Bull. Hist. Chem., VOLUME 25, Number 2 (2000 2
"WHAT'S IN A NAME?" FROM DESIGNATION TO DENUNCIATION - THE NONCLASSICAL CATION CONTROVERSY*
St ph n . W n n r, W r t r l t hn In t t t
In l , Chr t ph r . W l n, b r f th outbursts by Brown's obduracy and duplicity (7)—all h In ld r p t Un v r t C ll , nd n n ll, n t pr tt p t r , n th t th ntl nl l (UC , p bl h d p p r n th r rr n nt n hl rtl tt f rv rd t t b pt t f " bn r r f r f th t rp n d r v t v , ph n h dr hl l p h l (8 ." In th r j d r v f th r d ( . h r t n b l n t th l f ll d nt r p d Arn tt nd r r v n r d th W n r M r n r rr n nt , h h r h r t n f h th r t n t t t d n n t n f "p th t r z d b h n n th rb n rb n b nd l t n l l n ( ." rth r r , th t p t r t nt r tr n f r d nt pr d t . h r r h v l tl p t nt l f r n r t n nt nt n. A r n nt h d l n p zzl d nd f n t d r n r ntl 8 p p r n th r h t p l n n l l h t , n th r pr nt d h ll n t l n, n rb rn l t n ( , nd d th n d t r n t l tr t r l th r , h h r t d p n th p t l t f nd t n pr f nd d r nt n t r f r . l t l nv r n . r th UC r p, r rr n nt p r t r b tt l nd nt r r b tt l pp r d pr v d d h hl v bl t t f th r l tr n th r r l t r ( 0 . f r n h tr . nt x p f In ld l I th r n th n t b n d b n x n t n f Structure and Mechanism in Organic Chemistry r th ntr v r , th r th n t t ll t n fr t h n d v t d t W n r M r n r rr n nt (2 . h " f th t p rf l nd " ( nd l r t t tl f W l n p p r nn n d t rt I f r n r n h tr f ht n n th lv n f r n th "U f I t p n Ch l t n ." h nd ft n nd n f d f h n? h r p n n n nt tbr f WW II t h v pr l d d t n bl . Wr t n n 6 ll t n f j r n th pp r n f f rth r p p r n th r . tr b t n t th d p t , rtl tt pr d th n r d r th dl n pr n ht t f n n l d f v l n th r nd lv l h n t nt n n t th rn p n r " f th t th t th ntr v r h d ff rd d ( . r th h p rf l nd nd p r n l t n r n h tr t r l t ndp nt, I b l v th t n n l f th p ( , 4 ." Wh t v r th t t f th d p t d n , th d n ntr b t t r nd r t nd n f n nv t v th v d r dl l l. h pr n n f nt : p l n r, . C. r n, t h v b n th th p t f n xp r nt l t hn n th n t r t f th r b rb d nt . In n l t d f r t n h n br t d x pl h d b J. D. Roberts of preparing to "trample some wonderful and complex little th h ft n t t n j r bd pl n flowers with his muddy boots (5, 6)." Lest one con- th n r n h tr clude that Brown was more sinned against than sinning, th r l f C ld W r f nd n n th v l t n f his antagonists claimed to have been provoked to these p t W rld W r II h tr 124 Bull. Hist. Chem., VOLUME 25, Number 2 (2000)
the relationship of alternate theoretical formalisms ization (2). However, even this modified classical to differing representations of molecules and the scheme cannot accommodate the stereochemical results. conflicts that arise when new forms of represen- Wilson's ingenious but tentative solution to this tation are introduced dilemma was to propose that C-6 became only partly In this paper concentrate on the last of these issues; I bonded to C-2, while remaining partly bonded to C-1 as hope to treat others in subsequent publications. well (Fig. 2a). In this bridged cation the positive charge would then be divided between C-l and C-2. More- Wilson's results and his associated interpretation over, the electron pair originally binding C-6 to C-1 of them are as follows. Under the catalytic influence of would now be shared among or delocalized over three hydrogen chloride, camphene hydrochloride ionizes with centers—C-1, C-2 and C-6. Wilson proposed this delo- unexpected rapidity and also rearranges, producing only calized cation not as a fleeting transition state but rather one of two possible isomeric rearrangement products as a reaction intermediate, long-lived on the molecular (Fig. l). The speed with which the starting material lost time scale. The partial bonding between C-6 and C-l chloride ion suggested to Wilson that the organic cat- would preclude nucleophilic attack on C-1 from the endo ion, the direction, thus camphenyl ion, explaining the must be unusu- anomalous ally stable. It also formation of seemed highly only one prod- likely that the re- uct stereoiso- arrangement took mer. The pro- place following posed inter- ionization, after mediate which the rear- would thus be ranged cation re- neither a captured chloride camphenyl ion to give nor an isobornyl chlo- isobornyl ion ride. The entire but rather a process was re- mesomeric versible, with ion, i.e., a equilibrium fa- resonance hy- voring isobornyl brid of the chloride. Wilson two. Ever sought an ex- mindful of planatory mecha- what we r . Cl l h n nism that was in might call the harmony with two signal findings: the ionization of cam- Lavoisier gambit—seize the nomenclature and hearts phene hydrochloride was much faster than anticipated and minds will follow—Ingold in 1951 named these (kinetic anomaly), and only isobornyl chloride was species n rt t ions (12). formed, although its isomer, bornyl chloride, was the more stable of the two (stereochemical anomaly). Fig- The novelty of Wilson's explanation lay in the idea ure 1 shows a "classical" mechanism for the rearrange- that the s electrons of the C-1/C-6 bond could be delo- ment. However, this mechanism explains neither the calized over more than two centers. The division of kinetic nor the stereochemical anomaly. The first bonding electrons into two types, s and p, had been anomaly can be accounted for by assuming that chlo- worked out several years earlier by the theoretician Erich ride ion loss and carbon skeleton rearrangement are con- Hückel (13). By treating the s electrons like the local- certed; that is, the C-6/C-2 bond is formed simulta- ized electron pairs of classical Lewis theory, while al- neously with cleavage of the C-2/Cl bond, thus bypass- lowing the p electrons to be delocalized over more than ing free camphenyl ion altogether. Such an assisted ion- two nuclei, the Heckel theory nicely rationalized the well ization would indeed result in an accelerated rate of ion- established reactivity differences between single and Bull. Hist. Chem., VOLUME 25, Number 2 (2000) 125
multiple bonds. The theoretically sophisticated knew Winstein and the Wilson paper arrived at Harvard at that the s/p division was but one possible representation about the same time in the fall of 1939. Winstein was of multiple bonds, and that a strict boundary between taking up a National Research Council fellowship in localized and delocalized electrons was illusory. How- Bartlett's laboratory, fresh from doctoral and ever, since the thesis of a qualitative difference between postdoctoral work with Howard Lucas at Caltech, where s and p electrons helped make sense of a raft of chemi- he studied metal ion-alkene complexes and neighbor- cal and spectroscopic data, it came to be accepted as an ing group participation, both subjects with close affini- accurate description of the actual state of affairs. ties to the Wagner-Meerwein rearrangement (19, 20). Winstein apparently first read Wilson's piece on Sep- Incorporating the s/p dichotomy within the already tember 22, 1939. In the course of the following three very successful conventions for drawing molecular struc- weeks he wrote out no fewer than 33 pages of notes on tures was not easy, and representing electron delocal- this paper and the antecedent literature, including deri- ization was particularly tricky. In the case of benzene it vations of the kinetic equations and verification of the required at least two Lewis structures (resonance struc- calculations. He even went so far as to check the tures), both of which were fictional. The properties of Eastman catalog for the prices of camphene, borneol, the real benzene molecule are such that a single Lewis and other compounds necessary to continue the project structure is inadequate to express them, and resonance (21). It seems fair to conclude that Winstein was not theory is one way of dealing with that inadequacy with- only deeply impressed by the Wilson paper but was out abandoning classical molecular representations (14, making definite plans to pursue his own investigations 15). in the area. Wilson was proposing analogously that there was He did not act on this plan for almost a decade. only one organic ion involved in the camphene hydro- However, starting in 1949 at UCLA, Winstein began chloride rearrangement, that the ion was easily formed publishing solvolytic studies of a simplified version of because it was resonance stabilized, and that this stabi- Wilson's molecule that retained its most important struc- lization required the delocalization of a pair of s elec- tural feature, the strained bicyclic ring system (Fig. 2b) trons (Fig. 2a). Wilson's proposal occupied a mere line (22). He mustered an assortment of kinetic, stereochemi- in his paper and was only put forward as a possibility, cal, and theoretical tools to establish the reality of cat- but its initial reception was apparently cool (16, 17). In ionic intermediates with delocalized s electrons. In con- addition to struggling with the correct structure of the current investigations at MIT and then at Caltech, Rob- ionic intermediate, Wilson also had to decide how to erts used 4C labeling to uncover the full complexity of represent it. Literally pushed off to the side of the para- the rearrangements taking place in norbornyl and other graph, the representation he chose was unusual (Fig. 2a). cations. Winstein thought the array of evidence strongly Double brackets were not very common and in this case supported a delocalized structure for the norbornyl cat- rather unclear as well. This ambiguous representation, ion but, nonetheless, was cautious in terms of extending coupled with Wilson's statement (1), that "it is possible both the concept and the terminology. He observed that that [the intermediate ion] is mesomeric between [the (22b): camphenyl] and the corresponding isobornyl structures," (emphasis added) gives his presentation a very tenta- [t]he evidence for an unclassical (sic) structure for tive air. the norbornyl cation lends credence to the earlier sug- gestion of Christopher Wilson of a possible mesom- Subsequent to Wilson's publication, a half dozen eric cation from camphene hydrochloride. Such a or more papers about bridged ions appeared that sought formulation, while again not required by any one re- to extend the s delocalization concept to carbonium ions sult, takes account the most simply of products and reactivities....The number of known cases of car- in general (18). Although the bridged ion thesis was bonium ions the stereochemistry of whose reactions gaining favor, the authors of these papers reported no is best accounted for, under some circumstances, by new experimental work and many of them overlooked so-called non-classical structures, is still Wilson's contribution. One might have concluded, a small....Thus it remains to be seen how general this decade after its publication, that Wilson's hypothesis had situation may become. produced but a small ripple in the rising tide of physical It was Roberts, struggling with the "chimerical" organic research. In the mind of Saul Winstein, how- cyclopropylmethyl cation, who had coined the term ever, it had produced much more than a ripple. Both nonclassical (23) as a successful alternative to Ingold's 126 Bull. Hist. Chem., VOLUME 25, Number 2 (2000)
synartetic — "earthy English [...] preferable to graceful It was not long before Brown and Ingold came into Greek," in Bartlett's felicitous phrase (although Bartlett conflict over Brown's theory of steric strain (5). In es- made it abundantly clear that he found "nonclassical" sence, the theory holds that the course of a reaction can very infelicitous) (24). There were in fact many organic be profoundly affected by crowding in the reactants, ions and radicals alleged to be "nonclassical." Indeed, products and/or intermediates. Camphene hydrochlo- in the eyes of critics this wholesale baptism was a symp- ride is just such a crowded reactant (Fig. 1), and the loss tom of the im- of chloride during precision and ionization would trendiness of partly relieve that the concept. crowding. Thus, However, crit- the unusually high ics and advo- reactivity of cam- cates alike phene hydrochlo- have agreed ride could be ex- that the plained by the re- 2-norbornyl lief of ground state steric strain, cation is the Figure 2a. Int r d t n th r rr n nt f C ph n dr hl r d crucial case, without the invo- and this dis- cation of any spe- cussion will concentrate on the initial phase of the con- cial electronic effect. By the time Brown publicly chal- troversy over this species. lenged the nonclassical ion hypothesis during a Chemi- cal Society meeting in 1962 at Sheffield, England, the Opposition to the notion of s-electron delocaliza- focus of the battle had become the norbornyl system tion appeared in the form of an alternative, more "clas- (25). The two positional isomers of 2-norbornyl chlo- sical" explanation for the kinetic and stereochemical be- ride, exo and endo, differed in reactivity by a factor of havior of presumed nonclassical ions. Starting in 1944 several hundred. To Winstein this clearly signaled that at Wayne State and continuing at Purdue, Brown began loss of chloride from the exo isomer led to a single or- to investigate the ganic ion stabi- role of steric ef- lized by s-delo- fects on organic calization (22). reactivity. Steric Brown's counter- effects are very proposal was that classical in that the reactivity of they depend the exo isomer only on the size was normal and and relative po- that of the endo sition of isomer retarded nonbonded at- by steric effects. oms in the same In Brown's molecule and Figure 2b. W n t n nd r tr t r f r th nt r d t fr S lv l f model endo ion- may be treated x 2 rb rn l Chl r d ization would independently of lead to an increase in steric congestion, whereas exo ion- the nature of the bonding. Brown had become disturbed ization would not. Furthermore, Brown insisted that the by the widespread explanatory power granted to the elec- rearrangement could be accounted for by an equilibrium tronic theory, as a result of which "many phenomena between two distinct, "classical" organic cations rather which today are recognized as resulting from steric than requiring a single, delocalized cation (Fig. 2b)(26). forces were attributed to the operation of purely elec- tronic factors....attention to the role of steric effects in Brown's opposition to nonclassical ions puzzled as organic chemistry sank to a very low ebb (25)." Begin- well as provoked many of his opponents. In the research ning with very modest means, Brown embarked on a that ultimately earned him the 1979 Nobel Prize, Brown lifelong effort to give steric effects their due. explored the organic chemistry of diborane, B 2H6 . This Bull. Hist. Chem., VOLUME 25, Number 2 (2000) 127
compound had long posed a structural conundrum, one lishment" (32) shared ideas about structure and reactiv- that was finally solved by assigning it a pair of two- ity that were much more indebted to quantum mechan- electron three-center B-H-B bonds. In other words, ics than were Brown's, there remained the troublesome diborane exhibited just the kind of s-electron delocal- fact that there was more than one way to harness quan- ization that Brown refused to recognize in nonclassical tum mechanics to chemical ends. carbocations. In response to charges of inconsistency, Nye has described in detail how two rival paths Brown pointed out that diborane exhibited its delocal- formed and diverged in the 1930s (15): valence bond ized structure only in the gas phase, while in solution (VB) theory, with which resonance theory is closely al- where most nonclassical ion chemistry was being stud- lied, and molecular orbital (MO) theory. While there ied, diborane reverted to more "normal" modes of bond- was more than a little sniping between the principal play- ing (25). He also asserted that he was not opposed in ers over which approach led to "true understanding," principle to the concept of s-delocalization; the experi- for many chemists the crucial issues were more prag- mental data just did not support it. Brown likes to in- matic: how well the methodology accorded with long voke Occam's Razor and to claim that "Nature is simple established chemical concepts, and how effectively the (27)." One might, however, see the problem as not how calculations produced theoretical parameters of interest simple nature is but how subtle. and reproduced important experimental measurements. Brown's inability to accept the nonclassical ion The VB/resonance approach had a clear advantage with hypothesis was rooted in the intertwined strands of his respect to the first criterion. Pauling insisted that reso- entire scientific career. In order to quantify steric ef- nance theory had purely chemical roots, and he was as fects, Brown studied a variety of equilibria among Lewis well its very persuasive advocate. In that respect he far acids and bases. The results led him to the general con- outshone Robert Mulliken, the champion of molecular clusion that, while Lewis acids formed strong complexes orbital theory (33). When it came to the second consid- with donors of the n-class and weaker complexes with eration the outcome was considerably less clear and members of the p-class, "donor-acceptor interaction has depended on the nature of the molecules under study, never been demonstrated for saturated alkanes or the properties being examined, and the skill and inge- cycloalkanes, such as would be involved in the exten- nuity of the theoretician. sion of participation to the proposed s-class (28)." In Although hardly lacking in theoretical sophistica- addition, Brown and his coworkers had formulated an tion, the Hughes-Ingold group was quite committed to important extension of the Hammett equation (29), it- resonance terminology. Saltzman has described how self based in "classical" resonance theory, which rested Ingold's development of mesomerism actually antici- on a strict division between s- and p-electrons (30). pated many resonance concepts (34). The smooth blend- Thus, one important issue at stake was the viability of ing of Ingold mesomerism and the Pauling/Wheland the venerable and successful classical system of repre- resonance made that commitment perhaps inevitable senting molecular structure. The system had managed (35): to incorporate the Lewis electron pair bond and its nu- merous implications. It even accommodated resonance h n pt n f th th r f r l htl theory, although that development came perilously close pr d d ( 26 th d v r ... f th f nd nt l to stretching the system to its limit. Further erosion of n f n f nt l r n n f r th f r t n f v l nt b nd n ( 2 . v r, d r n th f l the distinction between s- and p-electrons could be seen l n r , t b v d nt th t nt l r as possibly undermining one of the most compact and n n h l l l r n f n f r th t d powerful qualitative tools available to the organic chem- f t n f v l nt b nd h h d r b d n th ist. th r f r .. Brown was heavily outnumbered in this fight; sev- At l t one of Ingold's admirers, the author of an influ- eral commentators likened him to "Horatio at the bridge ential textbook, seemed to harbor doubts about the util- (31)." Poised against him was an international coali- ity of MO theory for most chemists (36). tion consisting of Hughes, Ingold, Dewar, Bartlett, Rob- It is certainly not the case that MO theory lacked erts, Cram, and Winstein, soon to be joined by a number adherents in the UK. The British theoreticians Christo- of equal and lesser luminaries. But within the allied pher Longuet-Higgins and Charles Coulson (37) were camp there were differences that were themselves of pioneers in applying quantum mechanics to chemical great significance. For if the physical organic "estab- problems. One of their younger colleagues who very 128 Bull. Hist. Chem., VOLUME 25, Number 2 (2000)
strongly believed in the power of MO methods in or- Winstein's conceptions of homoally tic resonance and ganic chemistry was Michael J. S. Dewar (38), but he homoaromaticity demonstrated the power of MO theory was on the "wrong" (i.e., Robinson's) side of the quite dramatically (43). Robinson-Ingold dispute (39). As a result the relations In the US enthusiasm for MO theory was an inte- between Dewar and Ingold were cool at best, and often gral part of a general sense that American physical or- worse (40). Dewar eventually migrated to the US, hav- ganic chemistry had come of nd at least the ing already had a major impact on leading edge Ameri- equal of that of the English school. No one questioned can physical organic chemists, who were chafing at the Ingold's singular role in bringing the field to center stage limitations of resonance theory. worldwide (44). At the same time, his often imperious Because of its qualitative aspect and its use of clas- manner did not sit too well with the ex-colonials (45). sical structural representations, resonance theory is very Ingold's practice of aggressively coining and promot- appealing. Once the rules for manipulating these for- ing his own systems of nomenclature was particularly mulas have been mastered, one is able to rationalize a effective at raising hackles on this side of the Atlantic large body of experimental data with amazing ease. Yet, (46 : as one presses the technique, it becomes necessary to In th l tr n nt rpr t t n f r n r t n keep adding ad hoc rules and hypotheses to explain, for rt n Én l h h t h v b n p n r . h r example, why benzene is aromatic but cyclobutadiene v ht r n ll h v b n r rd ll r is antiaromatic, or why cyclopropenyl cation is isolable v d n [th US] f pr nt d nd t v l nd n but cyclopropenyl anion is not. MO theory can ratio- t r h n n r ll n n. nalize these differences without resorting to ad hoc hy- Beyond resenting Ingold's linguistic hegemony, many potheses (41). Americans felt that his views had become dogmatic on some issues and impervious to revision (47). Within For Winstein and Roberts, then, as for Brown, the this context Roberts' assessment of the significance of 2-norbornyl cation was a hook on which to hang a much Ingold's achievements is perhaps not quite so startling larger agenda. The Californians were intent on alerting (48): organic chemists to the benefits of abandoning resonance for molecular orbital theory. Roberts has described how h th n th t d pr d b t ph l r n difficult it actually was to use the seductively simple h tr nd th In ld r th t t t rr bl p rt nt n , nd t t r ll d dn t d resonance approach (42): h f r r n h tr . And th r r th r ... h d dn t nd r t nd h t h reasons for Roberts' reservations become clearer l n t l n b t, p rt l rl th b nz n . l n ld , W ll, v t t r n n when he lays out his vision of a characteristically Ameri- f r f b nz n , nd th r n rl th . And can style of physical organic chemistry (49): h d, If th r n rl th , th r b th rtl tt t n t l f r ph l r n h p rt nt, th l l r p t f th t t . h l r n h tr n b n f r , nd ll n t b l th r f r . b d ld r t n...[b] t th r r n n p nd nd r t nd th r l t f th p r t tr t r , nd ld t ff th h lf. h n v r d n th n l n b n t h h lp n th . p l th d d n t t l z th p l h r t r t Thus, the nonclassical ion controversy was not only f r n h tr , h h ll t t l r about the scope of electronic theories in organic chem- l l t pr v p rt l r nd f n pt . istry; it also concerned the claimed superiority of one Of the many conclusions that might be gleaned from of the two prevailing theories. While resonance is a revisiting the nonclassical ion ntr v r , one is very very useful tool for the explanation of experimental find- familiar. Struggles among chemists over competing rep- ings, MO theory is in many ways more effective for resentations are often protracted and intense. Since at exploration of potentially new phenomena. After An- least the time of Lavoisier, chemists have known that drew Streitwieser arrived in Roberts' laboratory as a symbols do not merely describe preexisting entities but postdoctoral fellow concentrating on MO calculations, rather help create and shape them. Ingold's command- he and Roberts proceeded to have a "wild time....One ing position in physical organic chemistry is due in part or the other of us would draw some new structure. I to his astute recognition of that fact (50). The ways in remember doing things that hadn't been contemplated which chemical bonds are represented are as much a before....Anything that we could do, we would do (33)." matter f nt nt n th r that specify how they Bull. Hist. Chem., VOLUME 25, Number 2 (2000) 129
are constituted (51). Thus, even when the protagonists Precisely so. Sp n f h t d nt b rt M z r agree on major conceptual issues, such as the superior- r n th " h r l" l pr p l th I cation, ity of MO v r VB methods, there is ample room for b rt p t h f n r n n f th d p pr bl dispute over representational issues. In this instance as ( 8 : well, nonclassical ions served as lightning rods. Dewar It was especially important as the opening of the suggested in 1946 that rb n n nd r n r r Pandora's box of an extraordinarily difficult and r n nt ld b r pr nt d p pl x nd subtle problem—a problem concerned in an impor- f rth r l b r t d th pr p l n h 4 t xtb tant way with what we mean when we write chemi- ( 2 . h , f r r, W n t n l f r th n n cal structures on paper. l l n t r f th n rb rn l t n d d n t n t t t " j r ntr b t n t h l th r " W n t n r l nf r n r pr r pr p l . A ACK OW E GME S rd n t r, W n t n h t l t t rd p pl x n t nl n n r t j n r ll M h f th r r h f r th p p r rr d t h l (53): th th r nj d th h p t l t f th n In t [it] had unfortunate consequences for organic chem- t t nd v n f S l S n nd n t istry, because the large majority of 'nonclassical d r n bb t l l v t C l f rn In t t t f h earbocations' are, in fact, p-complexes and their n l . h th r th n r f r M r f r chemistry can be interpreted much more simply and nv t n h t pr nt th t r l t th xt r A rd effectively on this basis than it can in terms of the S p . r f r n G rtl r n r l h r d obscure 'dotted line' representation that Winstein introduced... (Fig. 2b). n b r f his nt rv th , nd h t th r th r f r M rt n S ltz n ff r d h lpf l r t t rpr n l , W n t n h d r rv t n b t f th r n l dr ft. I th n th C lt h Ar h v nd r t v nd th d . Aft r pr n r th p rt nt f Sp l C ll t n , Un v r t " llf l l t t v d n f th v h n l r h br r , UC A, f r p r n t t fr th r b f h l b nd n , p ll fr th l h ld n . n ll , I xpr r t t d t r f r l r rb t l v p nt," W n t n pl n d b t th rb rt C. r n, G r A. Ol h, nd hn . b b "n v l nt rpr t t n , n v lt ft n b n rt f r n r l h r n th r p n n nd r ll h v d b r b t t t n f n l n f r x t n th . t n xpl n t n nd pl n p pl x nt r pr t t n f r v r th n n v bl ( 4, ." Ar nt b t th p r r t f n n t t n l nv nt n v r n th r r ft n t v t d b p r n l E E E CES A O ES p nd pr r t l . n th l , d ff r nt n An rl r v r n f th p p r pr nt d t th v nt n n l d t d ff r nt t . A r d t t xt r A rd S p t th A r n Ch l d nt tt nd n n r n th n n l l n pr b S t t n n ll , M r h , 8. l n p r d t n v v r p rt nt xp r nt l ppr h t th pr bl b h h d n 1. T. Nevell, E. de Salas, and C. L. Wilson, "Use of Iso- Str t r th n r ntl p bl h d thr d n n l topes in Chemical Reactions. Part I. The Mechanism of the Wagner-Meerwein Rearrangement. Exchange of MO tr t r f r th 2 n rb rn l t n ( 6 . Radioactive Chlorine and of Deuterium between Cam- Intr d n pl nn d r f rt l n " h phene Hydrochloride and Hydrogen Chloride," J. Chem. Soc., 1188-1199. n l l I n r bl ," th d t r f Chemical and , 2. C. K. Ingold, Structure and Mechanism in Organic Engineering News r t ( : Chemistry, Cornell University Press, Ithaca, NY, 1953, [t] n n t xp rt in carbonium ion chemistry, 482-494, 510-523. the nonclassical ion problem may seem largely one 3. E. M. Arnett, T. C. Hofelich, and G. W. Schriver, React. of notation. ...But the root of the problem goes much Intermed., 8 , 3, 189-226. It also became a veritable deeper than notation and nomenclature, or the topic industry, spawning two books, innumerable review ar- could surely not have absorbed so much of the ener- ticles, and an immense body of primary literature that gies of some of the leading physical organic chem- constituted a sizable fraction of the papers published in ists for more than 15 years. J, Am. Chem. Soc. during the 1960s and 1970s. 130 Bull. Hist. Chem., VOLUME 25, Number 2 (2000)
4. A trenchant overview may be found in W. H. Brock, The length in the 1980s, believes that Wilson's claim was Norton History of Chemistry, Norton, New York, NY, not made in jest (private communication). 1993, 558-569. (Note that on p. 567 norbornane should 18. A summary with references is in S. Winstein and B. K. be norbornene.) Morse, "Solvolysis of Optically Active 5. D. A. Davenport, "On the Comparative Unimportance a-Phenylneopentyl Derivatives," J. Am. Chem. Soc., 2. of the Invective Effect," CHEMTECH, 8 , 526-531. 74, 1133-l . 140. 6. A letter from Brown's wife, Sarah, charges that at an 19. S. Winstein, B. K. Morse, E. Grunwald, K. C. Schreiber, Organic Symposium, Saul Winstein had said that Brown and J. Corse, "Driving Forces in Wagner-Meerwein Re- "was arguing like a shyster lawyer" (Chem. Intell., , arrangements," J. Am. Chem. Soc., 2, 74, 1113-1120. 5 (l), 4; Brown himself made the same claim to me on 20. P. D. Bartlett, "The Scientific Work of Saul Winstein," August 20, 1998). In the same letter George Olah is ac- J. Am. Chem. Soc., 2, 94, 2161-2170. cused of calling Brown "senile;" in conversation with 21. Winstein Papers, Department of Special Collections, me (May 28, 1998) Olah admitted to making a com- University Research Library, UCLA, Collection 1290, ment he later regretted but one not nearly so derogatory. Box 18. It would seem that even recollections of the controversy 22. (a) Preliminary communication: S. Winstein and D. S. have the potential to be controversial. Trifan, "The Structure of the Bicyclo[2,2,1]2-heptyl 7. J. D. Roberts, The Right Place at the Right Time, Ameri- (Norbornyl) Carbonium Ion," J. Am. Chem. Soc., 4 , can Chemical Society, Washington, DC, 1990, 72-74; 71, 2953. Full papers: (b) "Solvolysis of endo-Norbornyl 250-253. Arenesulfonates," J. Am. Chem. Soc., 2, 74, 1147- 8. P. D. Bartlett, interview with Leon Gortler, March 7, 1154; (c) "Solvolysis of exo-Norbornyl p- I984. Bromobenzenesulfonates,"J. Am. Chem. Soc., 2, 74, 9. G. M. Kramer, C. G. Scouten, R. V. Kastrup, E. R. Ernst, 1154-1160. and C. F. Pictorski, "Ludwig Boltzmann and the 23. Ref. 7, pp 80-82. The term first appeared in print in I. D. Norbornyl Cation," J. Phys. Chem., 8 , 93, 6257-6260. Roberts and C. C. Lee, "The Nature of the Intermediate 10. M. Saunders, "Response to 'Ludwig Boltzmann and the in the Solvolysis of Norbornyl Derivatives," J. Am. Norbornyl Cation,' " J. Phys. Chem., 0, 94, 6509- Chem. Soc., , 73, 5009. 6511; E. Spohr and M. Wolfsberg, "Entropy and Degen- 24. Ref. 11,p 65. erate Rearrangements," 6511-6512; G. M. Kramer and 25. H. C. Brown, "Strained Transition States," in The Tran- C. G. Scouten, "Configurational Entropy of Fluxional sition State, Chem. Soc. Spec. Pub. 16, London, 1962, Molecules," 6512-6514. 140-162. 11. P. D. Bartlett, Nonclassical Ions: Reprints and Commen- 26. H. C. Brown, The Nonclassical Ion Problem, with com- tary, Benjamin, New York, NY, 1965, v. ments by P. v. R. Schleyer, Plenum Publishing, New 12. F. Brown, É. D. Hughes, C. K. Ingold, and J. F. Smith, York, 1977, Ch. 6, 8. "Wagner Changes, Synartetic Acceleration and 27. That these themes constitute a leitmotif for Brown is Synartetic Ions," Nature, , 168, 65-67. attested to by numbers of his graduate students and 13. J. A. Berson, "Erich Hückel, Pioneer of Organic Quan- postdoctoral fellows: Remembering HCB: A Collection tum Chemistry: Reflections on Theory and Experiment," of Memoirs by Colleagues and Former Students of Angew. Chem. Mt. Ed. Engl., 6, 35, 2750-2764. Herbert C. Brown, Dept. of Chemistry, Purdue Univer- 14. G. É. K. Branch and M. Calvin, The Theory of Organic sity, 1978. Chemistry: An Advanced Course, Prentice-Hall, New 28. H. C. Brown, "The Nonclassical Ion Problem," Chem. York, NY, 1941, 73-81. Eng. News, Feb. 13, 1967, 87-97. 15. M. J. Nye, From Chemical Philosophy to Theoretical 29. L. P. Hammett, Physical Organic Chemistry, McGraw- Chemistry: Dynamics of Matter and Dynamics of Disci- Hill, New York, 1940, 184-207. plines, 1800-1950, University of California Press, Ber- 30. H. C. Brown and L. M. Stock, "A Quantitative Treat- keley, CA, 1993, 239-261. ment of Directive Effects in Aromatic Substitution," Adv. 16. Wilson told Michael Dewar that "he had difficulty in Phys. Org. Chem., 6 , 1, 35-154. publishing it, even as a single sentence in his paper:" M. 31. That precise characterization is used by both Roberts J. S. Dewar and A. P. Marchand, Annu. Rev. Phys. Chem., (Ref. 7, p 89) and Brock (Ref. 4). 6 , 16, 321-346. P. Both Brown and others have referred to Brown's feel- 17. In a letter to Derek Davenport, Wilson claimed that ing of exclusion from the bicoastal American physical Ingold had not initially supported the mesomeric ion organic network: H. C. Brown, interview with L. Gortler. hypothesis. This statement is challenged by K. T. Leffek, June 7, 1982; Ref. 7, p 250. who asserts that Wilson was "not a man to allow a punch 33. According to Roberts, "...if Pauling had really pushed line to a story to be diminished by a strict adherence to the molecular orbital theory, and Mulliken had pushed the truth" (Sir Christopher Ingold: A Major Prophet of resonance, I just sort of have the feeling that we may Organic Chemistry, Nova Lion, Victoria, BC, 1996, 149- never have heard much about resonance" J. D. Roberts, 150). Martin Saltzman, who interviewed Wilson at Bull. Hist. Chem., O UME 2 , Number 2 (2000) 131
interviews with L. Gortler, January, 1979 and January, 46. P. . Bartlett, review of Watson (Ref. 36): J. Am. Chem. 1981. Soc., 8, 60, 2278. 34. M. D. Saltzman, "C, K. Ingold's Development of the 47. Hearing that Ingold had been ignoring one of his impor- Concept of Mesomerism," Bull. Hist. Chem., 6, 19, tant discoveries, the special salt effect, Winstein observed 25-32. that "a good many Americans think he has been an ass 35. Ref. 2, p 82. in his recent papers on ion pairs and solvolysis. In fact, 36. "Both the electron pair method and the molecular or- I'm building up to murder him" (letter to E. F. Jenny, bital method are approximations, and the exact truth January 13, 1958, Winstein Collection, UCLA, Box 2). probably lies intermediate between them. The latter can- 48. Ref. 33, p 94. not he described as a wave-mechanical theory of valency, 49. Ref. 42, p 32 however, and the conception of the 2-electron bond as 50. Ref. 15, pp 269-271. originally put forward by G. N. Lewis still provides the 51. B. T. Sutcliffe, "The Development of the Idea of a Chemi- most useful picture of the molecule for the purpose of cal Bond," Mt. J. Quantum Chem., 6, 58, 645-655. interpreting chemical phenomena" ( . B. Watson, Mod- 52. Ref. 38, pp 211-213. ern Theories of Organic Chemistry, Oxford University 53. M. J. Dewar, A Semiempirical Life, American Chemical Press, Oxford, 1937, 14; emphasis added). In the sec- Society, Washington DC, 1992, 46-47. ond edition (1941) the underlined phrase was replaced 54. S. Winstein, review of Dewar (Ref. 38): Chem. Eng. by a more neutral one. News, 4 , 27, 3440. 37. C. A. Coulson, Valence, Oxford University Press, Ox- 55. The early evolution of different representations for the ford, 1952. 2-norbornyl cation is described by G. D. Sargeant, "The 38. M. J. S. Dewar, The Electronic Theory of Organic Chem- 2-Norbornyl Cation," in G. A. Olah and P. v. R. Schleyer, istry, Oxford University Press, Oxford, 1949. Ed., Carbonium Ions, Wiley-Interscience, New York, 39. M. D. Saltzman, "The Robinson-Ingold Controversy: 1972, Vol. III, 1103-1106. Precedence in the Electronic Theory of Organic Reac- 56. This intriguing story and much else relating to the non- tions," J. Chem. Educ., 80, 57, 484-488. classical ion controversy may be found in B. . Coppola, 40. M. J. S. Dewar, "Some Comments on 'A Semiempirical "Deeper Beneath the Surface of the Chemical Article:
Life, — Chem. Intell., , 3 (l), 34-39; K. J. Laidler, Richard G. Lawton and the Norbornyl Cation Problem," K. T. Leffek, and J. Shorter, "Some Comments on "Some Chem. Intell., 8, 4 (2), 40-49.
Comments on 'A Semiempirical Life, — an Article by 57. Ref. 28, p 87. Michael Dewar," Chem, Mull., , 3 (4), 55-57. 58. Ref. 7, p 64. 4 . . D. Roberts, Notes on Molecular Orbital Calculations, Benjamin, New York, 1961. A OU E AU O 42. J. D. Roberts, interview with Rachel Prud'homme, 1987, Caltech Archives, Pasadena, CA, 47. See related com- Aft r p td t r l r t th Un v r t f rh , ments in Ref. 38, p 15. UK St ph n . W n n r ( h. ., nn lv n th 43. S. Winstein, "Nonclassical Ions and Homoaromaticity," Ed rd h rnt n t W r t r l t hn In Quart. Rev. Chem. Soc., 6 , 23, 141-176. 44. Winstein expressed great pride in "join[ing] Ingold and t t t h r h n r f r f Ch tr . Hammett on the list of recipients" of the American r r h nt r t nt r n ntr l l r n r nd Chemical Society Norris Award (letter to C. R. Hauser, l tr n tr n f r. In th p t d d h h b n April 13, 1967, Winstein Collection, UCLA, Box 2). r n l nv lv d n th h t r f h tr nd 45. Davenport very aptly chose the title, "C. K. Ingold: th 2000 Ch r f IS . In dd t n t nd t Master and Mandarin of Physical Organic Chemistry," n h t r l r r h, h h t ht n r nt tl d for his retrospective survey of Ingold's accomplishments "C ntr v r n Ch tr " th ll t C lt h (Bull. Hist. Chem., 6, 19, l-98). nd W I.
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