Martin Saltzman
Providence College Providence, Rhode Island 02918
J. J. Thomson and the Modern Revival of
Throughout the nineteenth century a hitter internecine warfare existed among chemists as to the nature of chemical affinity. Jons ~ a k o bBerzelius proposed (1819) that the origin of affinity was electrical in nature, a deduction based on the results of the electrochemical studies of Humphry Davy. To Berzelius the dualistic conception of chemical affinity seemed the preeminent rationalization (1).
If these electrochemical views are correct it follows that all chemical forces depend solely on two apposing farces, positive and negative electricity. Whence it follows that each compound substance, regardless of the numher of its constituent principles, may be divided into two parts, of which one is electrically positive and one is negative.
The dualistic theow. implied that affinity was the result . of charges and these only, and when applied to the realm of organic chemistry it failed miserably to account for the course of certain- substitution reactions. Dumas had shown, for example, that hydrogen in acetic acid could he replaced by chlorine to form trichloroacetic acid, which was absolutely contradictory to dualistic theory since hydrogen was positive and was being replaced by a negative chlorine. Thus by the middle of the nineteenth century dualism had been discarded in favor of a unitary theory principally advocated by Dumas, Gerhardt, and Laurent. Chemists were faced with two divergent theories of valence, one recognizing only the polar properties of honds and the other only the nonpolar. The picture of affinity became muddied in the last decade of the nineteenth century with twin discoveries of the electron (1894) by J . J . Thomson (1856-1940) and the unusual properties of complexes as determined by Alfred Werner. Thomson's Discovery of the Electron and the Revival of Dualism
Throughout the first two decades of the twentieth century Thomson was preoccupied with attempting an explanation of the nature of the chemical hond. His first recorded s~eculationsappeared in 1904 (2) in the printed text of t'he Silliman ikctures he delivered in 1903. To Thomson. the discoverv of the electron meant that Berzelius was vindicated-all bonding occurs by development of charge ( 2 ) . The ability of an element t o enter into chemical combination depends upon its atoms having the power of acquiring a charge of electricity. The view that the forces which bind together the atoms in the molecules of chemical compounds are electrical in their origin was first proposed by Berzelius. Chemists, in general seem, however, to have made little use of this idea, having apparently found the conception of "hands of affinity" more fruitful. Thomson interoreted the hond of the chemist to he a tube of force emanating from one atom and terminating in the other. thus constituting" a . nhvsical . link. The number of tubes would determine that mysterious property of atoms, valence, and the length, the degree of ionization. "In the electrical theory, however, there is a difference
between the ends, as one end corresponds to a positive and the other to a negative charge" (2). The loss or gain of a corpuscle of electricity (electron) created charged species and hence after almost a century dualism was resurrected. Harry Shipley Fry (1878-1949), a n American advocate of Thomson's dualistic theory, stated in its defense (3)
... that, if the farces which hold the atoms together in electrolytes are electrical, then the same forces must he assumed to hold in combination the atoms constituting molecules of nonelectrolytes.
Hence, it may be maintained that chemical reactions which involve the dissociation of molecules, either of electrolytes or of nonelectrolytes, are, let us say, electronic. Thomson's thinking was to undergo a n evolution which allowed for this dualistic theory being modified, hut the proposal of 1904 was most readily adopted by organic chemists without modification. This revival of dualism was eagerly seized upon by a school of chemists in the United States which advocated an interpretation of organic reaction mechanism based upon the "electronic conception of valence." Dualism and Orientation in Benzene
In a series of papers commencing in 1912 (4) Harry Shinlev Frv a t t e m ~ t e dto a n ~ l vthe dualistic theorv of Thomiondtothe prohiem of ori&tion in benzene (3) No single subject in the history of modern chemistry has received more attention than that of substitution in the benzene nucleus. Concerning the introduction of two and three substituents, the foremost authority, A. F. Holleman (5) (1911),states that he has found 1,300 memoirs. Despite the immense amount of work accomplished in the field, Hollernan states that no rule or theory has as yet furnished a satisfactory explanation of suhstitution in the benzene nucleus. To Fry the theory of Thomson would predict that the benzene nucleus has positive and negative charges residing on alternant carbon atoms. This seemed quite reasonable, since J. J. Thomson had stated in 1907 that "atoms of one and the same kind may become either positively or negatively electrified and "those with charges of opposite sign would combine to form a diatomic molecule" (6). Evidence for species such a s positive chlorine had been furnished by Noyes and Stieglitz (7) and thus bolstered Thomson's contention. Fry introduced the term "electromet" to account for the nature of the benzene nucleus as interpreted by KekulC.. The presence of a suhstituent would favor one electroner over another as the suhstituent would determine the distribution of the charge. Thus a halogen would he negatively charged and the nitrogen of a nitro group would he positive and fix the signs in the ring. Negatively charged suhstituents would produce hydrogen atoms with positive signs a t the ortho and para positions, which would be replaced by a reacting electrophile. A positively charged suhstituent adjacent to the ring would change the sign preference and thus cause the substitution to occur a t the meta position. The results Volume 50, Number 1, January 1973
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were consistent with observations except for the nagging evidence that ortho and para products were found in mixtures that should be exclusively meta. Problems with Literal Dualism Vorlander (8) stated that the observation of Fry led to a set of consistent rules based on polarity of the suhstituent. Whereas Fry's application of dnalism produced what appeared to be the answer to a question that had long perplexed chemists, the Americans Nelson and Falk (9) were to reach an abyss in their application of dualism. To Nelson and Falk. the chemical bond was a comulete transfer of electrons producing charged species. One can see the objections to this interpretation of dualism in the comparison of carbon tetrachloride and sodium chloride. In their electrochemical interpretation, chlorine should he negatively charged in carbon tetrachloride just as in sodium chloride. The behavior of chlorine in carbon tetrachloride is in complete contradiction to that of chlorine in sodium chloride. Further problems can be seen in the bonding in methyl chloride, in which carbon has a net negative charge of 2 and chlorine a net charge of -1, which balance out the three positive charges of the three hydrogen atoms. But from which particular atom did the electrons come could not be answered. Atoms could be simultaneously losing and gaining electrons. Clearly, this literal interpretation of J. J. Thomson's. theory proved inadequate. By the year 1914, all the evidence pointed to the nature of the chemical bond being electrical. Are these electrons completely transferred or is there some mechanism by which bonds may be "electrical in their origin, binding the two systems together without a resultant charge on either system?" (10). Almost simultaneously the dualistic theory after having evolved for a century floundered in its application to systems which are relatively nonpolar, i.e., organic molecules. Vestiges of this new resurrected dualistic theory of affinity remained in the thinking of organic chemists. In the period between 1920 and 1925 several theories, sometimes labeled as theories of "alternate polarities," were proposed by chemists of the English school principally by Lapworth (11). Robinson (12), and 1,owrv 1131. --, . , The theories of "alternate polarities" recognized the neeessitv wstulatine what anneared to be full-blown elec~ ~ of ~ ~ .. trical charges in the reactions of conjugated systems to explain the behavior of these systems. The view of Fry was modified by Lapworth and Robinson in that they proposed electrical charges developed a t the moment of reaction in conjugated systems. The course of the reaction would be determined by the most electronegative element in the system, which could serve as a key. Lowry recognized the distinction between single and double bonds when he stated in 1923: "whilst a single bond may be either a covalency or an electrovalency, a double bond in organic chemistry usually reacts as if it contained one covalency and one electrovalency." Thus Lowry successfully interpreted the essential compatibility of polar and nonpolar bonding in an organic system. A reaction of a conjugated system involved the ability to develop a series of alternating charges in the activated state, and in the "resting" state the system was a series of alternate single and double bonds. All these theories recognized the polar nature of organic reactions and the need to account for it. ~
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Evolution of Dualism The application of the modern dualism of J. J. Thomson to the interpretation of organic reaction mechanism led the way to and proved the validity of the electronic theory of bonding as enunciated by G. N. Lewis and I n ing Langmuir. The morass that literal dualism led to when applied to organic systems in the twentieth century 60 / Journalof Chemical Education
made it evident to G. N. Lewis that "two entirely distinct kinds of chemical union, one for polar and the other for nonpolar compounds, was repugnant to the chemical instinct which leads so irresistibly to the belief that all types of chemical union are essentially one and the same." Lewis then made the suggestion that the best view of bonding was not to "regard the atom as a unit, but rather if we might ascertain where the charge or charges resided within the atom itself?" (14). Without the speculations of these proponents of dualism, the acceptance of Lewis's view could have been placed in jeopardy. The great insight of Lewis was to gather these divergent views into a cohesive theory of bonding that could apply to all situations (12). Of late the electrochemical theories have come once mare into prominence, but there has always been that antagonism between the two views which invariably results when two theories are mutually exclusive, while both contain certain elements of truth. Indeed we mav now see that in the intermetation which we are now employing the two theories need not be mutually exclusive, but rather complement one another.
Thomson's speculations about the chemical bond had not become petrified after 1904 but had evolved along parallel lines to those of G. N. Lewis. Thus in 1907 it was perceived by Thomson that "it is possible to have forces electrical in their origin binding two systems together without a resultant charge on either system" (15). However, this view, as well as those he expressed in 1914, was not as clear-cut and applicable to the course of reactions as the comprehensive dualistic prospective. By 1914, Thomson had modified his view further in keeping with the current studies of atomic number and periodic properties to suggest that there is a ring of electrons, 0-8 in number, a t the surface of an atom which is responsible for affinity. An atom with less than eight electrons was considered mobile and capable of exerting an influence on another atom, while those with eight were saturated and unreactive. "Thus we see that an atom may exert an electropositive valency equal to the number of mobile corpuscles in the atom, or an electronegative valency equal to the difference between eight and this number." The disparent behavior of electrolytes and nonelectrolytes Thomson rationalized thus (15) We should expect that in compounds of violently contrasted elements the atoms in the molecule would be charged some with pmitive, others with negative electricity. Molecules of this type I shall eall ionic molecules, to distinguish them from molecules in which atoms are not charged. The process by which atoms get charged I shall eall intramolecularionization. In the view of Thomson
. .. chemical compounds may be divided into two great classes-in one class the atoms are electrieallv neutral, in the other they are charged-and the properties of compounds are strikingly different according as they belong to one class or the other. The former class of compounds comprise those (16) . . . in which the tendency of the electronegative atom to acquire a corpuscle is not sufficiently powerful to drag a corpuscle from the atom of the electropositive element.
How prophetically close was Thomson to the electronic theory of bonding as proposed by G . N. Lewis in 1916! Literature Cited ( I ) Thomron,J.J . . P h d Mop..1.217 (19041. (1) B ~ ~ Z ~ J. ~ J.. U SI,orboh , i Kemicn. I v (Paris, 18311; franalatod in "A SourccRmk in Chemistry.'' McCraw-Hill. New York. 1952, p. 61.
Fly, H. S.. "The Electronic Conception of Valenee and the Constitution of Bonrena,"Longmans. Green andCo.. London and N e w York. 1921, p. 75. (41 Fly, H. S.. J A m w Cham. S m . 34. 634 (19121; 36, 248. 262. 1038 (19141: 31. 855 (19151. (51 Holleman. A. F., "Die direct Einfuhrung "on Suhstituenton in den Bendkern," Verf, Leipzig, 1910. (61 Thomson. J. J.. "Electricity and Matte~,"CharlesSoibner's Sons. New York, 19W.
(3)
n 114
(71 N o y e . W. A . J. Amrr. Chem Soe. 23, 4M (19011: Stheglitz, J.. J Amer. Chem. Soc.. 23.797 (i9011. (81 Vorllnder. D.. and Siebert, E.. B m 52. 282(1919l.
(9) Nelson, G., andFdk. C..J.Amrr. C h e m Soc., 32.1637 il910l. (101 hwis, G. N.. "Valence snd the Structure of Atom." Chemical Ca.. Now York. 192.3. p. 74. (111 hpwa*h. A . M S ~noc. . ~ ~ ~ ~m i l h.sot ~64, iii. ~ 2 119~01. t ~ ~ ~ i (12) Robinsan, R..Mem. P l o c Mnnche8l~rLil Phil. Sot. 64, iu. 2(19MI. 1131 lawy,T..J. Chem. Soc.. 123.826119211. (141 Leuis, G. N..op. 0.. p.74. 1151 Thamsan. J. J.. "Corouscular Thmrv of Matter." C h a r k Seribner'l Sons. New York. l&.p.'lM. ' (161 Thornson. J.J.. Phd M a g , 27.781(19141
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