4 CINTAS onsid ered withou ulny mas Only the magnitude of a ch clea Arguments based on enantiomeric evidence of chirality contents.and hence of chiral e res ACKNOWLEDGMENTS The issu of chirality,its history and significance has the py y pe talented colle to all of then in particular Mark tha al. flhksandcistobalWeina,he ction ctions in sym LITERATURE CITED rea ctions,where the exc 1.eEIInfelicusermc omenreChirality19 lem nuence by the 3.H matic terms in orga jority rule as rathe depends s GP.B Appl Chem 1996.68219 nistry.IUPAC recommend rding t general sym e of for an d s the but eithe the av s ymmety of the lt this ple and the se d are isom entropy and degree of symmetry.Thus,even though the ystem may be very 13.Cune P.Su CONCLUSIONS We have long misused the terms chiral and chirality to 161 and practitio 3171317 orchiral discrimination.which inundate the hemical literture suggest ignoring the more accurate and pre cise terms that charac 19. 20 ony of s hem rtainythe hall mark o 21 flue 8800 200417719-73. e pr vide 22 Chirality DOI 10.1002/chir obvious as chirality is by definition a geometrical property of molecules or objects, thereby requiring an algebraic treatment of models viewed as collections of points consid￾ered without any mass. Only the magnitude of a chirality function could be used as a quantitative measure of chiral￾ity.17,18 Arguments based on enantiomeric excesses as well as chiroptical and other spectroscopic measures as evidence of chirality contents, and hence of chiral change, amplification, or discrimination, may certainly be flawed. One should note the dichotomy between chirality per se and the physical observables that support the existence of that concept.19 Consider for instance the fact that chiral molecules may have null optical rotation (within conven￾tional detection); reductions in symmetry going from mol￾ecules to crystals; or processes, often highlighted as chi￾rality memory reactions, where the enantiomer excess of substrate and product does not match at all.20 Semantics may be particularly problematic with chiral polymers, in which a particular handedness is usually influence by the statistical domain of chiral or achiral units. However, a ma￾jority rule could lead to decreased control of the helical sense and therefore of optical activity. The conflict sug￾gests that chirality is not an intrinsic property of the chiral moiety but rather depends on the molecular environ￾ment.21 Chirality has also remarkable relationships with con￾cepts such as similarity and entropy, not usually handled in a chemical context. According to a more general sym￾metry evolution principle, for an isolated physical system the degree of symmetry cannot decrease as the system evolves, but either remains constant or increases; the degree of symmetry has to do with the initial and final symmetry groups, it does not refer to any symmetry of the states of the physical system as the latter evolves.15,22 As a result, this symmetry evolution principle and the second law of thermodynamics are isomorphic; paradoxically a monotonically ascending correlation exists between entropy and degree of symmetry. Thus, even though the chirality of a system implies the lack of certain symmetry elements, chirality and degree of symmetry of an evolving system may be very different things.18 CONCLUSIONS We have long misused the terms chiral and chirality to denote systems evolution and molecular dynamics, which deviate from the conceptual basis of chirality embedded in geometry. Some readers and practitioners will feel that, at least intuitively, expressions such as chiral reaction, chiral recognition, or chiral discrimination, which inundate the chemical literature, suggest any enantiomeric (perhaps stereoisomeric?) bias without major headaches. But, why ignoring the more accurate and precise terms that charac￾terize stereoselective and asymmetric transformations, as well as the nature of molecular interactions? Chirality is certainly the hallmark of stereochemistry, but it can also be too much of a good thing. A few adjectives (enantio selective, diastereoselective, enantiomer, diastereomer, stereoisomer), combined as judiciously as possible provide the best illustration in the present context. In doing so, one does justice to the original Kelvinian concept, empha￾sizes the true origin of the stereochemical outcome, and uses clear and consistent definitions as properly vindicated by stereochemistry educators. ACKNOWLEDGMENTS The issue of chirality, its history and significance has been the subject of past and recent discussions with many talented colleagues; to all of them, in particular Mark Green, Joseph Gal, Meir Lahav, and Cristobal Viedma, the author offers his heartfelt thanks. LITERATURE CITED 1. Eliel EL. Infelicitous stereochemical nomenclature. Chirality 1997;9: 428–430. 2. Gal J. Problems of stereochemical nomenclature and terminology, Part: 1 The homochiral controversy. Its nature, origins, and a pro￾posed solution. Enantiomer 1998;3:263–273. 3. Helmchen G. Glossary of problematic terms in organic stereochemis￾try. Enantiomer 1997;2:315–318. 4. Mislow K. Stereochemical terminology and its discontents. Chirality 2002;14:126–134. 5. Eliel EL, Wilen SH. Stereochemistry of organic compounds. New York: Wiley; 1994. p 5. 6. Moss GP. Basic terminology of stereochemistry. IUPAC recommenda￾tions. Pure Appl Chem 1996;68:2193–2222. 7. Gal J. Carl Friedrich Naumann and the introduction of enantio termi￾nology: a review and analysis on the 150th anniversary. Chirality 2007;19:89–98. 8. Wade LG Jr. Precision in stereochemical terminology. J Chem Educ 2006;83:1793–1794. 9. Eliel EL, Wilen SH. Stereochemistry of organic compounds. New York: Wiley; 1994. p 154–155. 10. Mannschreck A, Kiesswetter R. Differentiations of enantiomers via their diastereomeric association complexes—there are two ways of shaking hands. J Chem Educ 2005;82:1034–1039. 11. Craig DP, Mellor DP. Discriminating interactions between chiral mol￾ecules. Top Curr Chem 1976;63:1–48. 12. Agranat I, Sarel S. Introduction. Reflections on chiral discrimination. Enantiomer 1996;1:249–250. 13. Curie P. Sur la syme´ trie dans les phe´ nome`nes physiques, syme´ trie d’un champ e´lectrique et d’un champ magne´ tique. J Phys Theor Appl (Paris) 1894;3:393–415. 14. Renaud P. Sur une ge´ ne´ ralisation du principle de syme´ trie de Curie. Compt Rend Acad Sci Paris 1935;200:531–534. 15. Rosen J. Symmetrie in science: an introduction to the general theory. New York: Springer; 1996. 16. A´ valos M, Babiano R, Cintas P, Jime´ nez JL, Palacios JC. Symmetry breaking: an epistemological note. Tetrahedron: Asymmetry 2004;15: 3171–3175. 17. Buda AB, Auf der Heyde T, Mislow K. On quantifying chirality. Angew Chem Int Ed Engl 1992;31:989–1007. 18. Petitjean M. Chirality and symmetry measures: a transdisciplinary review. Entropy 2003;5:271–312. 19. Weinberg N, Mislow K. On chirality measures and chirality proper￾ties. Can J Chem 2000;78:41–45. 20. Cozzi F, Siegel JS. Chirony of stereochemical metaphors. Org Biomol Chem 2005;3:4296–4298. 21. Cheon KS, Selinger JV, Green MM. Counterintuitive influence of mi￾croscopic chirality on helical order in polymers. J Phys Org Chem 2004;17:719–723. 22. Rosen J. The symmetry principle. Entropy 2005;7:308–313. 4 CINTAS Chirality DOI 10.1002/chir