provocative opinion A Critical Point Orestes J. Gonzalez History of Science Department, Harvard University, Cambridge, MA 02138
"The resolution of sodium ammonium tartrate, accomplished by Pasteur in 1848.. is narticularlv remarkable because such instances . arr rarr; in a centur).~,lsut,sequentresenrrh, o n l y ninr othrreratnples have been mrountwed in which cryirnllmtwn . . . affords a conglomrmw of sutliricntly larae rrytalid~spla)~n!: hemlhedr) ro permit their segregation by hand picking under a lens" ( 1 ) . ~
~~
These days we often see organic chemists developingmost ingenious molecular arravs in order to test the nrincides unherlying organic chemistry. Whether rare or ingenious or whether natural or "artificial." chemical processes are repmtnhle; nevertheless, it seems important w keep in mind the limitations imposed 1)srhr high dern.1: of complt~xityin organic chemist6 especially that associated withthe origin of optical activity in natural substances (L-aminoacids, enzymes, steroids, etc.). This activity constitutes an inconspicuous point of the Theory of Evolution (2). In this communication, several approaches to the origin of optical activity are examined, and the difficulty of recent progress in stereochemistry with regard to this puzzling problem are discussed. T o an organic chemist the sameness of physical and chemical properties-except for the ahilly to rotate the plane of polarized light-of enantiomeric compounds is, one could say, intuitive and unquestionable. A similar assertion can he made about one of the fundamental principles of organic chemistry: any attempt a t the synthesis of a compound containing an asymmetric center from an optically inactive reagent(~)leads to aracemic mixture. So fundamental are these principles that to attribute the origin of optical activity in systems of any kind a t all to a spontaneous resolution (21, or to a preferred crystallization of one enantiomer (21, or to a preferred decomposition of a labile chiral compound (2) are almost discordant to the organic chemists. To suggest such explanations then, reveals a desperate effort to justify a theory that generates difficulties within science itself. In fact, many times the question is also ignored or avoided. To invoke a spontaneous resolution of organic compounds could be equated with a revival of the theory of spontaneous generation and to speak of a preferred crystallization of one enantiomer is even worse. In the rare case mentioned in the beginning, Pasteur (3) did not provide an example of spontaneous separation because in the absence of an external agent, i.e., the "hand picking under a lens," the mixture remained a mixture. "It is only when the crystals separate a t a
temperature below a critical transition point that hemihedral crystals composed respectively of molecules of the dextro and levo salt separate to give a crystal mixture known as a conglomerate" ( I ) . Whose or what hands were nresent in the primitive earth to pick levo compounds from t'he conglomerates? In addition, when crvstallization is conducted in a temperature range outside the required transition point, "the crystals have the same form and are symmetrical; they show no sign of hemihedrism" (I).Was it possihle to have condomerates in the primitive earth? - If the origin of optical activity of any system whatsoever is to he explained through chemical processes one must confront the problem of either an absolute asymmetric synthesis, i.e., a synthesis in which no previously existing optically active compound has been introduced a t any stage or a resolution by an external physical agent. With the development of organic chemistry in the 20th century the question is limited to the finding of that external physical agent capable of inducing an absolute asymmetric synthesis or of resolving a racemic mixture. And here is where the most unrewarding attempts have been made. The idea of an absolute asymmetric synthesis by the action of circularly polarized light was ventured by Van't Hoff in 1894 (4). It is conceivable that a prochiral molecule could react preferentially on one of its faces after absorption of circularly polarized light, hut such an effect has never been ohsewed in a perfectly planar prochiral molecule (5). Some old claims (1930) were found erroneous (6).Two recent examples with extremelv small ontical vields have been claimed. hut thev stress that asymmetric -induction was observei thanks to a "conformational effect" in the prochiral substrate (5). Attempts to induce asymmetry under the simultaneous influence of combined electric and magnetic fields have been ruled out by the theoretical basis of such experiments (7).Low optical yield photoresolutions of racemic mixtures with circularly polarized light have been claimed (21, hut the complexity andlor the steric requirements demanded by the systems are such that they are irrelevant with respect to to an explanation of the origin of optical activity in nature. I t seems, therefore, that the principle of maximum entropy in chemical reactions is again manifested in the production of racemic mixtures. In addition, it is known that whenever possible, racemization does occur. "Random physical changes
Volume 62
Number 6
June 1985
503
resulting in a consistent pattern of increasing order in a segment of the universe would constitute an extraordinarv exception to the second law of thermodynamics" (8).To CAIfor a non-reoeatable statistical miracle of this sort would at least fall outside the realm of ohjecriw scicntiiir knor%lrdg(.. Even if the relevance of such idealistic vxwriment.; (ssnthesis and/or resolutions) is not questioned, the optical yields induced by circularly polarized light are so minute that to attribute the origin of optical activity in nature to this kind of ohotochemical processes is doubtful. Furthermore, the latter claim is self-contestable b e c a ~ ~ tht. s e amount uf nonpolarized mrli;lti(m thar existed in the r~rimit~ve earth would have sufficed to induce any photochemical reaction before there was a chance to interact with the in vitro formed circularly polarized light. The fact is that some troublesome gaps in the Theory of Evolution have been overlooked. In addition. if the term evolution is meant to imply that the physical universe is a selfgenerating, self-sustaining system, one could say with R. V. Young (9) that "this latter sense is increasingly incompatible with current developments in science itself and patently false on logical and philosophical grounds." Thus, besides the ahove-mentioned examole in s u ~ o o rof t the second law of thermodynamics, ernpi;ical data provided by astronomers clearly suggest that the space-time continuum, of which our earth is an infinitesimal part, had a definite beginning (10). This implication often is overlooked because science can neither do nor say anything about events that occur only once (11). Moreover, it is undeniable that the relationship between
504
Journal of Chemical Education
an effect and its cause is one of real dependence. Hence, if anything-the physical universe, for example-is considered as cause of itself, one would encounter a logical contradiction, i.e., an entity having a capacity to act before, at least ontologically speaking, it actually comes into existence. I would prefer not to dwell on further wbilosoohical considerations heresince they can he found in &i~ter;lepth elzeu,here ( I ? ) . Instead, I will concludc hv. savlnr . . thar ewlution is indeed a possible explanation for certain aspects of reality, but a clear indication of the limits of science out to be made.
Literlure Clted (1) Fiwr.L. F..andFieser,M.:'Advancedo~icChcmis~y,"Reinhold Publishin~Carp.. London, 1961, p. 71. (21 Balvoine, G., Moradpour, A., and Ksgan, H. R., J Amar Chsm. S a c , 96, 5152
(5)
London, 1898. Kwsn, H. B., and Fiaud, J. C., Topics in Sfereochem.,10.175 (11YiS). W.,sndLuthe,H.,Chem.Bsr,104,358 (1971).
( 6 ) Boldt,P.,Thilecke,
(7)
MadadC. A., Moseawite, A , Wynherg, H., and Meuwe8e.F.. Tetrahedron Lett., 1063 ,.-,,,.
(81 O'Reilly, S.,"BioeUlic%sndtheLirnilpof Science."Chrisfendom Publiesfions,Fmnt Royal, 1980. pp. 5659. (9) Young. R.V.Jr., private communication. (10) Weinberg, S., "The First Three Minutes? Editions du Seuil. Paris, 1978, pp. 175176.
(111 Monod, J.,"ChsnceandNecessity."A l f d Knopf, NwYork, 197L.pp. 145-117,and R. Jestmw. "Gd and the Astronomecs." Warner B n k s , New York, 1978, pp. 1011x3 (12) Jaki, S. L., "The Road of Science," Univ. of Chicago Press. Chicago, 1978. pp. 292-
293.
