactions. Stoichiometry is founded on the law of conservation of mass where the total mass of the reactants equal ...

ratio of enantiomer, crystallizing in Sohncke space group are sometimes included in kryptoracemic compounds although they are not strito-sensu kryptoracemic.

Etymology

The term (kryptoracemate) was coined by Ivan Bernal who employed this term during a meeting of the American Crystallographic Association in 1995. The name is made of '' (from

Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Dark Ages (), the Archaic peri ...

: κρυπτός,

romanized Romanization or romanisation, in linguistics, is the conversion of text from a different writing system to the Roman (Latin) script, or a system for doing so. Methods of romanization include transliteration, for representing written text, and ...

: "the hidden one")'' and ''

.'' It comes from the fact that the racemic composition is "hidden" in a Sohncke space group (usually enantiomerically pure).

Frequency of kryptoracemic compounds in organic crystals

There is no space group restriction for the crystallization of racemic compound crystallizing either in centrosymmetric or in non-centrosymmetric space group (SG). The frequency of organic racemic compounds in the

Cambridge Structural Database The Cambridge Structural Database (CSD) is both a repository and a validated and curated resource for the three-dimensional structural data of molecules generally containing at least carbon and hydrogen, comprising a wide range of organic compound ...

is summarized in the following table: Kryptoracemic compounds are thus very rare and represent circa 1% of the racemic compounds. The frequency of kryptoracemic compounds in the whole organic

was estimated to circa 0.4% to 0.8%. A review covering organometallic compounds with a stereogenic metal atom sorted a list of 26 possible kryptoracemic compounds.

References

Stereochemistry {{chemistry-stub