CHIRALITY AND CHEMICAL PROCESSES ros Yet another potential problem linked to the term chiral rightha clear what wea are about.It fers to both)In most s.the authors refer to an asy discrimination alludes to differenc ewemltenteCianiofoneenantiomerofagireng eR sus R bot lenot ealed in both 2D and 3D spaces.However.heterochira one Mo there is ctions may lead to a mes resu eterochir Note that the 6enenchiralamdahirnlemties g t and S e tin (or hings n two nons ever,as n ted by these authors.these meanings be vels of struc fusing as only a areful reading reve the les from achiral pr rs or having mer o r.of various cules that are chiral but they wed in the y or may not be enanti omer well that the propertie e trans of thei components and th state of the syste in oth as tn hiral p or a mixture of ativ for e on e th a part ne e wel as by sepa tion ochiral interactions maydi that the terms dia suc Thto be the ded mean s of chiral or edconversion a ster mer.but the is ARE THERE MEASURABLE CHIRALITY CHANGES crimination,chiral re involved refers iral ontent.This subjec the lact th an analys scope of th tan not er reaction 4: tereochemical education emphasizes the role of diaster ormati measurable change inter to thent ons. acquired am must be found in the effects produ the tions.The molecula situation shor ld the fore be denoted that the causes that e symmetrica Chirality DOI 10.1002/chiroscopic sample of molecules (in stark contrast to an indi￾vidual entity that may be labelled as left- or right-handed). If a reaction is claimed to be chiral, it is not completely clear what we are talking about. It is not the reaction that is chiral, but the material present (substrates, products, or both). In most cases, the authors refer to an asymmetric induction or asymmetric transformation (accepted names in the IUPAC glossary, even though numerous reagents possess C2 symmetry and are not asymmetric in an etymo￾logical sense)6 leading to an enantioenriched sample (i.e., with complete or partial enantiomeric excess). Moreover, one often discloses that behind expressions like chiral dis￾crimination or chiral resolution there is actually formation of homochiral or heterochiral aggregates. Note that the term homochiral does certainly imply a nonracemic sam￾ple (made up of molecules with the same sense of chiral￾ity), whereas a heterochiral sample is still chiral, but race￾mic. It is equally relevant in this context the conceptual dif￾ference between enantiomorphism (or enantiomerism) and chirality, again misused as synonyms. The former refers to the relationship between two nonsuperposable mirror-image objects, while chirality denotes the phenom￾enon that indicates the existence of such objects.7 In the light of this observation, expressions like chiral synthesis, chiral reaction, or chiral transformation are extremely con￾fusing as only a careful reading reveals the meaning intended by the authors; formation of a single chiral ster￾eoisomer or, of various molecules that are chiral but they may or may not be enantiomers. Another compelling reason to use the never-obsolete concepts of asymmetric synthesis and asymmetric trans￾formation is their functional character. They tell us how an achiral precursor or a mixture of stereoisomers give rise to a single stereoisomer, for instance on introducing new chiral elements (e.g., reactions at prochiral centers) as well as by separation following equilibration such as in crystallization-induced enrichments. It is in such contexts that the terms enantioselective or diastereoselective appear to be the correct adjectives for the intended mean￾ing. The term chiral has merely a structural connotation (a feature equally applicable to the cases of chiral or asym￾metric centers)8 as chirality does not involves any ideal￾ized conversion leading to a stereoisomer, but the distinc￾tion between the self and nonself; i.e., one knows what stereoisomers, chiral or achiral, may exist as the result of a given transformation. It is also pointed out that the imperfect use of chiral dis￾crimination, chiral recognition, or chiral amplification to name a few, stems from the fact that there is nothing chi￾ral per se about these processes, which are exhibited by chiral substances but caused by diastereomeric, not enan￾tiomeric interactions.9 It is perhaps an irony that, while stereochemical education emphasizes the role of diaster￾eomeric transition structures in asymmetric syntheses or biological activity resulting from drug-enzyme interac￾tions,10 research scientists have acquired a particular am￾nesia overlooking the origin of such molecular interac￾tions. The molecular situation should therefore be denoted as stereoisomer discrimination (vide infra), either enan￾tiomer or diastereomer discrimination.11 Yet another potential problem linked to the term chiral discrimination is that discrimination may actually occur in the presence of achiral species. Thus, enantiomer discrimi￾nation refers to measurable differences between homochi￾ral and heterochiral interactions (R


R against R


S), while diastereomer discrimination alludes to differences between the interactions of one enantiomer of a given spe￾cies with the two enantiomers of a different species (i.e. RI


RII versus RI


SII). Collectively, both can be denoted as manifestations of stereoisomer discrimination and revealed in both 2D and 3D spaces. However, heterochiral interactions may lead to a meso aggregate and as a result, enantiomer discrimination would involve a difference between chiral and achiral entities. Agranat and Sarel have suggested to replace chiral discrimination by chiral dis￾tinction invoking that, while discrimination is associated with differences between two things as being equal, dis￾tinction implies the lack of resemblance between two like things.12 Chiral distinction might then be equivalent to dia￾stereomeric interactions in a stereochemical context. How￾ever, as noted by these authors, these meanings appear to be rooted in human psychology and are too broad to be universally accepted in a scientific context. Since chirality can be observed at different levels of struc￾ture and organization, processes leading to chiral supramole￾cules from achiral precursors or having chiroptical properties different from those of their chiral monomers are likewise viewed in the literature as examples of chiral amplification or discrimination. We however know well that the properties of chiral substances depend on the composition and proportion of their components and the state of the system, in other words, the system does not behave as the sum of the individ￾ual components. As a representative example, formation of helical structures with a particular handedness is again a manifestation of stereoisomer discrimination where homo￾and hetero-chiral interactions may differ substantially with respect to their source and energy. This situation does not imply a chirality change, but a different stereochemical out￾come (hence asymmetric or stereoselective, not chiral). There is no such a thing called chirality transfer as we can￾not transfer handedness on shaking hands. ARE THERE MEASURABLE CHIRALITY CHANGES? A final point that deserves attention and discourages the use of chiral still further when transformations or synthe￾ses are involved, refers to the chiral content. This subject obviously requires an analysis beyond the scope of this ar￾ticle, although a few considerations also evidence the improper usage of, for instance chiral reaction or chiral amplification. Intuitively, one could interpret that such transformations involve measurable changes in chirality content or, alternatively in symmetry contents according to the symmetry principle: the elements of symmetry of the causes must be found in the effects produced; although the effects produced may be more symmetrical that the causes that produce them.13–16 However, quantification of chirality in terms of chiropti￾cal or thermodynamic parameters is not immediately CHIRALITY AND CHEMICAL PROCESSES 3 Chirality DOI 10.1002/chir