Marlin Saltsman
Prov~denceCollege Prov~dence,Rhode lslond 02918
James Bryant Conant and the Development of Physical Organic Chemistry
Few chcmist,~have had as varied and distinguished a career as that of James Bryant Conant. I n his recently published autobiography "My Several Lives" (I), Dr. Conant has describcd the cra in which he was active as a chemist as "not a great period in the history of American organic chemistry." This statement reveals a considerable amount of modesty by the pcrson who was so instrumental in the pioneering efforts to apply thc tcchniqucs of physical chcmistry to thc study of organic rcactions. With thc passing of time the contributions of James Bryant Conant to the devrlopmcnt of physical organic chcmistry have been somcwhat relegated to obscurity. I n the pcriod betweeen 1919 and 1934, over one hundred papers emanated forth from Harvard chemical laboratory in so many divcrsc areas that a reference to a t least one of thcse is to bc found in practicallyevcry chapter of Louis Hammet's pioneering volume "Physical Organic Chcmistry" (2). To discuss even a part of thcse studies would require a considerable monograph, and so only thosc showing a bridging of thc gulf of what Hammct has said "was almost a point of honor with both physical and organic chemists to profcss ignorance of the other's ficld" u d be revicwcd. This wedding of physical and organic chemistry by Conant was due in part to thc influence of two men, Thcodorc Richards and Emil ICohler, during the undergraduate ycars at Harvard. Conant had all intentions of pursuing doctoral studies in the arca of physical chcmistry undcr the dircction of Richards, when into his life entered Emil P. ICohler, the ncw professor of organic chcmistry. Having time for an undergraduate research project, Conant drcided to work with Icohler who was highly rccomrnended by his fellow students. "What was intended as an exploration of a neighboring ficld turned out to be my vork as a chemist." For his doctoral disscrtation (1916) Conant decided to present a dual thesis, doing a project with Richards in electrochemistry and a problcm in cyclopropane chemistry with ICohlcr. This unique combination was almost unprecedented and placed Conant as hc described it into "a rather special situation." The first evidence of the "rather special situation" was the appearance bcginning in 1922 of a series of papcrs applying the principles of electrochemistry to organic compounds. I n collaboration principally with Louis Fieser (5), the redox potentials of a series of quinones were measured. The results indicated that the clcctrochemical bchavior of organic compounds are in every way analogous to the wcll defined inorganic processes. Of particular interest was the study of irrcversiblc reductions (4) and oxidations (5). An example of an irreversible electrochemical reduction is that of 1.4-unsaturated diketones in acidic media.
The process consists of a reversible first step which was capable of being studied followed by an irreversible final step. In the study of many of the oxidations by electrochemical methods, Conant correctly predicted the intervention of free radical intermediates. Thus, w,B unsaturated ketones produce pinaconcs by thc transfer of an electron to the carbon atom of the carbonyl group, producing a free radical capable of dimerization. Phenols and naphthols produce dinuclear products as the result of the intervention of phenoxy radicals. The physico-organic techniques acre applied to the study of the rcdox potential of the hemoglobin-metbemoglobin system (6). One of the areas most vividly associated vith the chemical years is the study of the factors which influcnce the formation and stability of free radicals. Conant devcloped a convenient method of preparing substituted ethanes by the treatment of carbinols in acidic solution with vanadous chloride (7). A series of substituted dixanthyls (8) were prepared containing saturated non-aromatic groups, and their influence on the rate of dissociation was noted. Previous to t,his work only stable radicals of the triarylmethyl typc had been prepared principally by Gombcrg. The following ordcr of reactivity [CloH7> C6Hu > CBHU, (CH&CH] > (CsH&HJ > ( ~ - C & H ~ , ~ - C ~ H I ~ , C ~ H : , , C ~ H (CH2)3)was found. The influence of the secondary group on the rate of dissociation was attributed to steric factors causing crowding, a most remarkable cxplanation for 1926. Preparation of di-tcrt-butyl tetraphenylethane (1928) which was extremely labile, confirmed Conant's hypothesis that the "lability of a C-C linkage is to a large extent a function of the branching of the carbon chain." The results indicated that in non-induced free radical reactions, the rate controlling step is the dissociation into free radicals, in this case followed by the rapid reaction with oxygen. Thr ratcs of dissociation of substituted dixauthyls vere determined (1929) by using the absorption of oxygen as a measuring technique. The rates of dissociation of dixanthyls, where R=CH8, C2H5, C3HI, n-C4Hg, i-CIHn, were slow enough - to make accurate measurements possible. I n the years between 1931 and 1933, Conant studied the addition of free radicals (9), and the polymrrization nrocess (10). He noted that radicals combinr ~, with conjugated alkenes by 1,4 addition. With maleic anhydride 1,2 addition occurs, and Conant speculated briefly about the possible radical nature of thc Diels-Alder reaction. Among the achievements of the chemical ycars werr numerous pioneering structure-reactivity studies. I n a series of papers between 1924 and 1925, Conant interested himself in the question of relationship brVolume 49, Number 6, June 1972
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tween the structure of an organic halide and the speed of its reaction (11). As a model, he chose the reaction observed by Finklestein of the displacement of a halide by an acetone solution of iodide ion. Of particular interest was the study of halides of the type ACH2C1, ACHZCHZC~, and ACHzCHzCHzClin order to discover whether or not the "alternating polarity predicted by various electronic theories of valence would manifest itself in alternations in the reactivity of the halogen atom." Quantitative measurements of this second-order reaction were performed where A was CsHsCO, CzH,COO, CsHs and hydrogen. Whereas the above groups make AC1 unreactive, they activate ACHzCl by their "negativity." Conant found that as the suhstituent is moved to the p-position the reactivity falls to that of the normal halide. That the behavior of activating groups in nucleophilic displacements does not extend past the a-carbon atom was shown by an extended series of compounds. Secondary and tertiary halides were found to be less reactive and cyclohexyl was unreactive. I n those last years, (1930-33), several of Conant's most eminent pupils obtained their doctorate under his direction and mention should he made of their important contributions. With George Wheland (1g) the properties of very weak acids were studied. Equilibrium constants for the reaction of an alkali metal with a series of hydrocarbons, alcohols and amines by an indicator method were determined. The values ranged from a pKa of 16 for methanol to upwards of 35 for diphenylmethane. Paul Bartlett (13) engaged in a study of the kinetics of and structural influences in carbonyl addition reaction involving semicarbazone. The results indicated that the bimolecular addition
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was the rate determining step, with the dehydration following instantaneously. Acid catalysis was noted and the dependence of the rate of condensation with acidity was worked out. Various experiments were performed showing a wide variation in rates of formation as well as the equilibrium constant with structure. Frank Westheimer (14) one of Professor Conant's last students, extended the study to the rates of condensation in a variety of buffer solutions and confirmed the mechanism postulated. Although, Professor Conant's active chemical career ended when he was only forty his contributions during this brief period certainly qualify him for the appelation of one of the founders of the discipline of physical-organic chemistry. Literature Cited (1) CONANT. J. B., " M y Several Lives," Harper and Row. New York. 1970. L. P.."Physical Organic Chemistry." MoOraw-Hill. New (2) HIIMBT, York. 1940. (3) CONANT.I. B.. AND FIESER,L. F.. J . A m y . Chem. SOC..45, 2194 (1923): 46, I858 (1924). I. B., el al, J . Amar. Chem. Soo., 46, 1254 (1924); 48, 1066, (4) CONANT, 2468(1926): 49,1083(1927). J. B.. d ol, J . Amer. Chem. Soe.. 48, 3178, 3220 (1926): 52, (5) COXANT. A", \-"-",. no?", (6) CONANT. I. B.. A N D PAPPENEEIMER,A . M., J . Bid. Chcm.. 98, 57 (1932). and references therein. (7) con ha^. J. B., AND SLOAN,A. W., J . Amor. Cham. Soe.. 45, 2466
(1923). (8) CONANT. J. B.,el n l , J . A m r . Chcm. Soo.. 47, 572. 3068 (1925): 48, 1743 (1926).49,2080(1927). 50,204 (1928); 51, 1925 (1929). J. B.. el a!, J . Amw. Cham. Soc.. 53, 1941 (1931): 55, 3475 (9) CONANT. (1933). J..B., el al, J . A m ? . Chem. Soe.. 52, 1659 (1930): 54, 628 (10) C O N ~ T (1932). J. B., 91 a1, J . Amer. Chcm. Soc., 46, 232 (1924); 47, 476,488 (11) CONANT. (1925). (12) CONANT, J. B., A N D WREUND, G.,J . Amer. Chem. Soc., 54, 1212 (1932). P . D., J . Amw. Chem. Soc., 54, 2881 (13) CONANT,J. B.,AND BARTLETT, (1932). R.,.J . Amar. Chcm. Sor.. 56, 1982 (1934). (14) W ~ s ~ n s r x e F
