Martin Saltzman
Providence College Providence, Rhode Island 02918
I
Arthur Lapworth The genesis o f reaction mechanism
In attempting to assess the beginnings of the mechanistic view of organic reactions, the contribution of Arthur Lapworth (1872-1941) merits a considerable degree of admiration long overdue. Robert Robinson has written of Arthur Lapworth, "he contributed in particular, a series of fundamental observations which workers in various parts of the world have expanded into whole fields of investigation, and amongst the mass of information, vhich was organic chemistry, he picked out certain laws, a t first of quite limited application, which he and others have been able to fuse into more general principles of great scientificsignificance" ( 1 ) . Lapworth received his collegiate education at St. Andrews and Birmingham, and obtained his DSe (1895) at the University of London in the lahoratory of Professor H. E. Armstrong. A five-year stay at the School of Pharmacy in Bloomshury was followed by his appointment in 1900 as head of the Chemistry Department of Goldsmiths' College, an institution primarily devoted to training persons in art and textile manufacture. These years at Goldsmiths' College were marked by many of his greatest triumphs in the study of reaction mechanism which will subsequently be examined. An opportunity for professional advancement presented itself in the form of a Senior Lectureship at t.he University of Manchester. After his arrival in Manchester in 1909, it was a matter of only four years before Lapworth succeeded W. H. Perkin, Jr., as the Professor of Organic Chemistry. This position he held until 1922, when he relinquished it to allow for the promotion of his colleague, Robert Robinson. He then assumed the chair of Physical and Inorganic Chemistry, which carried with it the administrative responsibilities for the University Laboratory. Ill health terminated his active chemical career in 1935. "It is now possible to see how his ideas, always many years ahead of those of his contemporaries, played a leading part in the contribution which organic chemists have made to the revolution of chemical thought"(&). The insight of Arthur Lapworth may he attributed to his early (1901) advocacy of the interpretation of reaction mechanism on the basis of ions. "It is to electrolytic dissociation, often doubtless in extremely minute amount, t,hat the majority of changes in organic compounds may be most probably assigned"@). Lewis and Lagmuir's theories were fifteen years or more in the future! Lapworth's most spectacular success in the elucidation of reaction mechanism dealt with those transnovel approach of applying kinetic methods wherever 750 / Journol of Chemical Education
possible. Thus in the pioneering study of the formation of cyanohydrins, the loss of color of the yellow camphor quinone substrate with time was used to study "the effect produced by small quantities of agents which might not improbably influence the speed of the reaction" (4). Acids seemed to prolong the reaction time while bases and salts, especially thosc of weak acids, produced a large rate acceleration. Assuming that the reaction was ionic in nature and reversible, both quantum jumps in sophistication, it must proceed by an initial addition to the carbonyl group of cyanide ion. Lapworth proposed that the carbonyl group was polarized, because of the "residual affinity" of oxygen, and thus reacted to form an ionic complex, the whole process being revcrsihle. 0-
The second step of the reaction must according to the kinetic evidence be a rapid reaction of the ion nith a proton source such as water, alcohol, or an acid. \
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0'CN
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OH
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The acceleration of the reaction by agents which could produce an increase in cyanide ion concentration "makes it appear probable that the formation of cyanohydrins is to be represented as a comparatively slow union of the negative cyanide ion with carbonyl, followed by almost instantaneous combination of the complex with hydrogen2'(4). I n order to prove that the initial complex of carhonyl and cyanide was a stable entity, Lapworth reacted various carbonyl compounds with an excess of I
