Alkyl Cations: The First 30 Years James G. Traynham Louisiana State University, Baton Rouge. LA 70803 Frank C. Whitmore's 1932 paper, "The Common Basis of Intramolecular Rearrangements" (I), certainly affected then current and subsequent discussions of organic reactions. Some reviewers have even referred to "Whitmore's theory" rather as though it were the genesis of the concept of carbon cations (Z),in spite of Whitmore's crediting several earlier authors for "all the essentials of the mechanism proposed." Like some other influential chemists, Whitmore articulated clearlv a broadlv useful c o n c e ~ tillustrated with rather simple, commonplace compounds, and that at a pro~iti0uStime. Rut alkvl cations heran 30 vears earlier. In their informatiie hook, ~ s s a &the ~ s History of Oraanic Chemistry in the United States, 1875-1955. Stanley &d Ann ~ a r b e iattribute l the first proposal of carbocations to American chemist Julius Stieglitr in 1899 (3).Stieglitz's speculation about the possible occurrence of "positive carbon ions R3C+" was very cautious in tone and seems rather incidentally related to the interpretation of specific experimental data (4). Two vears later (1901). .. Arthur Lapworth. well ahead of most ofjlis contemporaries in concerns aboutreaction intermediates. soeculated that "it is to electrolvtic dissociation, often doibiless in extremely minute amount, that the majority of changes in organic compounds may be most probably assigned" (5). All of the illustrative reactions cited by Lapworth, however, involve carbanions or other anions; no carbocations appeared in any guise in that paper. The first firm interpretation of experimental data in terms of carbocations came three years after Stieglitz's speculation, in a paper hy Adolf Baeyer and Victor Villiger in 1902 (6). Renortine on the develonment of color in some soluti& of &iphenYlmethane derikatives, they likened the reactions of triphenylmethanol with sulfuric acid and of triphenylmethyl chloride with aluminum chloride to salt formation by metal hydroxides. Although they did not use a formula representation for a carbon cation, they did, in their discussion. focus on the metallike character of the carbon center and coined the term carbonium for it. Just two months later separate papers by Paul Walden and Moses Gomberg in the same journal reported similar studies of triphenylmethyl compounds, including the electrical conductivity of the halides in liquid sulfur dioxide solutions (7, 8). Gomberg used the formula 43Cf in his paper and proposed the name carbyl salts, which never caught on. A clear carhocation formulation for triphenylmethyl salts had been launched bv the indeoendent but virtuallv simultaneous publications of three investigators. Five vears later (1907) James F. Norrisreported his investigation of the relative rates of reactions bf alcohols with hvdrohalic acid solutions (9). Althouph he acknowledged ~ a e ~ e rworkon 's triphenylmethyl chloiide and his "proposal of hiscarbonium theory", Norris was quite hesitant about applying the concept to his reactions. H; recognized considerahle parallel between the behavior of triphenylmethanol
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BBsBd on a paper presented at the Frank C. Whltmore Centennial Symposium, 194th National Meeting of the American Chemical Society, New Orleans. LA, September 1987: paper HlST 058.
and tert-butyl alcohol toward HX, acknowledged that the reactions would probably he of the same nature, but was troubled bv the lack of color with the tert-butvl arouu. Norris remaindd uncertain ahout mechanism, h u t h e codcluded that "the reaction which takes place in the replacement of an alcoholic hydroxyl group by halogen . is not an ionic one." During the two decades after Baeyer's initial description of carhonium salts, several German chemists extended the investiaations of the colored substances including recordings ofibsorption spectra, isolation of crystalline salts, and cryoscopic measurements with sulfuric acid solutions (i factor = 4 for &COH) (10). But there was little or no evidence that carbocations were expected outside the triarylmethyl system. Clear formulations of the triarylmethyl cation appeared in these papers, however, and by the early 1920's, the c o n c e ~ of t such a cation seemed soundlv s u ~ ~ o r t ebvd experimental data and accepted. The forthrigki extension of the concept to short-lived reaction intermediates-alkyl cations without aryl groups-was broached by another German chemist, Hans Meerwein. In spite of the clarity of Meerwein's presentation and the persuasiveness of his argument as we look back from our . ~erspective, most of the chemists . contemporary with him were apparently not quite ready for such intermediates. In a long paper in 1922, Meerwein and Konrad van Emster reported a rather remarkable physical organic study of the eauilihrium isomerizations of some chlorobicvclohe~tanes (il). They found that the rate of rearrange&ent or camphene hydrochloride into isobornyl chloride was strongly dependent on the dielectric constants of the 13 solvents used. To account for this relationship, they inferred that the rate of rearrangement must be dependent on the degree of electrolytic dissociation of the substrate and that the transition from one isomer into the other is not a migration of choice, but a rearrangement of the alkyl cation. The lack of perfect parallel between rearrangement rate and dielectric constant actually strengthened rather than weakened their alkyl cation proposal, hecause the solvents that were out of line (methylphenol, sulfur dioxide, methoxybenzene, and diethvl ether)-either faster or slower rates of rearraneemenithan would have occurred with a perfect parallel-fad already been demonstrated to have a similar out-of-line effect on the formation of colored complexes of triphenylmethyl chloride (IOa). Parallel solvent effects were taken to demonstrate parallel processes, namely, ionization. Further, salts that we would identifv as Lewis acids were found to catalyze the rearrangements. In their summary they stated ,the point without any hedping: "The rearrangement of isomeric chlorides into each other follows only after prior ionization; it consists therefore only in a regrouping of the cation." In a footnote the authors wrote t h a t o n e can be inclined to see parallels with the isomerizations of propyl hromide into isopropyl bromide and of isohutyl hromide into tert-butyl bromide. Two vears later. Meerwein and co-workersr e ~ o r t e din two papers further investigations of rearrangemenMsomerization phenomena in the camphor series (12). Using esters as well as chlorides as substrates, they studied rates of racemization of isohornyl compounds and concluded that the great-
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er the ionization ability of the ester, the greater its racemization rate. By careful work, they linked racemization and isomerization through common intermediates and excluded symmetrical compoinds such as tricyclene from those reaction paths; the alkyl cation was the necessary and satisfactory intermediate. The authors' picture of the actual transformation of one carbocation enantiomer into the other is not satisfactory to an organicchemist today, hut that facet of the paper hardly affects the elegance of their analysis. At about the same time. T. M. Lowrv sueeested a t one of the Solvay conferences that organic compounds enter into reaction only by the formation of very short-lived ions a t the moment of reaction (13). His view, however, did not seem to echo or influence that of most chemists during that decade. English-language chemists could have bien aware of Meerwein's insights in spite of any language barrier. Abstracts of his papers in the 1923 and 1924 volumes of Chemical Abstracts included not only a summary of the transformations reported but also the carbocation picture of the rearrangements (14). Yet the proposal seems to have had sparse impact during the latter half of the 1920's even on chemists investigating reactions that we consider carhocationic. In one paper a t the time Christopher K. Ingold did cautiouslv refer to "the ~ o s i t i v echaree left on C, bv the seDaration o i the anion" in-allylic rearrangements, b u t a nonibnizing, bimolecular process for the rearrangements was also described as an alternative (15). "We hope a t a later date", he wrote, "to be able t o suhmit further evidence on this question." In another paper presaging his subsequent development of SNl-SN:! descriptions of reaction mechanisms, Ingold considered two mechanisms possible for the hydrolysis of henzyl chloride: one, in neutral and acidic solutions, involvine henzvl cation formation and the other. in stronelv alkalinesolutions, not (16). At about the same time, A.M. Ward acknowledged that his kinetic data for hvdrolvsis of some suhstituteduhenzyl chloride would fit an ilkyl eation mechanism. but he stronelv Dreferred one with a bivalent carbon inteimediate (17).- - Howard J. Lucas is designated one of the forefathers of physical organic chemistryby Leon Gortler (18). His papers during the 1920's on alkene additions made a clear analysis of competing theories for those reactions--electron displacement hy alkyl groups vs. alternating polarity-in favor of electron displacement, but he did not mention any alkyl cations (19). Even his 1930 paper describing his now wellknown Lucas test for alcohols does not mention alkyl cations or other mechanistic proposals (20). In 1928 C. W. Porter's hook on Molecular Rearrangements was published (21). In a review of that book (22), James B. Conant. alreadv one of the most eminent oreanic chemists in the united States, characterized the subject of molecular rearrannements as the "familv skeleton in the organic chemist's F~oset.Everyone knows that rearrangements occur but one is rather loath to talk about them." While welcoming the book as the first one dealing with rearrangements and their mechanisms, Conant significantly tempered his praise because there was "no discusiion in the book of Meemein's work on the mechanism of the Wagner rearraneement. This work seems to the reviewer t o he one of the outstanding developments in the last ten years and to provide the experimental basis for a discussi In of the mechanism of a large class of rearrangements." A. W. Stewart's 5th edition of Recent Aduunces in Organic Chemistry (23). published in 1927, presented an extensive summary (23a) of Lowry's suggestion (13), three years earlier, that organic compounds react through ions, but, like Porter's book (21). it did not mention Meerwein's ex~osition of mechanism of rearrangements. I t did, however, in-the last
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paragraph of a chapter on "organic chemistry in the 20th century", offer an interesting speculation that was shortly vindicated (23b): It mav be emohasized in this nlace that in the near future the study of quite simple reactions will offer many poinu, of interest. We are far too apt to be captivated by the application of old reactions tonew syn1heres;and it reemslikely that more interesting and useful work could be carried out by an examination of even such obvious problems as the hydration and dehydration of simple organic compounds. ~
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The connection of the concept of carbocations with experimental data was first made in 1902 for triarylmethyl systems (6). By various lines of investigation, triarylmethyl cations became acceptable, even familiar. Only a few perspicacious chemists, however, seemed willing before the 1930's to make much use of the concept to illuminate rearrangement mechanisms. In some papers acknowledgment of the idea of carhon cations as reaction intermediates was followed hv a firm reference for an alternative without an experimkntal bas;s for the choice. Resistance was weakeninn. however. Bv 1932 a unifvine mechanism for rearraneeme& was a t le&t considered pLobable by Robert ~obins-on if not conceived so s~ecificallvas bv Frank Whitmore. In Robinson's two lectuies published that year as a brief outline of an electrochemical theory of organic reactions, he discussed Wagner-Meerwein transformations and said (24): "Ionization as in Meerwein's theory, loss of elements of halogen acid and decomposition of C-NR3JOH or C-NzJOH are a few of the possibilities that have been realized; the fundamental mechanism is probably identical in all cases, the migratory group being anionoid." Then, 30 years after Baever's introduction of carbocations to account for behavior i f triphenylmethyl compounds and a decade after Meerwein's ~recocious~ r o ~ o sof a lalkvl cation intermediates in rearrangements o i h&yclic com~ounds,Whitmore's landmark publication on rearrangements in acyclic systems struck (I). Apparently the timing was just right. Literature Clted 1. Whitmtmtm.F. C. J. Am. Chem. Sac. 1)32,54,32744uW. 2. (a) For e m p l e , Alelander, E. a. Rimiples ofIanie Organic R e o e f i o ~ Wilily: ; New York, 19M. p p 44-45. (b) A collesgue of mine (N. H. Fischer) r e d s that, aa a atvdent at the Universitv,Tilbineen.Dermanv.in Ute I W s , he mtaluht"Me~r-
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9 N . m r .I F Am Chrm J 1907..W.627 642 (bJ Kchrman.P. C k m B e r . 1922. '10. la8 Hmrzxh. 4. Corm. Ber IOll.:.l~.Zi::C1612. .iB,Sl:-Sll lc, t1ufmnnn.K A , h#rmrcuthor.HChem R r IWe.4P.UI5WPE5 a, H
