Martin D. Saltzman
Providence College Providence, Rhode Island 02918
Benzene and Octet Theory
Benzene, initially isolated from coal in 1825 by Faraday, has tantalized and exasperated the imagination of chemists as few molecules have in the history of chemistry. From its original discovery until 1910, almost 1300 items concerning this molecule had already appeared in the chemical literature. In the attempt to elucidate the structure and properties of benzene one can see the rise and fall of numerous theories of valence. The electronic theory of valence embodied in the rule of two of G. N. Lewis ( 1 ) [1875-19461 and its more expanded form, the octet theory of Irving Langmuir (2) [1881-19571, were considered to he very radical when they first appeared. In part, the acceptance of the octet theory rested firmly upon its application to the benzene problem. The latter part of the 19th century saw a spate of papers providing empirical rules of substitution in the henzene nucleus. Two of these are particularly noteworthy, those of Huhuer (3) appearing in 1875 and Nolting's (41 in the following year. They described how substituents on the ring would direct entering groups to the ortho, para, or meta positions. The results of these early studies indicated that strongly negative groups such as the nitro or carboxyl, for instance, always led to meta substitution; whereas positive, neutral, or weakly acidic functions such as amino, methyl, phenolic, or halogen favored the ortho or para position. Further empirical rules were contributed in 1887 by Henry Armstrong (1848-1937), one of the monumental figures in the development of British chemistry. Armstrong recognized that groups monovalently attached to the henzene nucleus were ortho-para directing, whereas multiplybonded groups were meta directing although some exceptions were noted (5). Many of the empirical findings were summarized in the rule of Crum-Brown and Gibson (61, in which it was stated that if a group was more stable in compounds with hydrogen than hydroxyl, i.e., HCI-HOCI, it was then ortho-para directing. This can he contrasted with the meta directing pair HONOz-HNOz. This and other rules which appeared sporadically, for example, Vorlander's (7) contribution that positively charged groups adjacent to the ring were always meta directing, offered a guide to the placement of entering groups. These were extremely useful to the synthetic chemist; yet they lacked any rationale to account for their success. The structural proposals for benzene did little to solve the puzzle of reactivity. The common basis for all these structural suggestions was an effort to explain how the six extra valences of benzene led to its special stability. Among the most important of these proposals were those of KekulB, Baeyer (81, Armstrong (9), and Thiele (10). KekulE., in 1865, made the original proposal of the cyclohexatriene structure. In 1872 he was forced to present the dynamic oscillation hypothesis in order to offer an answer for the lack of two ortho isomers. Many chemists were quite content to use the KekulB structure as originally presented without the oscillation thesis, as it seemed to answer many of their questions about henzene. Baeyer and Armstrong, skeptical of KekulB's radical proposal, independently proposed structures which could account for the unusual properties of benzene. Their 498
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of the
model accounted for the symmetry relations in benzene by p m ~ o s i n ethat the fourth valence, or affinitv of carbon as it &as cdled, was directed towards the center of the ring. This led to the term "centric" formula (9). I venture to think that a symbol free from all objection may be based on the assumption that of the 24 affinitiesof the six carbon atoms, 12 are engaged in the formation of the carbon ring and six in retaining the sin hydrogen atoms, in the manner ordinarily supposed; while the remaining six react upon each other. -acting towards a center as it were, so that the "affini-
ty" may be said to be uniformly and symmetrically distributed. Ohvious difficulties arise when one tries to write a centric formula for napthalene, as the actual bond is not expressed. The structural theory of Johannes Thiele (1865-1918) presents a glimmering of what would be some three decades later. Thiele (10) in 1899 proposed the principle of partial valencies; valence was capable of infinite division. If one examines double bonds as they are in conjugated systems, there should he residing on each carbon atom of the double bonds, according to Thiele's thinking, a partial valence, indicated by dotted lines CHq-+I-CH,
..
.,
..
..
In the fully coniueated system the partial valences on C-2 and C-3 n&tuail;satur&e each other and form according toThiele an "inactive double bond"
Why does 1,4 addition occur in dienic systems? This can be readily seen by the assumption that only the terminal carbons have any free affinity with which to interact with an attacking group. The terminal carbons have only partial affinity, and thus make a demand for affinity on the double bond joining C-1 and C-2 as well as that joining C-3 and C-4. These consequently rupture, and the free affinity thus generated a t C-2 and C-3 will combine to form a double bond. The observed course of 1.4 addition is thus explained. Thiele's theory of partial valencies when applied to henzene offered a way of explaining the observed heat of combustion of benzene. The numerical results were consistent with nine single rather than three single and three double bonds. In a cyclic conjugated system such as benzene the possibility of free affinity is absent since there are no termini. Every atom in the doubly bonded system having some partial valence character is free to saturate its neighbor and thus make the three sets of double bonds inactive. When this occurs it is impossible to distinguish between any of the bonds.
Q Thiele's proposal was eminently attractive as it offered a way of avoiding the uncomfortable "oscillation hypothesis" of KekulB. The equivalence of the two ortho carbon atoms is explained as well as the unusual properties of the
benzene ring due to the saturation of the double bonds. As satisfying a preelectronic formulation as Thiele's proposal was, it failed to explain why cyclooctatetraene prepared by Willstater in 1911 behaved as a simple alkene though i t seemed to be a fully conjugated system. In many ways partial valencies led to the same conclusions as the wave mechanics of the 1930's. A renewal of interest in the connection between valence and electricity occurred with the discovery of the electron in 1897 by J. J. Thomson. Previous excursions into the realm of electricity and valence had been made earlier in the century by Berzelius. All of these had led to a morass of difficulties when applied to the bonding of nonelectrolytes. In 1904, Thomson (11) proposed an electronic interpretation of valence based on the ability of an atom to transfer an electron and hence form a bond. In Thomson's system all bonds would therefore be essentially polar, though one could, of course, rationalize an incomplete transfer and the absence of ions. The polar theory of valence when applied to the properties of benzene met with what seemed initial success. Early interest in the structure of benzene was found primarily among the German chemical fraternity, but by the beginning of the 20th century there was greater interest in synthesis and less in theory. In the United States, with only a nascent chemical tradition, the taste for theorization and the application of new ideas was rapidly developing. Great strides were to be made on native soil during the first two decades of the 20th century in the application of electronic conceptions of valence. Many American chemists eagerly seized the Thomson polar hypothesis as a means of rationalizing the course of organic reactions, and one in particular was a young chemist a t the University of Cincinnati, Harry Shipley Fry (18781949). Fry, influenced by Thomson's prestige and the rigor of his arguments on behalf of the polar hypothesis, saw in the electron theory of valence a rationalization for the pattern of substitution on the benzene nucleus (12). The polar hypothesis predicted alternations of polarity in the carbon atoms of the benzene nucleus, if indeed, the bonds were the result of electron transfer
Y H H+ *"
- + -
H
Substituents, by their polar character, could alter the nature of the charge distribution and hence influence the site of substitution. Atoms of electropositive character when attached to the benzene nucleus were unreactive and meta directing; while those with an electronegative atom adjacent to the ring were reactive and ortho-para directing. This may be seen in I and 11.
T*.;+
H+
H-
HI
H+ U
Polarity undoubtedly must play a role in orientation and thus it would seem to be impossible to dismiss the connection between electrons and valence. However, one feels rather uncomfortable in invoking a polar bond between carbon and carbon and between carbon and hydrogen even though it makes it possible to explain some of the observations about benzene. Similar excursions into the area of reaction mechanism based on the electron transfer concept were made exclusively by Americans; among these were William Albert Noyes (1875-1941), Julius Stieglitz (1867-1937), and K.
George Falk (1880-1953), principally in the years between 1910-1920. In this period, they produced many studies of reactions based upon this concept. The explanation of molecular rearrangement by this scheme, particularly by Stieglitz, even with its shortcomings, would act as a powerful influence later against the ready adoption of the octet theory in the United States. By the year 1916 the validity of the polar theory was in doubt and yet it seemed preeminently reasonable for certain systems just as Berzelius' dualism had a scant century earlier. In this year there appeared in the April issue of the Journal of the American Chemical Society uaoer - a . . that received iittle initial attention by the chemical world written bv a member of the Universitv of California faculty, ~ i l b e kNewton Lewis, entitled he Atom and the Molecule." Lewis proposed an electronic conception of valence that was all encompassing in its scope-the shared pair. As early as 1902, Lewis had been preoccupied with the periodic nature of chemical properties and its relation to the newly discovered electron. In discussing the periodic table in his lectures before his students at Harvard, where he began his academic career, Lewis became in'trigued with the recurrence of the number eight with regard to chemical properties, much the same as Newlands had four decades earlier. Lewis began to think of the arrangement of the electrons in the atom as perhaps being placed a t the corners of a cube, the number of cubes and electrons being determined by the position of the atom in the periodic table. The formation of ions and ionic compounds could easily be accommodated in this scheme by electrons being transferred to produce completed cubes. The cubical atom model, as Lewis referred to it, worked well for polar systems, but when applied to non-polar compounds there were deficiencies. From the years 19021913 Lewis seemed to have subdued his interest in valence theory, concentrating on his first interest, thermodynamics (17bJ. In 1913 simultaneous Daners in coniunction with (13, 14) his ~ e r k e l colleague, e~ ~ i l l i a m ' C . ' ~ r aappeared ~, discussing the apparent dichotomy of bonding-polar and non-polar. This would necessitate in their view the need for two entirelv different svstems of valence-a modern dualism. Lewis believed at this uoint that two entirely different systems of valence were necessary. How non-polar bonds could be accommodated within the cubical atom hypothesis was unsurmonntahle at this time. In 1913, he (13) made a vague reference to the non-polar bond as: "the number of positions, or regions, or points on the atom at which attachment to corresponding points on other atoms occurs." Lewis pondered the problem over a period of the next three years during which there appeared papers by J. J. Thomson (151, W. C. Arsem (16), and Alfred Parsons (17a), which all argued for an electronic conception of valency which did not require complete transfer of an electron. A detailed discussion of these oaoers . . and their influence on Lewis, appears in an excellent paper by Kohler (176). In these excursions into non-uolar valence, he saw the possibility that two electrons constituted a bond. Using the cubical atom as a model: if one joins a cube missing an electron with another in a similar situation, both will be sharing 8 electrons (176).
How does the shared pair preclude the necessity for a dualistic system of valence? As Lewis wrote (1) Great as the difference is between the typical polar and nonpolar substances, we may show how a single molecule may, according to its environment, pass from the extreme polar to nonVolume 5 7 , Number 8. August 7974 /
499
polar form, not per .salturn, hut by imperceptible gradations as soon na WP admit that an clectmn me\ he the common property of two atomic shells Lewis commented further (1) Two electrons thus coupled together, when lying between two atomic centers, and held jointly in the shells of the two atoms,
I have considered to be the chqmical bond. Lewis quickly abandoned the cubical concept in favor of the dot notation when he realized the difficulties inherent in using the cube for multiple bonds. In postulating the concept of the two electron bonds, Lewis was in direct opposition to the polar orthodoxy of his time: that the origin of the chemical hond was the transfer of an electron. The subtlety of the concept that a pair of electrons could encompass all the varieties of bonding was missed by the contemporary chemical community. In many ways Lewis was his own worst enemy in the sparsity of illustrations of his ideas in his paper. As Lewis commented in 1923 (18) In my original paper I contented myself with a brief description of the main results of the theory, intending at a later time to present in a more detailed manner the various facts of ehemistry which made necessary these radical departures from the older valence theory. This plan, however, was interrupted by the exigencies of war. . . If one searches the literature for the period 1916-1919 for applications of the Lewis shared-pair concept, this search would be in vain. European chemists were preoccupied with the Great War raging, with its concomitant difficulties of communication which may have been r e s p n sible for this course of events. However, Walter Kossel, a German physicist, published, in 1916, a view of atomic structure based on the importance of eight electrons. In the United States. which had been in the forefront of electronic theories of valence, there was little if any immediate reaction. W. A. Noyes, a n authoritative figure in valence theory does not even mention Lewis' theory in a 1918review lecture on valence theorv, tbouah a 1919 lecture hrieflv does. Noyes instead concentrated on the explanation of "alence along polar lines (19, 20). Thinking of reactions in terms of chareed s ~ e c i e sentering and leavine was far simoler than the gffic&ies of the sh&ed pair. The kev to chemical bondine would have lain buried in the chemical literature if it had not been for Irving Langmuir. Kohler (201 has recentlv. . orovided us with the correspondence between Lewis and Langmuir and a detailed studv of the orieins of Lanmuir's realization of the eeneral use of the shared-pair concept as well as the cool reception to the shared pair initially accorded Lewis. In 1919, Lewis not having further clarified his ideas, Langmuir began to wonder how the shared pair concept could explain such molecules as nitrogen and its oxides. His solution was to assure the applicability and the ultimate acceptance of the shared-pair concept (20).
-
A couple of weeks later, after I had developed the ldeas in regard to the arrangement of the electrons in what I call the third and fourth shells, I became impressed with the importance of the group of eight in the second shell rather than the group of 2,4,6. or 8. I started to say that I had developed what seemed to me to be new ideas as to the stability of eight (instead of 2, 4, or 6) elee-
trans in the second shell and then for the first time began to see the necessity of having all the electrons take part in the farmation of octets. I do not see that this idea is stated anywhere in your paper. I consider it an essential part of what I call the octet theory, and w~thoutit I do not see haw the equation 2p = 8n - e [Langmuir's method of determining the number of shared-pair bonds (pi from the total number of valence electrons fei and number of octets fnjl can he derived. These ideas were published in a very long paper by Langmuir in the Journal of the America0 Chemical Soci500 / Journal of Chemical Education
ety in 1919 (2) in which Langmuir introduced the two terms. octet and covalent hond. In a series of lectures. both domestic and foreign, Langmuir proselytized on be: half of the new unified theorv of chemical bondine. - Aooli.. cations, however, were few in number as the real meaning of the theory seemed to escape most chemists. The ultimate acceptance of the octet theory would depend primarily upon its application to the benzene problem. Some of these pieces would be contributed by American chemists, particulary those who had been associated directly with Lewis or by contact with his students. Others would later be contributed by a group of English chemists who were very willing to apply electronic speculation to reaction mechanism. The greatest difficulty, however, in accepting the octet theory was that it offered no better means of explanation of the course of reactions than the polar theory. Organic chemists of this period seemed to shy away from atomistics, as it was called-a far different state of events than today. In order to apply the octet theory the properties of this electronic arrangement had to be determined. Howard Lucas (1885-1963) was one of the important American contributors to the acceptance of the octet theory. Lewis, in 1916, had made a suggestion, later refined by his students Lattimer and Rbodebusch, concerning the possibility that the shared pair of electrons may undergo displacement, (known today as the inductive effect). In order to explain the enhanced acidities of rrhaloacids Lewis suggested (1) The electruna. heiny drawn towartla the TI. prrmir the ,pair o l elccrmns joining rhe methyl and iarbuxyl proups to npproach nearer tr, the methyl . . all the ~ltctronsmuw toward tlv letr. producing a greater separation of the electrons from the hydrogen of the hydroxyl, and thus a stronger acid. . . . It need only be home in mind that although the effect of such a displacement of electrons at one end of a chain proceeds through the whole chain, it becomes less marked the greater the distance. In a series of papers appearing between 1924 and 1926 (21) Lucas was able t o show that the electron-attracting ability of a substituent could alter the direction of addition to alkenes. Lucas further extended his theory to benzene by first preparing a table of electronegativities from the ionization constants of p-substituted benzoic acids using hydrogen as a standard. A detailed theory of substitution appeared in 1926 based upon the octet theory, with the newly included idea of electronic displacements~Lucas concluded that orientation in benzene was the result of the electronegativitv of a substituent: those more negative than hydrogen deactivate the ring and lead to meta substitution. Substituents less negative are activating and thus lead to orientation at the ortho-para positions. These conclusions were based upon an incorrect model of benzene proposed simultaneously by his colleape Linus Pauling (22). Pauling argued for a physical connection between the ortho-para positions in the ring but not the meta. Hence the inductive effect was transmitted in the order para > ortho > > meta. A substituent of greater negativity than hydrogen, such as halogen, should then deactivate the ring and lead to meta substitution-which it does not. Lucas was then forced to adopt the hypothesis that if a suhstituent had "unshared electrons on the atom joined to ring carbon," ortho-para suhstitution would result regardless of electronegativity. In a prophetic final sentence (21) he stated, "It is significant that the most strongly directive groups contain such electrons." A key part of the puzzle of aromatic substitution, the interplay of electrons under the influence of substituents, was now in place. The shared pair was capable of movement under the influence of a n electronegative substituent and yet the octet could he preserved. A theory originally born in America would find its ultimate validation and utility in the work of a group of En-
glish chemists. The versatility of the octet theory and its application to the benzene problem would manifest itself in the work of Thomas Martin Lowry (1874-1936), Robert Robinson (1886- ), and Christopher Ingold (1893-1970). Langmuir had in 1921 visited Great Britain to present his ideas a t the annual meeting of the British Association for the Advancement of Science, held in Edinburgh, Scotland. Robert Robinson, who was on the faculty a t St. Andrews, had been in attendance a t the meeting and may have seen that the Langmuir exposition of the octet theory was a new way of looking a t conjugated systems, particularly benzene. Until this fateful meeting, Robinson's thinking as well as that of his contemporaries had been deeply influenced by Thiele's concept of partial valencies. I n 1922 Robinson perceived that using the octet theory could enable one to arrive a t the same conclusion as Thiele without having to invoke a n infinite division of valences (23). Whilst all these theories and their corresponding devices and symbolization have proved serviceable as workable hypotheses, the connecting link in the form of a common physical basis is lacking and it is the object of the present communication to suggest that such may be found in the Thomson and LewisLangmuir theory of the atom. Rohinson had previously devised, in conjunction with his colleague Arthur Lapwortb (24), a system based on alternating polarities in conjugated systems. This system was in many ways reminiscent of Fry's but without the advocacy of completely full blown positive and negative charge to rationalize the effect of a substituent on the course of substitution in benzene. In the octet theory Robinson saw that electrons could be perpetuated along a conjugated system by "disturbances of covalency" and thus a new mode of electron action was proposed (23). One further advantage of the Lewis-Langmuir theory in relation to this subject may be noted. Experience has shown that the alternating effect is transmitted but feebly by saturated atoms, whereas it may be discerned at the end of long chains wholly comprised of unsaturated atoms. This is easy to understand in view of the fad that unsaturated atoms share more electrons in common than saturated atoms. There will he a greater mobility of electrons, and the octets, when farmed will have units at least which are not suhject to restraint, a candition which tends to stability. In discussing conjugation Robinson postulated that "three electrons shared by two atoms is a relatively stable configuration" and thus fell short of a satisfactory explanation. Robinson's difficulties with the double bond were relieved by a suggestion made in 1923 by Lowry (25). The crux of Lowry's thinking is embedded in the opening statement of his paper. The object of this paper is to suggest that, 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 covalencv and one electrovalencv . . . Their existence can he iustifird on the basis at the rlrcrronr~theor). of \.nlmey by asswung a complete i,ctet to rnrh negatwely charged atom and a sextet ofel~ctronatoearh positiwI>charged atom. The existence of this semi-polar arrangement, Lowry conceded, would result from the influence of an attaching polarizing group. Lowry plunged into the benzene problem using as his basis the semi-polar bond. To Lowry the structure of benzene could be that of Kekuli. or, for that matter any of the centric models, because it was not the resting molecule (as he termed it) but the activated state with its alternate electrovalencies which determine the pattern of substitution. In many ways Lowry's conception of benzene is the same as Fry's and the polar school, with the exception that the plus and minus signs now represent
the displacement of electrons of the octets binding the carbon atoms together in the ring. Lowry's scheme suffers the same shortcomings as that of Fry and Lucas, in that polarity alone will not predict the pattern of aromatic substitution. All the parts of the puzzle would fall into place in 1926 in simultaneous publications by Robinson (26) and Ingold (27). To this point the notion of electron displacement through inductive effects had been established by Lewis and Lucas. Lowry and Robinson had shown the possibility within the octet theorv of the movement of a air of electrons, or electromerish as it was to be duhded. Yet the connection of these two oossible modes of electronic interaction had not been made. By 1925 Robinson had become familiar enough with the octet concept to postulate a reason for aromatic character (28). The occurrence of many atoms in a molecule provides further opportunities for the emergence of electron groups of marked stability and, ceteris paribus, the possession of such groups confers chemical stability as shown, for example, by reduced unsaturation and a tendency to maintain the type. These are, of course, the chief characteristics of benzenoid systems, and here the explanation is obviously that six electrons are able to form a group which resists disruption and may be called the aromatic sextet . . . that six electrons in the benzene molecule produces a stable association which is responsible for the aromatic character of the substance. Then in 1926 both Robinson and Ingold saw the possibility that atoms in conjugated systems may either supply or withdraw electrons from their neighbors causing a permanent polarization, a process proceeding throughout the whole conjugated system (26). Conjugation occurs by virtue of electronic displacements which produce alternating polar effects as accessory in producing a means of adjustment of the disturbed covalency of the carbon atom. Ineold came to the realization that electrons mav un" dergo internal displacements along the conjugated system thus ~ r o d u c i n aa t the ortho and Dara . ~. o s i t i o na net excess of electrical charge (29).
The p-position is reached by mutually accommodating adjust-
ments of electrons from the two double bonds . . . This, it may be remarked. at least seems to exulain whv it is that in the absence of other disturbances the effect of weak o.p-directing group, like methyl, outweighs that of a strongly m-directing group, such as -NMes, in competition. . .
a
The subtle interplay of these effects was postulated to be a t the root of the pattern of substitution in benzene. Impressive correlations of experimental observations with theoretical data were made and thus the stage was set for the eventual acceptance of the octet theory. Solving the mystery of orientation in benzene had shown that valence bonds (shared pairs) were capable of stresses and strains. In response to the demands of reagents, dynamic movement could and would occur, a far more complex situation than had been though of just a decade earlier. In time, the use of the octet theory to explain reaction mechanisms would be quite common. In solving one problem, orientation in benzene, the octet theory proved quite adequate; yet i t was incapable of explaining the uniqueness of the aromatic sextet. The stage was now set for the new wave mechanics of the 1930's. Acknowledgrnenl
The author is very grateful to Dr. Robert E. Kohler, Jr. for sharing his expertise and many insights into the origin and development of the octet theory. Volume 51, Number 8, August
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