van't Hoff-Le Bel Centennial - ACS Publications

9 Has van't Hoff been Well Read and Understood by the Profession?

Downloaded by NORTH CAROLINA STATE UNIV on May 3, 2015 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/bk-1975-0012.ch009

P. H .

HERMANS

Marckhoek A-15,

Breda-Ginneken, Holland

At the occasion of the world wide held van't Hoff - Le Bel centennial commemoration i n 1974, the present highly aged author has been bestowed by with several marks of honor for having been one of the group of young Dutch workers, now called the "Delft School", who f i r s t irrevocably proved the non-planarity of the carbon rings beyond five carbons (H.G. Derx) and l a i d the fundamentals of Conformational Analysis by definitely proving the existence of a "barrier to rotation" (P.H.H.) as well as providing also the first quantitative measurement of a barrier to rotation on an exact thermodynamic basis (1, 2, 3). In this way, the early Dutch work i n question, which remained generally ignored for half a century, even i n a l l modern textbooks and h i s t o r i c a l reviews, was repeatedly brought to public attention. Although this (whatsoever belated) acknowledgement should be appreciated as such, the present author does not agree with location of the very origin of Conformational Analysis i n his own early work. In his personal view i t should be ascribed to van't Hoff himself, a fact which seems to have been quite generally overlooked by the profession as well as by i t s historians. It i s an undisputed fact, that the origin of stereochemistry should be traced back to the Dutch Chemist van't Hoff who shares this merit with the Frenchman Le Bel. Much less well known - at least less well remembered today i s , that the pioneering papers by van't Hoff at f i r s t have met with a fierce criticism, composed i n quite discourteous, i f not improper wordings, authored by a recognized organic-chemical "authority" of the time. It was the German chemist Kolbe, who published his rejecting and almost insulting c r i t i c i s m (in J. Prakt. Chem.). He even used i n i t the word "charlatanerie" (mountebankingness) to characterize his judgement! (It has meanwhile been revealed that, several years l a t e r , i n an advise related to van't Hoff's admission as a member of the Royal Academy of Science i n Amsterdam, the well-known Dutch professor of Chemistry Franchimont wrote about van't Hoff: "His work i s very 123

In van't Hoff-Le Bel Centennial; Ramsay, O.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

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superficial and he i s too soon contented with i t , — thoroughness, which one likes to see as a mark of lasting value, i s lacking. His conclusion: van't Hoff was not e l i g i b l e as a member of the Academy.) One of the most eye-catching aspects of the van't Hoff Le Bel concept of carbon chemistry was rotation about the single C-C bond. This concept was indispendable i n their doctrine i n order to avoid prediction of more isomers i n ethane derivatives, than those actually observed. From that very moment onwards (and for some mysterious reason) the concept of free rotation spread i t s e l f l i k e an ink blot through the whole of chemistry, even becoming a current term normally used by every chemist. This has been a f a t a l error which one should merely see as an invention ad hoc by unattentive colleagues, who could permit themselves superficial thinking. In this connection one should not forget that - at this timethe majority of so-called organic chemists were but poorly trained in physics and preferred to operate with easily understood qualitative ideas; quantitative thinking often but induced their aversion. It has been generally overlooked, at the time, (and I believe i t i s even s t i l l so at present), that van't Hoff himself has never intended to propagate free rotation. He was a far too clever and physically trained mind to do so! In fact, one has only read him superficially. I dare to claim this i n spite of the fact that i n the second edition of his famous book: "Oie Lagerung der Atome im Raume" (The position of atoms i n space) one finds even a chapter entitled "Free rotation" (4). In my personal view i t may, however, be taken as dead sure that - here - he has exclusively intended to refer to a geometrical aspect of the matter, and not to a really physical one. I suppose that - well knowing his customers of the time he held a pedagogical motive for acting so. He asked them to make a s p i r i t u a l somersault, and probably did not wish to take a r i s k of rendering i t too d i f f i c u l t for them a l l at once. If my supposition i s correct, history has well proven him to have done the right thing. Primarily, no misunderstandings nor problems should arise with regard to the number of isomers; the issue of isomers should not f a i l . Free rotation was a good guide to isomers. Irrevocable proof of my claim that van't Hoff himself was quite aware of the true situation correctly, i s found i n his book: "Dix années dans l ' h i s t o i r e d'une théorie" (published i n 1887) (5). Here, one finds a subchapter on hexa-substituted ethanes, along with a discussion of the isomers of malic acid, clearly demonstrating that proof. I shall quote some sentences from that writing i n order to enable everyone to judge for himself (free translation from French): "It i s very probable that among the numerous isomers rendered possible by rotation about the C-C bond, some are particularly favored and thus w i l l predominantly 11


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occur" - and further: "That i t i s not excluded that once, one w i l l be i n the position to establish which they are." van't Hoff then speculated which ones are probably thus favored i n the case of malic acid. He l e t himself thereby be guided by the thought that pronouncedly "negative groups" (as e.g., hydroxyl and carboxyl) w i l l avoid coming too close to each other. The preferred conformations which he then proposes are exactly the ones which we take for granted today. In my view i t i s here and nowhere else, where we find the very origin of "Conformational Analysis": with van't Hoff himself i n 1887. I want to ask the question i f any one has been taught these facts before. It would seem that they have been entirely overlooked. In no modern textbook have I ever found a single word about this matter. Contrarily, a l l modern authors, when introducing conformational analysis, start off with the statement that rotation i s not "free", thus emphasizing the suggestion that the concept of free rotation would represent a generally rusted-in error, not corrected u n t i l "modern times". This i s , however, as I hope to have pointed out, a l l but true. During about half a century the ship of Chemistry has towed this error along and has been held back i n her progress by missing an important guiding principle, u n t i l the "Delft School" definitely cut the towing cable, without being ever acknowledged or referred to for that essential action u n t i l another half a century later. Meanwhile numerous false p r i o r i t i e s i n the f i e l d of conformational analysis have been assigned to much later authors, and stubbornly transpired into the literature. A conspicuous example i s the predominant s t a b i l i t y of the chair form of cyclohexane, which was experimentally as well as theoretically well established by the "Delft School", as early as 1924, but " o f f i c i a l l y " did not emerge for several decades later (1).

Literature Cited 1. 2. 3. 4. 5.

Ramsay, O.B., Chem. Zeit. (1973) 97, 573. Chemisch Weekblad (1974), July. E l i e l , E . L . , Opening address to the Conference du Centenaire de le Bel, Paris, Sept. 17, 1974. van't Hoff, J.H., "Die Lagerung der Atome in Raume," Vieweg, Braunschweig, 1877. van't Hoff, J.H., "Dix Annees dans l'Histoire d'une theorie," P.M. Bazendijk, Rotterdam, 1887.


van't Hoff-Le Bel Centennial - ACS Publications

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