Louis P. Hammett Columbia University N e w York City
II
Physicd Organic Chemistry In Retrospect
W h e n 1 pickedthe title"Physica1 Organic Chemistry" for a book I was writing nearly 30 years ago, the name had not yet become a familiar one. So far as I know, 1 was indeed the first to use the name in the title of a book. But the subject was already in an active state of development. Yet when I had started to be a chemist 20 years earlier, the subject itself hardly existed. I do not think i t often happens that a branch of science grows from near nothingness to the present importance of physical organic chemistry within the professional lifetime of an individual. I n the feeling that there may he lessons to be learned from the history of this development, I have made it my subject. This will he a rather personal report: a story about things that have seemed important to me, and that have influenced my own thinking and attitudes. I do not know how to be impersonal about a development that has been a significant part of my professional and emotional life for 45 years-and still is. I n the preface to the book I said "For a time it was almost a point of honor with both physical and organic chemists to profess ignorance of the other's field." This represents litt.le if any exaggeration. To many physical chemists in the 1920's and early 19301s, the organic chemist was a grubby artisan engaged in an unsystematic search for new compounds, a search which was strongly influenced by the profit motive. And indeed I remember the remark of one prominent organic chemist that the great joy of organic chemistry was that one might start out to make a perfume and end up with a dye or a drug. On the other hand most organic chemists recognized a limited variety of physical chemists. One group sought to explain a phenomenon called a monomolecular reaction, whose very existence was in some doubt. Another studied the photochemistry of acetone. A third was concerned with electrolyte solutions so dilute that Wilder Bancroft could characterize them as slightly polluted water. I n terms of this kind of dichotomy my own situation in the early twenties could only be characterized as Award address on the occasion of the presentation of the James Flack Norris Award in Physical Organic Chemistry to Professor Hammett at the 151st Meeting of the ACS, Pittsburgh, Pennsylvania, March 1966. We print here the text of remarks prepared for oral delivery in the hope that readers will be able to share the pleasure of the listeners who thus heard a whole branch of chemistry become a personal autobiography. Portions of the text will form historical supplements to chapters in the forthcoming revision of Dr. Hammett's classic "Physical Organio Chemistry."
464 / Journal o f Chemical Education
ambiguous. I had behind me a year of organic research with Staudinger, a year and a half during the first World War of analytical control a-ork on organic materials and of research on cellulose acetate solutions, another year and a half of developmental research on dyes and pharmaceuticals, a doctoral research on the properties of the hydrogen electrode, and some years of teaching a course in qualitative analysis in which the classroom work consisted of an elementary treatment of the physical chemistry of electrolyte solutions. This history was the result of an almost ridiculous series of accidents, but it gave me a breadth of experience which was unusual. It had exposed me to some highly original and stimulating people: Kohler at Harvard; Staudinger at Zurich; Nelson, Beans, and Kendall a t Columbia. I was also profoundly influenced by four men whom I had discovered in my reading. Two of them, Hantzsch and Werner, I never met; the others, Lewis and Brpinsted, I met only many years later. I sometimes think we have forgotten that a young scientist can learn from an individual without doing a postdoctoral hitch with him. I n 1928 I published my first paper in the field of what is now called physical organic chemistry, an interpretation of Hantzsch's investigations on acids, and about the same time started two students, Dietz and Deymp, on experimental work on acid-base systems. Conant had been working in the field for some years, his first publication being Fieser's doctoral thesis in 1922 on the redox potentials of substituted quinones. Ingold's first quantitative paper was the Gane and Ingold report on the hydrolysis rates of esters of dibasic acids. James Norris published important pioneering work in 1927 and 1928. I recommend his paper on the quarternization of pyridine (1) to the earnest considerationof those chemists who still hope to explain all of the effects of changing solvent in terms of the dielectric constant. By the late thirties when I was writing my hook, the number of people active in the field had increased considerably. Conant had become President of Harvard, but his student Bartlett was a leader in the study of reaction mechanisms. His paper with Tarhell on the stilbene-bromine reaction ($) is the classical example of the investigation of a reaction intermediate by way of the distribution of the ultimate reaction product between various sustances. Pedersen at Copenhagen, Bell and Wynne-Jones in England, Kilpatrick in this country, all of whom had worked with Brfinsted, were active. So were Lucas at the California Institute of Technology and his student Young at UCLA, and Branch, Olson, and Stewart at Berkeley. People have sometimes told me that the book helped
to accelerate this developmcnt, cven to influence its direction. It would be nearer the t,nit,hto say that it rode the crest of a tide whose progress was inevitable. Indeed, I think some of the old guard would have described it a bit unhappily as "irresistible." The course of this tide reminds mc of a plot of the function e" against z, a very charact,erist,ic feat,ure of which is that, if one plots cnough of this function on any reasonable scale, there is a long range in which the ordinate cannot be distinguished from the s axis, but in which nevertheless it is really multiplying just as rapidly as it does later when the growt,h is obvious. One extremely important process that went on during the period when nothing secmcd to be happening was thc undermining of a number of obsessively held convictions. Some of these mcre: (1) there cannot possibly under any conditions be any relation between the rates of a group of reactions and the equilibria. of the same group; (2) it is scient~ifically immoral to talk about a reaction mechanism involving intermediates which cannot be isolated; (3) aridity is determined by hydrogen ion concentration, and a solntion of HCI in benzene is not acid becausc it docs not conduct and cannot therefore contain hydrogen ion; (4) there are two isomeric sodium salts of things lilic nitromethane or acetoacetic ester-onc wit.h sodium attachcd to oxygen, the other with sodium attached to carbon; (3) sulfuric acid converts alcohols t,o et,hers and olefins because it is a po~verfi~l dehydrating agent; (6) thirigs like bromine add simultancr~uslyt,o hot,h ends of an olefinic double bond; (7) entropy effc~:tscan be neglected in the problem of st,ruct,ureand rcact,ivit,y. Except for the last, these idcan nre as dcad as phlogiston; those of you who are less t,h:m 40 ycars old have probably never heard of them; but ~ ~ o of n cthcm died easily. Instead of describing the tlet,:~ils of their terminal illnesses, I prefer t,o talk about the growth of poskive ideas. Three regions of gro\vth seem t,o me to hnvc becn particularly important,. First Kinetic Investigations of Reaction Mechanism
One import,ant region involved thc (levelopnimt of complcte corifidence in the gcncralieed Law of Mass Action, which may he put: I n dilut,e systems the raic of every chemical rcaction is proportional t,o the prodnot of the concentratiotin of the suhst,ances which arc reaetiiig, and is indcpcnclent of the cori~:entrationsof all other sirbst,ances anrl of the presence or ahserice of all other reactions. This gericralization of the Guldberg and Waage observation of 181i7 was clearly foreshadowed in a remarliahle paniphlet hy van't Hoff in 1884, Eludes de d:ynamique ehimique. I n this he int r o h r e d the dassifiration of n:ar:t,ions into first, srcond, and third ordcr; hc revognizcd the rarily of thc third-order reaction arid t,hr ahsenl:e of higher orders than third evert whcn the st,oichio~nririlrcal:tion involves many n~olecules;and he con(.ludrd that in such cirses t,he rcar.tion musi talx plaw in steps involviug t,he formation and further reaction of interniediatcs. During thc 1890's ihc validity of the generaliao~l 1,:tw was n major suhjsot of invrstigation in the Ostwald s1.1iool of physical chemistry. In 1800 W1. 1iisti:tI i o ~ s k ypublished iu Russiitr~jourrials t,hc results of au invrstigation, suggested by Ostwald, whkh was au
excellent study of the kinetics of thc rcverihle reaction of ester formation and ester hydrolysis catalyzed by dilute acids. Its prime purpose was to deteniiine if the net rate can be represcnted as tlie algebraic sum of tlie rates of two completely indepenrlrnt but opposing reactions, and it succeeded admirably in demonstraliug that this is the case. The work was republished in Gcrman in 1898 (5). I n 1899, Bodenstein rerified the independence of the opposing react,ions Ha + IZ 2HI
=
in the gas phase (4). Incidentally, Bodellstein rrrognized how difficult it is t,n find liinctically simple real:tions in the gas phase; i.e., reactions !!-hose rate is proportional to a product of integral powers of roncentre tion. Much later, in a ret,rospet:tive lecture (j),he remarked that "abnormal reaotion courses" have again and again been observed, "especially ill gas reactions in which-in contrast to those in solutiou-they far outnumber the cases of derent' ones." A paper by Federlin (fi) reported work sponsored by Luther and influenced by Ostwald aud Bodenstein. This was a kinetic study of the reaction of IiSrOa and H3P03 catalyzed hy HI, which ronclutled that the reaction rate can he completely arconnted for as the resultant of two independent reactions of ronipanihle rat,e: one the reduction of persulfate by iodide, the othcr the oxidation of phosphite by iodine. I n an interesting discussion he remarlied that this kind of scquence was being talked about as an explanation of all catalysis, but that Ost~valddid not ngree and emphasized "that purely catalytic effects can ocrur." Indeed : Ostwsld is $80 of tho opinion that it is not possible to oplr~in negative catalysis through the assnmption of intermediates. I think however that there is no obstacle to the interpretntion of rate decreases by way of intermediate reactions, nnd would exchin this ss follows: The negative catalyst decreases, perhaps through combination, the conccnt,rittion of intermediates present. only in traces and produces therehy an observable decrease in rate.
I find it worthy of not,e hot,h that a student should disagree in the first person with the Professor in tlie Germany of that day, and that the disagreement, should he published in a journal founded by that Professor. I also note with interest that Federlin's nanie does not again appear in the indexes of the Zentralblatt. Yet only two years later Bredig (7) reported a stndy of the benzoin condensation, a reactiou in d i i c h two molecules of benzaldehyde i,ombine under the sprrific catalytic effect of ionized cyanides. He found the reaction to be second order in Fenzaldehyde, and to a good approximation first order in cyanide ion, and concluded under the heading "Die Frage nach clan Chemismus der Reaktion": "One must reject all hypotheses which are not consistent with the kinetic laws of the reactions." There is no hedging about t,his; the generalized Law of Mass Action is accepted without reservation or hesitation and is used with complet,e confidence as the basis for judgments about the niecha, nism. Bredig is not in the least worried by the fact. that he has to assume the formation and further reactiorl of My translation; theGerman word w a brauen. Volume 43, Number 9, September 1966
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465
intermediates whose presence he cannot directly detect, and he has not the slightest doubt that the catalyst is a substance which enters into reaction in the ratedetermining step and is later regenerated. Almost simultaneously two remarkable papers by Lapworth appeared (8). I n the first, on the basis of qualitative observations on the effect of acids and bases on the cyanhydrin reaction, he proposed that the reaction goes in two steps, a relatively slow first one R,CO
+ CN-
-
R2C(CN)O-
and a rapid second one, R,C(CN)O-
+ HCN = RC(CN)OH
The other paper was a quantitative study of the rate of hromination of acetone in acid solution. The stoichiometric process is CHZCOCH.
+ Br2
-
CHsCOCHIBr
+ HBr
but the rate is proportional to the product of the concentrations of acetone and hydrogen ion and is independent of the concentration of bromine. Lapworth concluded: I t may be ohsenred that the independence of the velocity of reaction on the concentration of bromine shows clearly, first, that the reaction proceeds in a t least two stages, in one or more of which the bromine is not involved, and, secondly, that in the stage or stages in whioh bromine takes part, the velocity of reab tion is so great that the time occupied is not measurable. . t h e observations as to the influence of acids of different concentration are best explained on the snpposition that in this reaction [the one thevelocity of which is measured] one hydrogen ion is involved. [Further] It is clem itlm, that the independence of the speed of reaction on the concentration of bromine shows that the velocity with vhieh the seeand farm of the acetone is brominated must be ir~comparahlygreater than that of the reverse change of the labile to thenormal form.
..
These papers of Federlin, of Bredig, and of Lapworth established the whole technique of the kinetic investigation of reaction mechanismon afirm basis. And then a remarkable thing happened; the whole development stopped almost completely for twenty years or more. There were a few kinet,ic investigations, but they were isolated, out of the main stream of chemical activity, and unnoticed by the great mass of chemists. I n 1919 at Staudinger's suggestion I studied intensively Henrich's "Theorien der Organischen Chemie," which was the bible of the theoretical organic chemist of that day, and remained completely innocent of the knowledge that mechanisms can he investigated by way of kinetic studies. I have recently looked over the 1920 edition of the same hook and, if the index can be trusted, it contains no reference to the Bredig or the Lapworth work to which I have just referred. Indeed, if there is any reference to a kinetic investigation of anything I failed to find it. Renaissance of Kinetics
And then just as suddenly Ingold and Bartlett and Pedersen began studying reaction mechanisms in the late twenties and early thirties. I am no more certain about the reason for thesudden upswing in this kind of work than I am about the reason for its sudden cessation a generation earlier. I n many ways the new group started out just where Bredig and Lapworth had stopped. They appeared to accept without question or comment the generalized Law of Mass Action; they 466
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Journal of Chemical Education
did not feel it necessary to isolate an intermediate in order to establish a mechanism. There were however still doubters. I n 1932, Bray wrote (9): It is generally realised by chemists that many homogeneous reactions take place in steps, and that the order of reaction, determined by rate measurements, designates the molecules that react in or before the rate determining step. However, there is undoubtedly much skepticism about the possibility of identifying the intermediate compounds formed in and after the rate-detemining reaction.
I suspect that the confidence of the men of the renaissance in solutions kinetics was considerably bolstered by developments in the parallel but usually isolated culture of gas kinetics, and especially by the work of Bodenstein. Bodenstein (5) had not only found and verified in a number of ways mechanisms which explained the complicated kinetics of the react,ions of hydrogen with chlorine and with bromine, but he had developed a technique of mathematical approximation, commonly (hut I think inaccurately) called the steady state method, which represented a great advance in the quantitat,ive treatment of complicated reaction systems. How much this did to trigger the renaissance in the study of kinetics and mechanisms of reactions in solution I do not really know. Chemists working on kinetics in solution and those working on kinetics in the gas phase have tended to be intolerant of each other and communication has been decidedly limited. Certainly however, many physical organic chemists in the early thirties knew of the Bodenstein approxim* tion. Shortly after this sudden rebirth of interest in reaction mechanisms in solution, two development,^ occurred which helped greatly to maintain the activity. One was the availability of isotopes. So far as I know the first use of isotopes in physical organic chemistry was in the determination by Polanyi and Szaho (10) that in the alkaline hydrolysis of amyl acetate the ether oxygen stayed with the alkyl group, not with the acyl group. The other was the recognition of the prevalence and importance in organic chemical reactions of the displacement reaction, the process A BC AB C, in which the rupture of the BC bond is coexistent with the formation of the AB bond. The idea of "simultaneous addition and dissociation" in connection with the Walden inversion had been proposed by Werner in 1911 (11) and was supported by Lewis in 1923 (123). But I'olanyi in 1931 (15) compared the value of 11 kcal for the activation energy of the reaction H H-H H-H H, which had heenobtained from the study of the catalysis by hydrogen atoms of the ortho-para hydrogen conversion, with the 100 kcal required for the dissociation of the hydrogen molecule. Eyring and Polanyi (14) showed that the idea of the displacement process can be given theoretical respectability in terms of ideas which had been presented by London (15). Both Polanyi (16) and Olson (17) applied this concept to the Walden inversion. The Walden inversion had been a source of mystification since its discovery in 1895; indeed it seemed to me during the thirties that some of the workers in the field felt a vested interest in maintaining the confusion and resented attempts at clarification. But with the new
+
-
+
-
+
+
ideas Hughes (18) had by 1938 brilliantly proved by the use of radioactive labeling that all kinetically second-order displacements on carhon invert the configuration. I n 1937 Hughes and Ingold (19) and a little later Steigrnan and Hammett (20) obtained evidence that the kinetically firsborder solvolytic reaction also goes predominantly with inversion although there may be much racemizatiou. With the clarificationby Cowdrey, Hughes, and Ingold (21) of the kinetics and the stereochemistry of the reactions of the halogen acids, the fog was in principle completely cleared away from the Walden inversion and the practicability of stereospecific synthesis had become apparent. Also in the early thirties, Polanyi extended the displacement reaction idea in the direction of predicted parallelisms between the activation energy of a series of reactions and the strengths of the bonds being formed and being broken, a direct relation for the former and an inverse one for the latter. And a statistical mechanical approach to the idea of reaction rate as the resultant of an equilibrium between reactants and a transition state and a universal rate of conversion of the transition state to reaction products was given detailed form by Eyring (22). Before this development, physical chemists could be and were scornful of the organic chemist for thinking that he could treat the effect of structure, of medium, and the like on rates and on equilibrium in the same terms. They no longer could do this. Correlations of Structure and Reactivity
A second extremely important region of growth involved the relation between structure and reactivity. As late as the mid-twenties, it was possible for thoughtful colleagnes of mine a t Columbia to argue that the organic chemists' pictures of molecular structure were merely a sort of shorthand for recording the reactions of the substances pictured. But by 1930 the pictures had been found consistent with evidence from X-ray and electron diffraction, from dipole moments, from monomolecular layers, and from spectroscopy. One could even set down figures of precision for the interatomic distances within molecules and for the spacefilling properties of molecules of various shapes and sizes. Lewis' remarkable insight in recognizing the electron pair bond in 1916 was at first in stark contrast to the prevailing ideas of physics, but the chemical and the physical points of view had been reconciled by 1928 through the development of quantum mechanics. Pauling's resonance concept in 1931 erased the last broad area of deep mystery in the valence theory of carbon compounds by accounting for the special properties of aromatic compounds. As a result of all this the chemist could in the early thirties have a sense of security in his models of molecular structure that had previously been absent or incomplete. Meanwhile English chemists, notably Ingold, had been developing a kind of confidence in the possibility of a priori prediction of the effect of structure on reactivity which I have never been able to share. I had, however, developed a strong hunch that the organic chemists' basic rule that like changes in structure produce like changes in reactivity could be made quantitative. By 1933 I was pretty weU convinced that this is in-
deed the case and that the relation involved is what is now called the linear free energy relationship. The parent of all relationships of this kind is the discovery by Br@nsted and Pedersen (28) of general acid and base catalysis and of the rule that the logarithms of the rate constants of the catalyzed reactions are linearly related to those of the acidity constants of the catalyzing acid or base. Pedersen ($4) in 1931 clearly recognized that this is a relationship between the rates and the equilibria of the same series of reactions, i.e., the proton transfer process. I n 1933 Pfluger and I (25) extended the idea by finding a linear relation between the logarithms of the specific rates of the reactions RCOOCHs
+ N(CH&
-
RCOO-
+ N(CHa).+
and those of the equilibrium constants of the reactions RCOOH
+ HnO
RCOO-
+ OHs+
i.e., a relation between the rate constants of one reaction and the equilibrium constants of a different but closely related reaction. And in 1935 both Burkhardt (26) and I (27) found a whole flock of linear free energy relationships in the reactions of substituted benzene derivatives. With respect to the benzene derivatives I was able (28) to systematize these relationships in a simple equation which seems to have met with considerable favor among chemists. It would be what Wright called hypocritical humility for me to pretend that I am not pleased that this is often called the Hammett equation. I can say honestly that I did not myself propose and that I have never consciously propagandized this nomenclature. There are two things I should like to emphasize about the equation. First, it was set up on a purely empirical basis. Indeed it is a much simpler relationship than valence theory current a t the time of its origin would permit, although it was later shown by Jaff6 (g9) to be consistent with molecular orbital theory. Until recently the statistical mechanics of the situation bothered me considerably, but I now see no serious diEculty. For me the empirical nature of the equation is a source of pride rather than of shame. I think we tend too much to deprecate the role of empirical generalization in the advance of science. In the second place, the equation is a first approximation, not an exact relationship. Again I make no apology. The setting up of approximate relationships is sound and effective scientific procedure. Through the contributions of Brown, Taft, Wepster, and of Tsuno we now know how to apply an effective second approximation, the Tsuno-Yukawa equation. It seems to me particularly important that this equation requires only two parameters to describe the interaction between a substituent and a reaction zone, not the four kinds of effect proposed in Ingold's publications or the half dozen or more involved in some current discussions of the effect of substituents on the reactions of benzene derivatives. The empirical relationship therefore sets definite boundaries to the ltind of theory that can usefully be employed. The Acidity Function
The third area of growth I should like to discuss Volume 43, Number 9, September 1966
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467
involves the properties of acids and bases. I n 1923, perhaps even a bit later, I was contentedly telling students in the course in qualitative analysis that HC1 is less acid in benzene than it is in water because it is less ionized, and that the failure of the beuzene solution to react with calcium carbonate demonstrated this. By 1927 I was satisfied in my own mind that: (1) water is a base in the same sense that ammonia is a base; (2) the so-called hydrogen ion in water solution is OHs+ just as it is NHl+ in liquid ammonia; (3) measured by any homogeneous equilibrium or rate phenomena HCI is more acid in benzene than it is in water; (4) since water is a base it masks or levels the differences in strength of strong acids, and prevents any determination in aqueous solution of the relative strengths of weak bascs. Consequently the study of strong acids or of weak bases requires the use of solvents less basic than water; ( 5 ) acid indicators and basic indicators are differently affected by a change in solvent; (6) the concept of hydrogen ion activity has no operational meaning, and a hydrogen ion concentration cell which includes a junction between solutions in two different solvents has at most a very vague significance. On the basis of these ideas an extraordinarily able graduate student, Alden Deyrup, and I set out to obtain a measure of the changing acidity of systems ranging from water to 100Yo sulfuric acid. We were able to find a series of basic indicators of overlapping range which made this possible and we described the scale of acidity thus obtained by the acidity function Ho. We also found that this scale helped materially in understanding the effect of strong acids as catalysts and in uprooting the myth that acid condensing agents are such because they are dehydrating agents. At the time Deyrup and I were doing these things, we had to use visual indicators and a visual colorimeter. We did not have access to any kind of spectrophotometer. Even when a decade later Flexser and I got the use of a spectrophotometer, it was one which required a week or so of hard eye-straining work to get the absorption spectrum one now gets from a recording instrument in a few minutes. I t was the limitations of the tools we had, not any deliberate choice, that led us to the selection of indicators most of which were nitroanilines. We now know from the work of Deno, Arnett, and others that this was fortunate, that if we had had a wider choice we might well have been completely bewildered. As it is we now have a clear pattern which is, I think, leading us to a useful understanding of the factors involved in many important systems. Again I feel no need to apologize for having set up initially what has turned out to he an incomplete treatment. My approach some 40 years ago to the study of these strong acid systems derived more than anything else from a series of papers by Hantzsch in the Zeitschrift fur Elektrochemie in 1923 and 1924, although I was also strongly influenced by Br6nsted and by Franklin. I did not happen to know of Lowry's contribution to what is often called the Br6nsted-Lowry theory of acids and bases until later. For my taste this would be the Hantzsch-Br#nsted-Lapworth-Lowrytheory of acids and bases if I felt it necessary to give the names of individuals to theories. I prefer to call it the protontransfer theory. Hantzsch was a remarkable scientist. By the stand468
/
Journal of Chemkol Educotion
ards of conventional organic chemists alone, his accomplishments were outstanding. But he kept making unconventional excursions into the field of physical chemistry which often irritated the physical chemists because the results were inconsistent with cherished opinions. They usually retaliated by pointing out that Hantzsch's quantitative methods mere sloppy. But Hantzsrh's work on acids, especially on solutions in sulfuric acid, was not only pioneering-it mas sound in concept, in execution, and in its conclusions. I owe it a great debt. You may therefore imagine the lift I, the beginner in science, got from a letter Hantzsch sent me after the appearance of my first paper on the acidbase question. This was to the effect that he was greatly pleased with the quantitative treatment I had given his ideas; that he had had great difficulty in persuading physical chemists to pay any attention to his work; that some of them might fail to note my paper; would I be good enough to send him some reprints so that he could be sure that this did not happen. A Look Ahead
The 395 page book I wrote in the late 1930's coutained about 40 pages concerned with structures as opposed to reactions, including 5 on nlolecular orbital theory. It contained 27 pages concerned with atom and radical reactions. By 1957 "Free Radical Reactions in Solution" descrved a BOO-page book by Walling, and in 1961 "Molecular Orbital Theory for Organic Chemists" deserved a 460-page book by Streitwieser. The major portion of my 1940 book, and indeed the almost exclusive content of the field of physical organic chemistry at that time, was the subject of rates, equilibria, and mechanisms of reactions of the heterolytic type. The growth of that subject has been only somewhat less spectacular than the growth of the suhjects of radical reactions and of molecular orbital theory. But the subject has changed a great deal. Much that was tentative 30 years ago has become assured, closer approximations have replaced relationships that were then crude, and both the subject and its practitioners have matured considerably. The time seems to me ripe for a thorough-going reassessment of this portion of physical organic chemistry, and such a reassessment has been my chief scientific activity during the past few years. I have found it a fascinating occupation, nearly indeed as exciting as was the preparation of the 1940 book. The field is still in fact far short of the dismal goal to which Katchalsky refers (SO): Whether we like it or not the ultimate goal of way science is to become a well-controlled apparatus for the solution of schoolbook exercises or for practical application in the constmction of engines.
That goal is still distant for this field when we can be as completely surprised as we were a few years ago by the discovery that reactions involving bases can go loL3times faster in dimethylsulfoxide than they do in methanol. And the time does not seem near when we shall he able to make a schoolbook exercise out of the prediction of what substance will he a catalyst for uvhat reaction. So for many years to come, I am sure, physical
organic chemistry will continue to he fun, as it has been during all the time I have been a chemist. Literature Cited
(1) NORRIS,J. F., AND PRENTISS, S. W., J . Am. Chem. Soe., 50, 3042 (1928). (2) BARTLETT, P. D., I N D TARBELL, D. S., J. Am. Chem. SOC., 58,466 (1936). W., Z. Physik. Chm., 27,250 (1898). (3) KISTIAKOWSKY, (4) BODENSTEIN, M., 2.Phy~ik.Chm., 29,295 (1899). (5) BODENSTEIN, M., 2.Elektrochern., 38,91 (1932). (6) FEDERLIN, 2.Physik. Chm., 41,565 (1902). (7) BREDIG,G., AND STERN,E., 2.Elektrochem., 10,582 (1904). A., J . Chem. Soc., 83,995 (1903); 85.30 (1904). (8) LAPWORTH, (9) BRAY,W. C., Chem. Rev., 10,161 (1932). M., AND RABO, A. L., Trans. Faraday 8% 30,508 (10) POLANYI, (1934). A,, Ber.,44,873 (1911). (11) WERNER, (12) LEWIS, G. N., "Valence and the Structure of Atoms and Molecules," Chemical Catalog Co., New York, 1923. (13) EVANS,M. G., AND POLANYI, M., Trans. Faradey Soe., 34, 11 (1938). M., 2. Phgsik. Chen., BIZ, 279 (14) EYRING,H., A N D POLANYI, (1931).
(15) LONDON, F., 2.Elektrochem., 35,552 (1929). (16) MEER,N., AND POLANYI, M., 2. Physik., B19, 164 (1932). A. R., J . Chem. Phys., 1,418 (1933). (17) OLSON, (18) See COWDREY, W. A., HUGHES,E. D., NEIELL, T. P., . ~ N D WILSON,C. L., J . C h m . Soc., 209 (1938). (19) HUGHES,E. D., AND SABPIRO,U. G., J . Chem. SOC.,1192 C. R., AND MISTER(1937); HUGHES,&. D., INGOLD, S.,J . Chem. SOC.,1196 (1937). MAN, (20) STEIGMAN, J., AND HAMMETT, L. P., J . Am. Chem. Soc., 59, 2536 (1937). (21) COWDREY, W. A., HUGHES,E. D., A N D INGOLD, C. R., J. Chem. Soc., 1208 (1937). (22) EYRING,H., J . Chm. Phys., 3,107 (1935). (23) BR$NSTED, 3. N., AND PEDERSEN, K., 2.Phgsik. Chen., 108, 185 (1924). J.Am. Chem. Soc., 53,18 (1931). (24) PEDERSEN~K., H. L., AND H,WMETT,L. P., J . Am. Chm. Soe., (25) PFLU~ER, 55,4079 (1933). G. N., Nature, 136,684 (1935). (26) BWRKHARDT, (27) HAMMEW, L.P., Chm. Rev., 17,125 (1935). HmME~T!L.P.r J.Am. Chem. 59, 96 (29) JAFFO,H. H., Chem. Rev., 53,191 (1953). A,, International Science and Technulogy, (30) KATCHALSKY, October, 1963,p. 43.
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