Homochiral, definition

Each of the terms expresses a relationship between objects, not a property of an object. By leaving out the term similar, the original definition has recently been shortened, and its scope narrowed homochiral objects are only (properly) congruent, whereas heterochiral (cnan-tiomorphous) objects are only improperly congruent -4. This shortened definition has invited misuse (sec Section 1.2.2.2.) by considering the terms as properties which would not have been possible with the addendum similarity. 

Unfortunately, perhaps because of a misinterpretation of a clear definition (see Section 1.1.3.4.), the term homochiral, describing a relationship and not a property between molecules (see Section 1.1.3.4.), has been frequently used since 1985 in the sense of the above enantiopure. As was aptly remarked20, this is inappropriate just as one does not call a notepad identical just because it is made up of identical sheets of paper . 

It is difficult to offer definitive interpretations for these trends observed in the reassembly-crystallization experiments. Entropy and enthalpy play significant roles in the pairing of homochiral and heterochiral assemblies [56]. It is possible that the slightly more elongated structure of 28 30 is entropically more favored in the crystallization process. Clearly, many more examples of supraminol assembly are needed to appreciate fully the subtle forces involved in their stabilities and crystallization properties. 

The definitions of chiral and the associated terms homochiral and heterochiral made their first appearance in a footnote of a lecture, entitled The Molecular Tactics of a Crystal, that Sir William Thomson, who had been elevated to Lord Kelvin in 1892, delivered to the Oxford University Junior Scientific Club on May 16, 1893. The famous footnote reads ... 

The term homochiral was introduced by Kelvin in the 1904 publication of his Baltimore Lecture of 1884 and represented a relationship between molecules (see Mislow10). Molecules are homochiral if they possess the same sense of chirality. For example, the right hands of a group of people are homochiral (or alike). More recently, and unfortunately, homochiral has been used in the sense of enantiomerically pure, i.e. one reads of a homochiral compound , which clearly violates the original definition. Since some journals permit the latter usage, readers should be aware of the potential for confusion. 

The final relative concentrations of the three catalysts are termed x, y, and z. We also used as an intermediate parameter, the relative amounts of heterochiral versus homochiral catalysts (P=z/(x-i-y)). The relative rate constant g and the equilibrium constant K (see Scheme 3 for definitions) are the key parameters for the calculation. An easy kinetic treatment first gave Eq. (2) relating eep j to the g and P values. 

The optical purity is usually, but not always, equal to enantiomer excess. In order for the two to be equal, it is necessary that there be no aggregation. It is possible, for example, that a homochiral or heterochiral dimer (see Glossary, Section 1.6, for definitions) would refract the circularly polarized light differently than the monomer (or each other). In 1968 [19] Krow and Hill showed that the specific rotation of (S)-2-ethyl-2-methylsuccinic acid (85% ee) varies markedly with concentration, and even changes from levorotatory to dextrorotatory upon dilution. In 1969 [20], Horeau followed up on Krow and Hill s observation, and showed that the optical purity (at constant concentration) and enantiomer excess of (5)-2-ethyl-2-methylsuccinic acid were unequal except when enantiomerically pure or completely racemic. This deviation from linearity is known as the Horeau effect, and its possible occurence should be remembered when determining enantiomeric purity by polarimetry. 

Homochiral has been used by some as a synonym for "enantiomerically pure". This is another usage of a term that should be discouraged, as homochiral already had a clear and useful definition, and using the same term to signify two completely different concepts can only lead to confusion. A better term for designating an enantiomerically pure sample is simply enantiopure. 

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