Norman 1. Allinger W a v n e Stote Universitv Dr'ro +, V c n ; i a n
I1
Conformational Analysis in the Elementary Organic Course
A s t,lir available iliforniation i n t,hr lield of organic clrcmistry continues t o increase, the tearhcr of thr first-year organic course is faced with an incrrasingly difficult prohlem as to which of the many varied aud interesting topics available are to he o~nit,t,ed cnt,irely, and which are t o he treated only very hriefly in the COII~SP. Since our understanding of thr thwcdimrnsional structure of organic molecules has hecomc nn1c11 morn detailcd than it was a decade ago, and sinrr tlie outcome of so many reactions is dependent on the rsact strueturn of the system, the subject of conformational analysis can scarcely be neglcctcd in any modern introductory course. The subject is now treated hricfly in most textbooks, and a rudimrntary coverage can hc prcscuted in two or thrrc lcctnro I1o11rs. r I lm prrl~istoricera of conformational analysis may hc considcrcd t o havc started with thc postulation of the tetrahrd~alcarbon by van't Hof and LeBel in 1874 ( I ) , since prior to that time thr possibility of the collsideration of conformations did not exist. Important early milestonns wrrc thc post,ulatc by Sachsn (1800) of hoat and chair forms for cyclohcxane ( 2 ) , and experimental evidcricc for this situation hy Iliicknl (102.5) in thc preparation of cis and trans isomers of a decalin-like ring system ( 3 ) . The real foundation of conformational analysis was, howcvcr, laid down ahout, I!):% when a number of physical cl~emists,in an rtrart. to csplain the discrrpancy hetwcrn the entropy of cthanc as found from heat capacity measurcmcnts and as calculatrd from spectral data, slowly came to t11c conclusion that there must hc a hindrance to the rotation about the carbon-carhon bond in ethane (4). ,Ihesc > tlicrmochcmical ~neasurcn~cnts sho~vctl that there must exist a torsional barrier to free r o t a t i o ~in~ ethanr which was threefold, sinusoidal in shape, and ~vliich had a height of ahont 2.8 kcal/mole (Fig. I). A similar barrier was found to exist in n-hutane, but hcrr t,he minima did not all havr t11c samc cncrgy. Supnrimposrd on tlrr torsional harrier to rotation about t,hr central hond was all additional van dcr Waals' repulsion hntwccn thr two methyls, such that thr anti-conformation gave an energy n~inimnm ahout, 0.8 kcal below that of the gauche miuilna (Fig. 2). I hr nrost successful applications of conformational analysis to date havc been with cyclohcxane rings. 'I'hr hasic ideas were introduced by Reckett, I'itzer, and Spitzcr in ]!I17 (5) but the real usefulness of thcsr ideas was first illnstrated in the important paper hy 1
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This article is based upon a paper presented ss a part of tltc Symposiun~on Three-llirnensionel Cllemistry before the Division o l Clmnioal Education nt the 144th Meeting of the Arneriwn Clleniral Scwiety, Los Angeles, Califurnie~,April, l!)(i:
