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82 Bull. Hist. Chem., VOLUME 37, Number 2 (2012)

JOHN WILLIAM BAKER AND THE ORIGIN OF THE BAKER-NATHAN EFFECT

Martin D. Saltzman, Providence College, [email protected]

The Baker-Nathan effect was a theory proposed by independent school in west London from 1909-1916. He John William Baker (1898-1967) and Wilfred Samuel was awarded a Royal Scholarship in which Nathan (1910-1961) of Leeds University in 1935 to allowed him to enroll at the Imperial College of Science explain certain anomalous results obtained in SN2 reac- and Technology in London. His exceptional aptitude for tions carried out in solution. The explanation produced chemistry was shown by his passing the intermediate by them was expanded to a general type of no-bond exams for the B.Sc. Degree by the end of 1916. In April known as by Mulliken in 1917 he was conscripted and assigned to the Royal En- papers published in 1939 and 1941 (1). Hyperconjuga- gineers where he was commissioned a Lieutenant. His tion is today defined as the conjugation of polarized major responsibilities were in the area of water purifica- sigma bonds and adjacent pi orbitals. This theory has tion. In May 1918 he was sent to Mesopotamia (now Iraq) been applied to a host of anomalous physical measure- where he was assigned the same duties. He was finally ments such as bond lengths and dipole moments. The demobilized in 1920 and returned to Imperial where he Baker-Nathan effect as it was originally framed is no was awarded his B.Sc. with first class honors in 1921. longer used in of more plausible explanations for The M.Sc. was awarded in 1923, a Ph.D. in 1925 and rate accelerations in solution. D.Sc. in 1928. His dissertation was nominally supervised by Jocelyn W. Thorpe (1872-1940) (2) but most directly This paper will first deal with the life of John Wil- by Christopher K. Ingold (1893-1970) (3). liam Baker, a name that is far from familiar to most chemists today. Those chemists who received their Jocelyn Thorpe worked in the laboratory of Victor training in physical in the pre-1975 Meyer in Heidelberg under the guidance of Karl von era probably remember the Baker-Nathan effect. The Auwers. He was awarded his D.Phil. in 1895 and then paper will then examine how Baker and his collabora- assumed a junior position at Owens College in Manches- tor Nathan discovered what was believed to be the first ter under W. H. Perkin Jr. (1860-1929). In Manchester in example of hyperconjugation. Next it will briefly discuss addition to his teaching duties he worked on a variety of the downfall of the Baker-Nathan effect as an explana- synthetic organic problems. He stayed until 1909 when tion of what happens in unexpected rate accelerations in he was awarded the Sorby Research Fellowship at the reactions in solutions. A brief discussion of the concept of University of Sheffield. Freed from any teaching or ad- hyperconjugation as used today will conclude this paper. ministrative duties, he was able to produce a large body of research which led in 1913 to his being offered the John William Baker professorship in organic chemistry at Imperial College. In Thorpe’s first year at Imperial, Ingold, after com- John William Baker was born in London on March pleting undergraduate studies in Southampton joined his 10, 1898, and he attended the Latymer Upper School, an Bull. Hist. Chem., VOLUME 37, Number 2 (2012) 83 research group and quickly established himself as both a acrimonious debate that occurred between Ingold and first class researcher and an innovative thinker. As part Robinson concerning the application of the electronic of the war effort Ingold was involved in the production theory to aromatic substitution in the late 1920s. of tear gas and other related work at the Cassel Cyanide company in Glasgow. When the war ended there seemed In 1930 Robert Robinson resigned his professorship little possibility of Ingold entering the realm of academia at University College London to become the Waynflete and so he stayed at Cassel for another two years. In 1920 Professor of Organic Chemistry at Oxford. Ingold was Thorpe was able to secure Ingold a position as a lecturer offered the position Robinson vacated, and he imme- in organic chemistry at Imperial. In the postwar years diately accepted the chance to return to London from there were two major research programs that Thorpe car- Leeds, which in the 1920s was a very grimy industrial ried on with the assistance of Ingold: the effect of city with very unhealthy air quality. The conditions of substitution on ring closure, which led to the Thorpe- Ingold’s employment at University College would not Ingold principle, and ring-chain tautomerism. let him bring any of his team with him. The only person who did go to London was Christopher L. Wilson who Baker was supervised by Ingold and the two papers was finishing his B.Sc. and wished to do his Ph.D. un- he jointly published with Thorpe and Ingold reflect der Ingold at University College. Ingold was strongly this program. The first was in 1923 and dealt with ring in favor of Baker becoming his successor at Leeds (6) closure, and the second in 1924 was concerned with but the professorship went instead to Frederick S. Chal- ring-chain tautomerism (4). In 1925 the possibility of a lenger (1887-1983), a natural products chemist from the university position of any sort for Baker was bleak to say Univeristy of Manchester. Challenger had joined the the least. As a matter of necessity he accepted the position staff in Manchester in 1920 and had worked with Rob- of Senior Chemistry Master at the North-Eastern County inson. Baker, Shoppee, and Burton among others were School in Barnard Castle in the northeast of England. in Shoppee’s words “left high and dry in the provinces” However his days as a schoolmaster were very brief due (7). Challenger had no problem with Baker continuing to the meteoric rise of Ingold in British chemistry. At his line of research in physical organic chemistry. Baker the unheard-of age of 30, Ingold was elected a Fellow would remain at Leeds for the balance of his academic of the Royal Society and was offered the professorship career, ultimately becoming the Reader in the Mechanism at Leeds University vacated by the retirement of J. B. of Organic Reactions. He did apply for several professor- Cohen (1859-1935). ships during his time at Leeds. In 1943 he was considered for the professorship at University College, Bangor, but Ingold was allowed to pick his own staff and he the appointment was given to Edward D. Hughes instead. assembled a first class group of investigators as junior Baker was never elected a Fellow of the Royal Society members. Baker was hired as an assistant lecturer, and either, which was surprising given his research output the group grew to have several future notables in Brit- and the difficult circumstances in which this research ish chemistry such as Harold Burton (1901-1966) and was conducted, namely economic depression, war, and Charles W. Shoppee (1904-1994). During Ingold’s brief six-year tenure at Leeds which lasted until 1930 when he post-war austerity. left for University College London, there were three on- In his career Baker published over 100 papers of going areas of research. The first was aromatic substitu- which 76 appeared in the Journal of the Chemical So- tion and the mechanism of aromatic side-chain reactions. ciety. In 25 of his J. Chem. Soc. papers he was the sole The second dealt with aniontropic change, which was author. In addition he was the author of four monographs part of his continuing interest in the ionic mechanism of and books. These were Natural Terpenes (1930) (8), Tau- tautomerism. The third research program dealt with the tomerism (1934) (9), Hyperconjugation (1953) (10), and effects of polar on the velocity and orienta- Electronic Theories of Organic Chemistry (1958) (11). tion in addition reactions to unsaturated systems. His Hyperconjugation has the following dedication in the Baker published six papers during the Ingold time at preface: “To Christopher Kelk Ingold in grateful appre- Leeds. Three of these dealt with the nature of the alter- ciation of his inspiring friendship throughout my career.” nating effect; one involved ring-chain tautomerism; and When Ingold left Leeds in 1930 Baker took over two involved ring formation (5). These were sufficient his teaching duties in the first-year organic chemistry for him to be awarded the D.Sc. degree by University of course. His approach followed that of Ingold and in es- London in 1928. Baker was never involved in the ongoing sence he was Ingold’s man in the northeast of England 84 Bull. Hist. Chem., VOLUME 37, Number 2 (2012)

Figure 1. Group photo from Baker’s time at Leeds for the physical organic approach to organic chemistry the mechanism of wear in engines. He died on August 6, rather than repetition of endless facts with no obvious 1961, from complications following surgery. connection which was so typical of the time. However Baker’s reverence for Ingold caused him to ignore other How the Baker-Nathan Effect Was important changes that were going on, and his course did Discovered not substantially change in content for over 27 years. What was revolutionary in 1930 was dated by 1957 In a paper published in 1935 in the Journal of the when the newly appointed professor of organic chemistry Chemical Society bearing the title “The Mechanism of Basil Lythgoe removed Baker from the course. This was Aromatic Side-chain Reactions with Special Reference to not taken very well by Baker, who resigned the Polar Effects of Substituents. Part V. The in 1959 ostensibly because of the ill health Polar Effects of Alkyl Groups,” (12) we find of his wife. When his wife died in 1962 he the first discussion of what came to be known moved to York where his only child, a son, as the Baker-Nathan effect. This paper was a lived. He remarried and became very active study of the Menschutkin reaction between in the Congregational church, devoting his p-substituted benzyl bromides and final years to church work. He died on May in dry acetone. This is an example of a class 12, 1967. of SN2 reaction where the reactants are both Wilfred Samuel Nathan was born in neutral. The results did not conform to the December 1910 in Cardiff, the son of parents usual order of reactivity expected. Ingold who immigrated to Wales from the continent. had discussed the effect of alkyl substituents He obtained his Ph.D. at University College, on SN2 reactions in terms of the positive Bangor, under the supervision of the noted Figure 2. Nathan’s photo (+I) at a saturated carbon physical organic chemist Herbert Ben Wat- from his obituary from the . Thus one would expect that the order son in 1935. With support from a Department BP Shield of alkyl groups based on releasing of Scientific and Industrial Research grant he abilities would be t-Bu > i-Pr > Et > Me > H. was able to work with Baker in Leeds for one This had been experimentally proven to be year. When this grant expired Nathan moved to Ingold’s the case in several studies done prior to 1935. The experi- group at University College London where he was sup- ments undertaken by Baker and Nathan introduced a new ported by a grant from Imperial Chemical Industries. In scenario in that the alkyl group was part of a conjugated 1936 he accepted a position with British Petroleum (BP) system. To rationalize their results Baker and Nathan at their laboratory in Llandarcy, Wales. He was trans- contemplated the possibility that a resonance effect or ferred in 1941 to BP’s facility at Sunbury-on-Thames to use Ingold’s terminology was also near London and spent the rest of his professional career operating in this case in addition to the inductive effect. there. At BP he worked in a variety of areas including Ingold in his 1934 Chemical Reviews paper “Prin- thermal diffusion, the behavior of waxy crude oils and ciples of an Electronic Theory of Organic Reactions” Bull. Hist. Chem., VOLUME 37, Number 2 (2012) 85

(13) discussed the various types of electronic effects that is removed by the hydroxide ion. As the C–H bond is could occur to explain the mechanisms of organic reac- being broken and negative charge is being built up, as tions. Baker and Nathan used the Ingold nomenclature in the case of the propyl group, the inductive effect of as the basis to explain the anomalous behavior of the the methyl will intensify the negative charge and retard methyl group (12): this being lost. The ethyl group operates in It is tentatively suggested that, when the methyl group such a way as to minimize the inductive effect and hy- is attached to a [italics in original], perconjugation will facilitate the breakage of the C–N the duplet of forming the C–H bond in this bond to the ethyl group and the formation of the double group are appreciably less “localized” than are those bond in ethylene. These effects are illustrated in the in a similarly placed C–C bond. This means that a following diagram from Baker and Nathan’s paper (12). methyl group, attached to the necessary system, is ca- pable of permitting electron-release by a mechanism which, in essentials, is a type of tautomeric effect,

The asterisk refers to a note at the bottom of the page which reads as follows: “The original suggestion that A further question that needed to be answered was such a polar effect is present was due to W. S. N.” whether the effect was, using the Ingold terminology, strictly a mesomeric effect (a permanent polarization How Nathan came up with the idea we do not know effect also known as the resonance effect) or one that but considering that Nathan was a relatively young man is an electromeric effect (a polarizability effect caused and had been a student in the early 1930s when both by an attacking reagent). In order to answer these ques- valence bond theory and theory had tions Baker, Nathan, and Shoppee conducted a study been developing, he may have been influenced by these “The Effects of p-Alkyl Substituents on Prototropy in new ideas. The caveat was made that this new effect was the Methyleneazomethine System” (14). Prototropy is in addition to the normal positive inductive effect (+I) a special case of the general phenomenon of tautomer- of alkyl groups. ism, a subject that Baker was very familiar with from The rationale that was presented by Baker was that his previous work with Ingold and Thorpe. if methyl increased the rate of SN reaction in benzyl sys- 2 Shoppee had shown in 1931 (15) that this reac- tems and tert-butyl had the least effect this must be due to tion was favored by electron-withdrawing substituents the lack of what came to be known as hyperconjugation. (negative inductive effect, –I). These new studies of When a conjugated system is not present, then the normal tautomerism in the azomethine system involved alkyl inductive effect occurs and the rates are reversed. This substituents only in the para position. Any alkyl group argument can be clearly seen in the following diagram would be expected because of its +I (positive inductive included in the text. The ordinary inductive effect is in- effect) to slow the rate of reaction as compared to the parent system. If the methyl group interaction involved a mesomeric (reso- dicated by fully pointed arrows and the hyperconjugative nance) effect (as in the diagram from Ref. 14) then effect by the dotted arrows. it would be expected to retard the reaction to a This new effect was used by Baker and Nathan in greater extent than the tert-butyl group. This indeed the same paper to explain the Hofmann degradation reac- is what was observed in the experiments performed. tion of quaternary ammonium hydroxides. When ethyl dimethyl propylammonium hydroxide is subjected to the Hofmann degradation reaction it produces ethylene as predicted. Baker and Nathan offered an explanation which invoked their newly discovered effect. The key to There was still a lack of a clear-cut experimental the path of the reaction is which of the hydrogen 86 Bull. Hist. Chem., VOLUME 37, Number 2 (2012) result which could show in a definitive manner that electron-demanding reaction, and there can hardly hyperconjugation was a real effect and not the result of be any reaction more suitable than a unimolecular some already known factor. Hughes, Ingold, and Taher nucleophilic substitution, where there is only a tackled this question in a paper “Mechanism of Substitu- single electron-transfer involving carbon in the rate- affecting stage. [italics in original] tion at a Saturated Carbon Atom. Polar Effects in Alkyl Groups, as Illustrated by Solvolytic Substitutions of p- The reaction that was chosen for study was the hydrolysis Alkylbenzhydryl Chlorides,” published in 1940 in the of p-alkylbenzhydryl chlorides in 80% aqueous acetone, a Journal of the Chemical Society (16). reaction known to be of the SN1 type. If the Baker-Nathan effect was real then there would be a competition between These investigators point out that in the displace- the inductive effect and the conjugative Baker-Nathan ment reactions in a series of simple alkyl bromides, the effect. The key would be in the transition state where the Baker-Nathan effect would operate in the same manner conjugative effect would be paramount in lowering the as the inductive effect. Thus it would be impossible to energy of activation by electron transfer. see if the effect is a real one. However, in the reactions of a p-alkyl benzyl bromide series, although the inductive The results obtained by Hughes, Ingold and Taher effect and the tautomeric effect (Baker-Nathan) would (Table 1) were consistent with the order that would be reinforce one another, the order of reactivity would be predicted if the Baker-Nathan effect existed. This order different depending on which effect predominated. Not was H < Me > Et > i-Pr > t-Bu. In addition to the large knowing whether the inductive effect or the tautomeric rate increase for Me vs H, all the other alkyl groups effect would be the most important, one would have dif- showed decreases with respect to Me in a regular man- ficulty in predicting the order of reactivity. If the Baker- ner as would be predicted by reduced hyperconjugative Nathan effect is not real or is real but very weak, then the ability. “We regard this as the first satisfactoryevidence predicted order of reactivity would be Me < Et < i-Pr < of the reality of alkyl conjugation as envisioned by Baker t-Bu (sequence I in the following figure taken from Ref. and Nathan” (16). 16), but if the Baker-Nathan effect is real and dominant, Table 1. First-order Rate Constants and Arrhenius the order would be H < {Me > Et > i-Pr > t-Bu}. Parameters for the Hydrolysis of Benzhydryl Chloride and its p-Alkyl Derivatives (Table I of Ref. 16)

p- H Me Et i-Pr t-Bu 106 k (s–1) at 0.0°C 2.82 83.5 62.6 46.95 35.9 10–11 A (s–1) 1.91 1.23 2.19 3.47 4.07 E (kcal mol–1) 21.0 18.9 19.4 19.8 20.05

One further paper published by Baker in 1942 (17) The original report of the Baker-Nathan effect (12) should be mentioned. This dealt with the reaction of was based on the study of an SN2 reaction in a series of to produce cyanohydrins as evidence for the compounds that did not give rise to large variations in the mesomeric (hyperconjugative) effects of alkyl groups. rates of reactions. For example, the difference between H Baker performed these studies to add further experi- and Me where one would expect both an inductive and mental evidence for the rate enhancement by methyl tautomeric (hyperconjugative) effect was only 1: 1.65. in contrast to tert-butyl when something other than the As Ingold and his co-workers put it, “It is impossible to inductive effect is at work. Lapworth (18) had previously be confident that so closely spaced a rate series means shown that in the equilibrium between p-substituted what it may appear to mean” (16). They explains the rationale for the study they are about to describe as follows (16): What we need in order to get over the difficulty benzaldehydes and their cyanohydrins that electron- revealed in all this work is a device for making releasing groups stabilized the and destabilized one of the four contributing factors (and it must be the cyanohydrins. Baker studied the series of p-alkyl one of the two which involves alkyl conjugation) benzaldehydes where hyperconjugation was possible in much greater than all others; and this can be done. the aldehydes but not in the cyanohydrins. He found that We decide [sic] to study reaction rate in a strongly Bull. Hist. Chem., VOLUME 37, Number 2 (2012) 87 the order of stability of the cyanohydrins with respect to nocent enough title of “Concerning the Baker-Nathan their corresponding aldehyde was in the order H > t-Bu Effect” offered an explanation based upon solvent effects > i-Pr > Et > Me. It thus seemed that hyperconjugation (23). In this investigation of p-alkylbenzhydryl chlorides was a real phenomenon. in 80% aqueous acetone it was shown that if the same Chemists increasingly used the Baker-Nathan ef- reaction is carried out in the gas phase the order of reac- fect to provide explanations for unusual patterns of rate tivity was reversed with methyl now showing the least enhancements and equilibrium as well as physical mea- reactivity. Since the structure of the reactants was exactly surements such as bond lengths, dipole moments, and the same the difference must be due to solvation effects heats of hydrogenation to name a few. Two reviews of and not to hyperconjugation in this case (23): hyperconjugation were published in 1945 in Chemical It should be made clear that the position being taken Reviews (19) and in 1949 in Quarterly Reviews of the is not that C–H hyperconjugation is necessarily of no Chemical Society in 1949 (20), which gave an overview consequence, although the concept is not needed to of how this effect was used to explain certain anomalies. explain the data cited here. The viewpoint is simply this: if the role of the solvent as discussed is of im- Table 2 lists references made to the Baker-Nathan portance then other modes of electron release by alkyl effect and hyperconju- groups in the order gation in some popular Table 2. Hyperconjugation references in some texts from 1943- tert-Bu > Me may texts of a more ad- 1974 (21). be relatively more important, even vanced nature pub- Text author and title Year of Number when attached to an lished from 1943 to publica- of pages electron-demanding tion devoted to 1974. This is by no system. means a comprehen- hypercon- sive listing, but it does jugation This paper did provide evidence that Remick, 1943 10 not mark the begin- the concept of hyper- Electronic Interpretation of Organic Chemistry ning of a decline in conjugation was taken Alexander, 1950 8 the appearance of the seriously as a means Ionic Organic Reactions Baker-Nathan effect of explanation in the Waters, 1950 8 in the chemical litera- chemical community. Physical Aspects of Organic Chemistry (4th ed.) ture. Indeed, Schubert Gould, 1959 11 and Sweeney appear In addition pop- Mechanism and Structure in Organic Chemistry to have inspired a ular undergraduate Fieser &Fieser, 1961 4 number of studies textbooks such as Advanced Organic Chemistry of solvent effects in Morrison & Boyd and Hirsch, 1974 8 connection with the Streitweiser & Heath- Concepts in Theoretical Organic Chemistry Baker-Nathan effect. cock of this period A search of SciFinder also mentioned the for the topic “Baker- role of hyperconjuga- Nathan” shows more papers from the 1960s than any tion as an explanation for certain anomalous experimental other decade and that about 1/3 of papers dealing with findings (22). “Baker-Nathan” were papers published after 1956 and also concerned with solvents (24). Invocation of What Happened to the Baker-Nathan Effect? the Baker-Nathan effect in the chemical literature has dropped to practically nothing over the last 30 years, Today the Baker-Nathan effect is an historical foot- and some of those few papers that continue to treat it note remembered by those who received their training also continue to investigate the extent to which it can in physical organic chemistry prior to the mid-1970s. be attributed to solvent effects. For example, Conney During the mid-1950s there was great enthusiasm for and Happer looked at chemical shifts in 13C NMR in using the Baker-Nathan effect as a way of explaining alkyl-substituted styrenes. They found that the order of unexpected results when inductive effects were thought substituent effect did not change in a variety of solvents to be the driving force behind reactivity. One of the first ranging from non-polar to polar aprotic to polar protic, to challenge this explanation was a paper published by a result they interpreted as hyperconjugative because the Schubert and Sweeney in 1956. This paper with the in- solvent seemed not to matter much (25). Ten years later, 88 Bull. Hist. Chem., VOLUME 37, Number 2 (2012) a computational chemistry paper by Exner and Böhm in Figure 3 are “Ngrams,” plots of the frequency of oc- reported hyperconjugative stabilization in p-alkylbenzyl currence of a word or phrase in the corpus of scanned cations but not in the neutral parent , con- books and journals at the Google Books project (27). cluding that the Baker-Nathan effect in solution kinetics The plots show that even at the height of their popular- must be a solvent effect (26). ity, Baker-Nathan occurred in print more than an order of magnitude less frequently than did hyperconjugation.

Figure 3. Ngrams for the terms (a) Baker-Nathan and (b) hyperconjugation (27).

The decline of the Baker-Nathan effect, then, does Both terms have declined in frequency, which seems to not appear to be a simple matter of displacement by a bet- suggest a move away from their subject matter in physical ter explanation. In addition to questions about the source organic chemistry. Whereas Baker-Nathan has all but dis- of the effect, other factors appear to include a restricted range of the term’s usage as well as larger currents in appeared, hyperconjugation continues to appear in print. physical-organic chemistry. It is difficult to be definitive, but it would seem that Some researchers have made a distinction between the disappearance of the Baker-Nathan effect from chem- the Baker-Nathan effect and hyperconjugation, treating ical literature is due to a combination of doubt over the the latter term as a more general or theoretical concept utility of the concept in its original and rather restricted and the former as a term of more restricted application. realm of application (reactivity in solution) and a shift Exner and Böhm, for example, consider the Baker- in research interests in physical organic chemistry more Nathan effect as an empirical term restricted mainly to widely. The concept of hyperconjugation is still used in trends in reactivity (26). A related usage point can be seen certain contexts, and it remains a useful concept in educa- in the title of Conney and Happer’s paper, which refers to tion (28), even though the original Baker-Nathan effect Baker-Nathan order, that is, to an empirical result (25). has long passed into history of physical organic chemistry as a footnote. Although there is still controversy among Regardless of just how individual researchers have theoreticians about hyperconjugation, Baker and Nathan used the terms Baker-Nathan and hyperconjugation, the as a result of their 1935 experiments opened a new way former was never as widely used as the latter. The graphs to explain many puzzling problems in organic chemistry. Bull. Hist. Chem., VOLUME 37, Number 2 (2012) 89

Acknowledgments 8. J. W. Baker, Terpenes, Methuen & Co., London, 1930. 9. J. W. Baker, Tautomerism, G. Routledge & Sons, London, The author is grateful for the help supplied by Pro- 1934. 10. J. W. Baker, Hyperconjugation, Oxford University Press, fessor R. J. Bushby of Leeds University concerning the Oxford, 1952. life of J.W. Baker. He is also grateful to Dr. Peter Morris 11. J. W. Baker, Electronic Theories of Organic Chemistry, of the Science Museum, London for finding the picture Oxford University Press, Oxford, 1958. of W.S. Nathan. He thanks the anonymous referee for 12. J.W. Baker and W.S. Nathan, “Mechanism of Aromatic thoughtful suggestions and the editor, Carmen Giunta, Side-chain Reactions with Special Reference to the Po- for assistance in focusing the final section. lar Effect of Substituents. V. The Polar Effect of Alkyl Groups,” J. Chem. Soc., 1935, 1844-1847. 13. C. K. Ingold, ”Principles of an Electronic Theory of References and Notes Organic Reactions,” Chem. Rev., 1934, 15, 225-274. 1. R. S. Mulliken, “Intensities of Electronic Transitions in 14. J. W. Baker, W.S. Nathan, and C.W. Shoppee, “Mecha- Molecular Spectra IV. Cyclic and Hyperconjuga- nism of Aromatic Side-chain Reactions with Special tion,” J. Chem. Phys., 1939, 7, 339-352. R. S. Mulliken, Reference to the Polar Effect of Substituents. VI. The C. A. Rieke, and W. G. Brown, “Hyperconjugation,” J. Effects of p-Alkyl Substituents on Prototropy in the Amer. Chem. Soc., 1941, 63, 41-57. Methyleneazomethine System,” J. Chem. Soc., 1935, 2. For a biography of Thorpe see C. K. Ingold, “Jocelyn 1847-1849. Field Thorpe. 1872-1939,” Obit. Notices Fellows Roy. 15. C. W. Shoppee, ”Symmetrical Triad Prototropic Sys- Soc., 1941, 3, 530-544. tems. VII. Analogy between Symmetrical Triad Systems 3. For further information on Ingold see K. T. Leffek, Sir and Aromatic Side-chain Reactivity, and the Effect of Christopher Ingold: A Major Prophet of Organic Chem- p-Substitution on Mobility and Equilibrium in the αγ- istry. Nova Lion Press, Victoria, BC, Canada, 1996. C. W. Diphenylmethyleneazomethine System,” J. Chem. Soc., Shoppee, “Christopher Kelk Ingold. 1893-1970,” Biogr. 1931, 1225-1240. Mems Fellows Roy. Soc., 1972, 18, 349-411. 16. E. D. Hughes, C. K. Ingold, and N. A. Taher, “Mechanism 4. J. W. Baker and C. K. Ingold, “Formation and Stabil- of Substitution at a Saturated Carbon Atom. XVII. Polar ity of spiro-Compounds. X. spiro-Compounds Derived Effects in Alkyl Groups, as Illustrated by Solvolytic from cyclo-Heptane,” J. Chem. Soc., Trans., 1923, 123, Substitutions of p-Alkylbenzhydryl Chlorides,” J. Chem. 122-133. J. W. Baker, C. K. Ingold, and J. F. Thorpe, Soc., 1940, 949-956. “Ring-chain Tautomerism. IX. The Mutarotation of the 17. J. W. Baker and M. L. Hemming, “Mechanism of Aro- Sugars,” J. Chem. Soc., Trans., 1924, 125, 268-291. matic Side-chain Reactions with Special Reference to 5. J. W. Baker, ”Ring-chain Tautomerism. XIII. Three- the Polar Effects of Substituents. XI. The Aldehydecya- carbon Ring-chain Tautomerism in a Bridged Ring nohydrin Reaction: Evidence for the Mesomeric Effects System,” J. Chem. Soc., Trans., 1925, 127, 985-990. J.W. of Alkyl Groups,” J. Chem. Soc., 1942, 191-198. Baker, “Formation and Stability of spiro-Compounds. 18. A. Lapworth and R. H. F. Manske, “Conditions Deter- XII. Further Evidence for the Multiplanar Configuration mining the Thermodynamic Stability of Cyanohydrins of of the cyclo-Heptane Ring,” J. Chem. Soc., Trans., 1925, Carbonyl Compounds. I. Some Effects of (a) Substitution 127, 1678-1682. J. W. Baker, “Chemistry of Polycyclic in Aromatic Aldehydes and (b) Ring Formation,” J. Chem. Structures in Relation to their Homocyclic Unsaturated Soc., 1928, 2533-2549. Isomerides. VI. Some Reactions of isoPhorone,” J. Chem. 19. C. L. Deasy, ”Hyperconjugation,” Chem. Rev., 1945, 26, Soc., 1926, 663-670. J. W. Baker, “Nature of the Alternat- 145-155. ing Effect in Carbon Chains. XII. Nitration of Some De- 20. V. A. Crawford, ”Hyperconjugation,” Q. Rev. Chem. Soc., rivatives of Methyl Benzylaminoformate,” J. Chem. Soc., 1949, 3, 226-244. 1927, 565-571. J. W. Baker and C. K. Ingold, “Nature of 21. A. E. Remick, Electronic Interpretations of Organic the Alternating Effect in Carbon Chains. XV. Directive Chemistry, John Wiley and Sons, New York, 1943. E. Action of Some Groups of the Form –CR′R′′·COR in R. Alexander, Principles of Ionic Organic Reactions, Aromatic Substitution,” J. Chem. Soc., 1927, 832-836. John Wiley and Sons, New York, 1950. W. A. Waters, J.W. Baker and I. S. Wilson, “Nature of the Alternating Physical Aspects of Organic Chemistry, D. Van Nostrand Effect in Carbon Chains. XVII. The directive action of the Company, 1950. E. S. Gould, Mechanism and Structure in Organic Chemistry, Holt, Rinehart and Winston, New groups ·CH2·CH2·NO2, ·CH:CH·NO2 and ·C(NO2):CHR in Aromatic Substitution,” J. Chem. Soc., 1927, 842-848. York, 1959. L. F. Fieser and M. Fieser, Advanced Organic 6. R. J. Bushby, Leeds University, Private communication, Chemistry, Reinhold Publishing Corporation, New York, 3/20/2012. 1961. J. A. Hirsch, Concepts in Theoretical Organic 7. L. N. Mander, “Charles Wiliam Shoppee, 1904-1994,” Chemistry, Allyn and Bacon, Boston, 1974. Biogr. Mems. Austral. Acad. Sci., 2003, 14, 509-528. 90 Bull. Hist. Chem., VOLUME 37, Number 2 (2012)

22. R. T. Morrison and R. N. Boyd, Organic Chemistry, Al- 27. Google Ngram Viewer, http://books.google.com/ngrams/. lyn and Bacon, Boston, 1959. A. Streitweiser Jr. and C. Ngrams were generated for the time period 1930-2008 Heathcock, Introduction to Organic Chemistry, Macmil- for the terms “Baker-Nathan” and “hyperconjugation” lan, New York, 3rd ed., 1985. on Oct. 18, 2012. 23. W. M. Schubert and W. A. Sweeney, “Concerning the 28. J. J. Mullins, “Hyperconjugation: A More Coherent Ap- Baker-Nathan Effect,” J. Org. Chem., 1956, 21, 119-124. proach,” J. Chem. Educ., 2012, 89, 834-836. 24. Search of SciFinder database for Research Topic “Baker- Nathan” carried out Oct. 18, 2012. About the Author 25. B. T. Cooney and D. A. R. Happer, “The Baker-Nathan Order: Hyperconjugation or a Solvent Effect?” Austral. Martin D. Saltzman is Professor of Natural Science J. Chem., 1987, 40, 1537-1544. at Providence College, Providence, RI. 26. O. Exner and S. Böhm, “Baker-Nathan Effect, Hypercon- jugation and Polarizability Effects in Isolated ,” J. Chem. Soc., Perkin Trans. 2, 1997, 1235-1240.

The American Chemical Society Celebrates National Historic Chemical Landmarks ® Chemists and Chemistry that Transformed Our Lives

2012 Landmark Designations: DayGlo Fluorescent : Brighter, Longer Lasting Color This class of pigments based on uorescent and polymeric AL HIS ON TO TI R materials was developed between the 1930s and 1950s by A IC N scientists at Switzer Brothers, Inc. (now Day-Glo Color Corp.). 1992 2012

C The Legacy of Rachel Carson’s Silent Spring H S 20YEARS K E Carson’s book led a paradigm shift in how chemists practice M R A their discipline and helped to establish a new role for chemists IC M AL LAND in investigating the impact of human activity on the environment.

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