A History of the Structural Theory of Benzene - American Chemical

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A History of the Structural Theory of Benzene—The Aromatic Sextet Rule and Hückel’s Rule Shigeaki Kikuchi Shibaura Institute of Technology, 307 Fukasaku-Tameihara, Omiya-shi, Saitama, 330, Japan Because benzene has the simplest and most fundamental structure of the aromatic compounds, it has been the starting point for studying them. The German organic chemist August Kekulé’s hexagonal structural formula became the conventional formula for benzene soon after he proposed it in 1865. The Kekulé formula was very effective for the study of aromatic compounds, providing the scientific foundation for the synthesis of dyestuffs, medicines, and other organic chemicals. This formula was considered insufficient as a structural representation of the chemical nature of benzene, however, thus stimulating further chemical study of the substance. Some progress was indeed made, but at least one great difficulty remained: to represent the bonding state of the benzene nucleus (i.e., to draw the fourth valences of the carbon atoms of the benzene ring). Known as “the benzene problem”, this became the critical question in the study of benzene and was widely researched. In 1931, Erich Hückel, who was then at the Technische Hochschule in Stuttgart, used quantum chemistry to solve the benzene problem, proposing his rule concerning aromaticity (1). This fact was not generally recognized, however, until the 1950s (2). One question I would like to deal with in this paper is what caused this lag of nearly 20 years. Since the 1950s, Hückel’s rule, the aromatic sextet rule, and Bamberger’s axiom have been generally accepted, and the history of the structural theory of benzene has come to be described in terms of three peaks: Bamberger’s axiom in the phase of the classical theory of valence, the aromatic sextet rule in the phase of the electron theory of valence, and Hückel’s rule in the phase of quantum chemical theory (3). Bamberger ’s axiom, which was proposed in 1891 by Eugen Bamberger of the Akademie der Wissenschaften in Munich, postulated that the central association of six valences was necessary to aromaticity (4). The aromatic sextet rule, first proposed in 1925 by J. W. Armit of the University of St. Andrews and Robert Robinson of the University of Manchester, stated that the association of six unsaturated electrons on the rings of benzene and related compounds was responsible for their stability (5). Hückel’s rule stated that conjugated, monocyclic coplanar organic compounds with (4n + 2) π-electrons (n = 0, 1, 2, 3, …) possess relative electronic stability and aromatic character. This description, however, still leaves some questions unanswered. For example, it is not clear why the “rule of six” was not generally accepted until the 1950s. It is also not obvious why Robinson expressed skepticism about his rule in the 1930s. Furthermore, the process of the revival of the rule of six has yet to be delineated. The main object of the present paper is to show why the rule of six rapidly lost its significance in the 1930s and why it has been reevaluated since the 1950s. To shed light on these questions, I have investigated changes in the way that rule has been regarded in chemical theory.

194

Bamberger’s Axiom as Predecessor of Aromatic Sextet Rule and Determination of Molecular Skeleton of Benzene by X-ray Diffraction The benzene problem can be illustrated as follows (6). Contrary to the addition reaction that the Kekulé formula with three double bonds would lead us to expect, the bromine atom readily substitutes for the hydrogen atom of benzene. In addition, benzene is much more resistant to oxidizing and reducing agents than normal unsaturated olefinic compounds. When these properties were discovered, it was thought that the Kekulé formula was inappropriate to represent the bonding state of benzene. Thus alternative formulas for benzene were proposed. The centric formulas for benzene (1), proposed by Henry Armstrong at City and Guilds of London Institute, Central Institution (later Central Technical College) in 1887 (7) and by Adolf von Baeyer of the Akademie der Wissenschaften in Munich in 1888 (8) were both intended to represent the strong central bond of the fourth valences of the six carbon atoms of benzene. Their real purpose, however, was to represent the peculiarity of the bonding state of the benzene nucleus.

1 Bamberger proposed the axiom of six centric valences as a way of applying the centric formula to the other aromatic rings. Using centric formulas, he tried to represent the bonding state of a benzene-like nucleus. According to him, if centric formulas could be used, it would be possible to express quite clearly the changes in chemical properties for such reactions as eq 1. Because of the deficiencies in his axiom (9), however, it did not gain recognition at the time. HC HC

CH

CH N H aromatic

H2C H2

CH

H2C

H2C

CH N H

H2

CH2

H2C

nonaromatic

CH2 N H

(1)

Johannes Thiele of the Akademie der Wissenschaften in Munich took up the benzene problem from another standpoint in 1899 (10). He attempted to represent the additive ability of the double bond by using partial valence. For example, according to Thiele, the addition reaction of the ethylene double bond could be attributed to the presence of partial valences that were not completely consumed by a chemical bond. Thiele maintained that the 1,4-addition of butadiene could be explained because the partial valences of the central two carbon atoms would neutralize each other and a new inactive double bond would be formed at that position. In

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order to resolve the benzene problem, Thiele applied his theory to the Kekulé formula. Thiele’s formula (2) showed the presence of six inactive double bonds in benzene. Thiele’s theory was invalidated after the preparation of cyclooctatetraene by Richard Willstätter of the Technische Hochschule in Zurich in 1911 (11), because it was olefinic but not aromatic. At that time, Willstätter concluded that the centric formula was the most appropriate for benzene. Although Bamberger’s axiom was proposed to express the bonding state of the aromatic nucleus, its real significance lay in pointing up the inadequacies of classical theory in expressing the delicate differences in bonding states.

or

formed by sharing a pair of electrons; the other was the octet rule—the concept of the octet as the stable arrangement for the electrons in an atom. This perspective attained the unification of the ideas of the ionic or polar bond and the nonpolar bond. Paralleling the development of the electron theory of valence, several electron formulas were proposed for benzene. In the 1910s, Harry Shipley Fry of the University of Cincinnati (16) and other chemists (17) applied the concept of the ionic bond to chemical compounds and proposed electron formulas of the ionic bond type for benzene (Fig. 1). Although it was not appropriate to apply this to electrically nonpolar benzene, the ideas of the electron shift and of the electric polarization of the molecule were carried on in subsequent electron theory as it sought to explain the aromatic substitution reaction.

2 The structural formula drawn in terms of a tetrahedral carbon model was called the space formula. Space formulas for benzene were proposed not only during the classical phase (12) but also after the electron theory of valence became predominant. They were discredited, however, after the structure of benzene was determined by the X-ray diffraction method. Using the X-ray diffraction method, the British physicist William Henry Bragg and others examined the structure of diamond and then of some aromatic compounds (13). The rings of the aromatic compounds were found to have a puckered or diamond-type structure. Thus such a model was considered to be the most promising model for benzene until 1928, when Kathleen Lonsdale of the University of Leeds definitively clarified the structure of the benzene nucleus (14). She examined the crystal of hexamethylbenzene using X-ray diffraction, clearly showing that it has a planar, regularly hexagonal structure. Development of Electron Formula for Benzene— Proposal of Aromatic Sextet Rule Soon after the discovery of the electron, the electron theory of valence emerged to surmount the difficulties present in the classical theory. The chemical formula with electrons was called the electron formula; it showed the number and arrangement of electrons (15). Roughly speaking, the electron theory of valence can be divided into two stages. First the idea of the ionic, or polar, bond was favored. Such a bond was thought to be formed by electron transfer between bonding atoms, meaning that a chemical bond was formed by the electric attractive force between oppositely charged atoms. In this stage, definite progress was made. For instance, the relation of electron transfer to oxidation and reduction reactions was clarified. The idea of the electron isomer (electromer), however, was not invariably effective to explain chemical phenomena. Electromers were isomers that had the same conventional formula but different electron distributions. In general, it was not possible to apply the idea of the ionic bond to nonionic or nonpolar bonds, which were characteristic of organic compounds. In the second stage, after about 1920, chemists adopted two principles. One was that of the shared electron pair bond—the idea that a single chemical bond was

A

B

C

Figure 1. Fry’s electron formulas for benzene (3 of 18 electromers).

Hugo Kauffmann of the Technische Hochschule in Stuttgart proposed an electron formula for benzene in 1908 (18). In his formula (3), the neighboring carbon atoms of the ring were combined by means of three electrons. He felt that his formula was suitable to transform Thiele’s formula into an electron formula, making it the successor to the Thiele formula. This type of electron formula found many advocates in the second stage (19). In 1926, however, Alfred W. Francis of MIT, who supported this formula (4), pointed out the difficulty of clarifying the distribution of the six third electrons in benzene (20), which he saw as the benzene problem in those days. Although Kauffmann’s idea had some advantages, the bond of three electrons was not adequate to show the difference in the chemical natures of benzene and cyclooctatetraene, for example.

C

C O O O O O O C O O O

O O O C

O O C O O C O O

3

H C H C

C H

H C

C H C H

4

In 1922, Maurice L. Huggins of the University of California applied Gilbert Newton Lewis’s “tetrahedral” model to benzene (Fig. 2) (21). His formula was a rewriting of an old space formula. Because his benzene ring was a wave or puckered ring, his formula was refuted in only six years, but his idea of a stable central bond of six electrons in some aromatic compounds was carried on in the aromatic sextet rule.

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Quantum Chemical Study of the Benzene Problem— Proposal of Hückel’s Rule

a

b

c

d

Figure 2. Electron formulas for benzene (a, b), pyridine (c), and pyrrole (d) by Huggins. Drawing b is the side view of benzene.

Although Armit and Robinson are usually credited with proposing the aromatic sextet rule in 1925, in fact it was first proposed in 1922 by Ernest C. Crocker of MIT (22). Like Huggins, Crocker applied the principles of electron theory to benzene, but he differed from Huggins in not adhering to Lewis’s “cubic” model. This enabled him to propose the idea that each of the monocyclic aromatic compounds shared the stable group of six aromatic electrons in the ring (Fig. 3). Crocker also noted the resemblance between his idea and Bamberger’s axiom. H C

H and

H

C

C C H

→ 6e

→

C C

→

H

H

Figure 3. Electron formulas for benzene by Crocker (left), Robinson, and Ingold (right).

In 1925, Robinson named Crocker’s stable electron sextet the aromatic sextet and noted it as a new electron group that should be added to such groups as the electron pair and the electron octet. Robinson then applied this new rule to the interpretation of chemical phenomena. The British chemist Christopher Kelk Ingold, who was then at the University of Leeds, also contributed greatly to developing electron theory, applying it in his study, too (23). Other types of electron formulas were also proposed for benzene (24), but none of them gained any importance. One of them, a formula presented by Linus Pauling of California Institute of Technology, is worth noting (25). His idea was very similar to the aromatic sextet rule in that the group of six extra electrons in the benzene nucleus was thought to be stabilized by an effect analogous to that in the rare gas elements. His formula found some supporters (26), but it was questionable with respect to the diagonal bonds in the ring. Pauling himself did not consider his formula essential to his quantum chemical treatment of benzene in 1933. By 1928, the benzene ring itself was found to be planar and regularly hexagonal, so inappropriate electron formulas for benzene lost their meaning. The aromatic sextet rule was a concept designed to gain an understanding of the structural characteristics shared by benzene and monocyclic aromatic compounds. It surpassed its predecessor, Bamberger’s axiom, in distinctly assigning the number six to the electrons in the aromatic nucleus and in recognizing this as a new stable group of electrons. By about 1930, this was one of the most promising hypotheses for the molecular structure of benzene (27), but the stability of the aromatic sextet remained to be explained. A short description published at the time by Wendell M. Latimer and C. W. Porter of the University of California is interesting because it seems to suggest a way to solve the benzene problem (28).

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Soon after the birth of quantum (wave) mechanics, physical chemists applied it extensively to chemical problems, including the benzene problem. After two preliminary examinations by others (29, 30), Hückel made great progress in researching the benzene problem in 1931. In his quantum chemical treatment of benzene, Hückel attached considerable importance to the aromatic sextet rule in the following respects. Firstly, he felt that the old benzene problem would be solved if the stability of the aromatic sextet (i.e., the six π-electrons in benzene) could be explained appropriately. Secondly, Hückel examined the valence bond (VB) and the molecular orbital (MO) methods (i.e., the two approximate methods of quantum chemistry), to determine which would provide an appropriate interpretation of that rule—an indication of which method would be best suited to the treatment of the π-electrons in organic compounds. Hückel concluded that because the MO method could explain the stability of the aromatic sextet, it would be better than the VB method for the treatment of the π-electrons. Using the MO method, the aromatic sextet was interpreted as a closed shell, analogous to one of the noble gas elements, the stability of which could be understood in terms of the nature of a closed shell. At that time, Hückel proposed what he felt was a more broadly applicable rule than the aromatic sextet rule. The next major step was taken by Pauling. Using the VB method, Pauling examined Kauffmann’s formula, which he rejected as unstable in 1931 (31). Next, in 1933, he applied resonance theory (VB method) to benzene (32). Like Hückel, Pauling examined the stability of the six π-electrons of benzene, but he concluded that benzene was stabilized as a result of the resonance of five canonical structures (Fig. 4). His results were questionable, however, because they indicated that cyclobutadiene was hypothetically more stable than benzene.

Figure 4. Five canonical structures of benzene.

In 1934, using a method analogous to the VB method, W. G. Penney of the University of Cambridge overcame the deficiencies in both Hückel’s and Pauling’s treatments (33). Whereas Hückel and Pauling had dealt with only the π-electrons, Penney treated both the σ- and π-electrons of benzene, augmenting their conclusions. According to Penney, the stability of benzene depended mainly upon the σ-bonds. In this respect, his study worked against the rule of six. Because of the criticisms expressed by Penney and also by G. W. Wheland at California Institute of Technology (34), Hückel could not maintain his old position. Although he acknowledged the inadequacy of his interpretation of the stability of the aromatic sextet, Hückel did not write about it explicitly (35). I have considered the change in his position with the aid of Figure 5, which is based on a figure he published in 1934. In 1931 he thought that the lowest excitation energy of benzene, the energy necessary to transfer an electron from the highest occupied molecular orbital to the lowest unoccupied one, was 2β, too large to allow benzene to become chemi-



cally active easily. This was how Hückel interpreted the stability of the aromatic sextet, but this view made no clear distinction between, for example, benzene and ethylene. Thus it was probably obvious to him that his idea could not effectively explain either the stability of the aromatic sextet or the differences in the properties of benzene, ethylene, and other organic compounds. Hückel repeatedly called chemists’ attention to the aromatic sextet rule, but he accepted Penney’s opinion in 1937 (36) and in the end was unable to provide sufficient proof of the significance of the rule.

the nitrogen atom and the second electron pair of the double bond are easily moved. Their displacements are shown by the curved arrows in formula B. As a result, a new electron distribution C appears, and displacement in the opposite direction occurs easily. In the end, the system is stabilized in a state intermediate between A and C, that is, in the mesomeric state. Robinson and Ingold (38) called this stabilization the “mesomeric effect.” (A) R2N

(B) R2N

C

C

C

O

C

C

C

O

(C) R2N

C

C

C

O

Figure 6. Electromeric change and the mesomeric effect.

In 1925, Robinson proposed the hypothesis below to explain the observed regularity of aromatic substitution reactions (39). His hypothesis, which was essentially the same as what is now called the electrophilic substitution reaction, was that the cationoid (now termed electrophilic reagent) approached the atom having the excess electron distribution and the substitution reaction occurred at that position. (The reverse situation was the nucleophilic substitution reaction, where Robinson’s anionoid, now termed nucleophilic reagent, approached the positively charged atom.) Robinson depicted his idea as in Figure 7. A little later Ingold also adopted this hypothesis.

Figure 5. Electron configurations.

Rapid Loss of Significance of the Aromatic Sextet Rule Robinson began to have some misgivings about the aromatic sextet rule in 1932 (37), which I feel represents a turning point in its significance. Robinson, and slightly later Ingold, began to develop electron theory for organic chemistry in the early 1920s. The fundamental concepts in Robinson’s theory were the inductive effect, electromeric change, and the mesomeric effect. Robinson gave the name “inductive effect” to the following phenomenon: the electrically more negative atom of two chemically bonded atoms attracts the electron pair of the bond, drawing it somewhat toward itself from the midpoint of the two atoms; for the more positive atom, the situation is the reverse. “Electromeric change” is the term Robinson used for the following type of electron displacement: the second electron pair of the double bond and also the lone electron pair are easily moved; as a result, such electrons tend to distribute evenly throughout the molecule, which stabilizes the molecular system. For example, in molecular system A in Figure 6, the lone electron pair of

Figure 7. Robinson’s hypothesis for the electrophilic substitution reaction.

As Robinson did not give up the aromatic sextet rule until 1932, his idea may be interpreted in relation to that rule as follows. After the activation of the aromatic sextet state, electromeric change places the excess negative charge on the carbon atom at the opposite position of the nitrogen atom (left above). Then the electrophilic reagent Dz+ approaches that position (right above). After the liberation of a hydrogen atom as a proton (left below), the reverse electromeric change returns the system to the initial stable aromatic sextet state (right below). Each symbol × designates an electron of the double bond in benzene. In 1932, Robinson expressed skepticism about the aromatic sextet rule, a change that I feel resulted from a disagreement between him (40) and Hückel (41) about the theory for the aromatic substitution reaction. In particular, their hypotheses were directly opposed with respect to the charge distribution of the substituted benzene ring. Robinson proposed the


197


charge distribution for the electrophilic substitution reaction seen in Figure 8. Conversely, quantum chemical treatment of the aromatic sextet that took into consideration only the inductive effect led Hückel, in 1931, to propose the opposite charge distribution for the same reaction (Fig. 9) (42). Because of this, Robinson began to have misgivings about the aromatic sextet rule. It was not a vital part of his theory, in which the inductive effect and electromeric change were more significant. From his point of view, the Kekulé formula was sufficient to explain the aromatic substitution reaction, and there was no need for the rule (43). X

Aromatic Seven-Membered Ring Compounds— Revival of Aromatic Sextet Rule

X

(ortho-para orientation)

(meta orientation)

Figure 8. Robinson’s charge distribution O

O

Cl

N

–16 +5 +5

+16 –5 –5

–1

+1

–1

+1

+8

–8

(ortho-para orientation)

(meta orientation)

Figure 9. Hückel’s charge distribution.

In 1933, Ingold criticized the aromatic sextet rule from the standpoint of mesomerism (44). Pyrrole, for example, had an aromatic sextet, but according to the concept of mesomerism, the normal state of pyrrole was the stable intermediate among five electron distributions (Fig. 10). In this case, the rule did not necessarily seem to lose its significance. Other cases containing 12 and 8 π-electrons (Fig. 11), however, dealt the aromatic sextet rule a fatal blow, because from the viewpoint of mesomerism, such systems were considered to be stable systems of 12 and 8 π-electrons. Thus the aromatic sextet state was readily and inevitably regarded merely as a case of mesomerism. 2e

2e

2e NH

NH

NH 2e

2e

NH

NH

Figure 10. Ingold’s five electron distributions for pyrrole.

(δ–)

(δ+)

O MeO

C

H

(δ+) X

(δ–)

(δ–) Y

pointed out that there was an inconsistency in their argument and that they had set their parameters arbitrarily (46). He also indicated that it was unclear whether the charge distribution of the benzene ring was significant to the aromatic substitution reaction or not, and that experimental evidence of the mechanism of such reactions was insufficient. In fact, it was not until about 1950 that the details of this mechanism were clarified experimentally (47). Hückel’s acceptance of mesomerism in 1937 was important, however, because it was an additional factor contributing to the fate of the rule of six.

(δ+)

C

O (δ–)

The revival of the aromatic sextet rule came from the study of new aromatic seven-membered ring (ASMR) compounds (48). It played an especially important role in research to discover the parent compound (i.e., the structure fundamental to them). The significance of the parent compound to ASMR compounds corresponds to that of benzene to benzenoid compounds. In 1945, Michael J. S. Dewar of the University of Oxford proposed a seven-membered ring structure for stipitatic acid (Fig. 12) (49). This compound was obtained as a metabolite of the mold Penicillium stipitatum in 1942, but no one had been able to ascribe a reasonable structure to it. He attributed the aromatic character of stipitatic acid to the resonance in the hypothetical tropolone structure (5). This was the first proposal of an ASMR compound and also the first hypothesis of a parent compound (Tropolone hypothesis). After Dewar’s initial postulate, chemists began to look for and prepare ASMR compounds. COOH

COOH O

HO

HO

O O

O

H

O

H

5

Figure 12. Resonating structures of stipitatic acid.

In 1948, Holger Erdtman of Kungl. Tekniska Högskolan, Stockholm, proposed a second hypothesis, i.e., the vinylogue hypothesis (50). He found γ-thujaplicin and ascribed the seven-membered ring structure, 6, to it. Differing from Dewar, however, Erdtman attributed its acid nature to its structure. According to him, its nature could be easily explained if the compound was regarded as a cyclic vinylogue of a carboxylic acid. Vinylogy was then a well-known phenomenon (51). Figure 13 illustrates his idea that γ-thujaplicin is a cyclic vinylogue made from a vinylogue [O=C–(C=C) 3 –OH] of a carboxylic acid [O=C–OH].

(δ+)

Figure 11. Ingold’s systems of 12 and 8 π-electrons.

In 1935, Pauling and Wheland used the MO method to attempt to give quantum chemical foundations to the theories proposed by organic chemists (45). Although they seemed to attain considerable agreement with organic chemists’ theories and experiments, their results were inadequate in some respects. In 1937, Hückel

198

OH

O

O

C

C

C

C

C

C

C

OH

O– H+ CH3

CH CH3

6 Figure 13. Cyclic vinylogue of a carboxylic compound.



J. W. Cook of the University of Glasgow proposed a third hypothesis at the tropolone symposium in 1950, although at the time Dewar did not consider it feasible (52). This hypothesis attributed the nature of the ASMR compounds to the group of six π-electrons in the ring, which, of course, was the rule of six. I call this third hypothesis the rule of six hypothesis. At the time, tropolone had just been prepared and experimental research on the ASMR compounds was only in the initial stage, so it was impossible to determine what hypothesis was the most appropriate for them. In order to compare the various hypotheses, research into the fundamental structure was continued in the direction of eliminating impurities—i.e., removing substituents step by step from the seven-membered ring. In 1951, Hyp J. Dauben, Jr., of the University of Washington (53) and W. von E. Doering at Hickrill Chemical Research Laboratory (54) prepared and examined tropone (7). Both of them, and especially Doering, thought the rule of six hypothesis (8) favorable. O

O

O

( ) 7

( )

or

6

( ) ( )

( ) ( )

7 8 Around 1952, a number of experiments on ASMR compounds gave support to the vinylogue hypothesis. For example, it was found that the nucleophilic substitution reaction preferred 1-, 2-, 4-, 6-positions and the electrophilic one preferred 3-, 5-, 7-positions. The electronic charge distribution that can be assumed from such experiments was, I have found, the same as that deduced from the vinylogue hypothesis (Figs. 14 and 15). O

OH

O (+)

1 2

(–) 7 3 (–) 6 5 4 (–)

of six π-electrons (eq 3). Thus he thought the third hypothesis appropriate.

O

O

OH

Cl

HO

HO

OH Br

i.e.

(–)

Cl–

(–)

OH

Br

HO Br

OH

Br

Br

(3)

Br– Cl

Br

Cl

After the above experiment, Doering prepared diazocyclopentadiene (9) (57). He then thought that a reasonable elucidation of its nature was possible if Hückel’s rule was effective. Presumably he thus came to believe firmly in Hückel’s rule. In 1954, Doering prepared tropylium cation (cycloheptatrienylium ion), C7H7+, the fundamental structure of the ASMR compounds (eq 4) (58). This preparation was, in a sense, the first verification of Hückel’s rule. N

N

N

N

N

N

N

N

a

b

c

d

9

OH H

OH

Cl

(+) H H

(+)

(+)

O

(2)

Br

Br

and

OH

Cl

SOCl2

Cl

Br

Cl

Cl

Br

HCl

H Br2

O

OH

(+)

(+)

or (+)

H

–HBr Br

Br

Figure 14. Charge distribution assumed from experiments.

O

O

OH

Br

Cl

(+)

(–)

Figure 15. Charge distribution deduced from vinylogue hypothesis.

At about same time, other experiments were carried out to determine whether tropolone was merely one of the hydroxylic derivatives of tropone (i.e., α-, β-, or γhydroxytropone) (see 5, α-hydroxytropone) (55). As a result, it was believed that the hydrogen bond had great influence on the nature of tropolone (see Fig. 12). This favored the tropolone hypothesis. Doering’s experimental study offered evidence for the rule of six hypothesis (56). In 1952 he discovered the nucleophilic exchange reaction of halogen (eq 2). This was not explainable in terms of the vinylogue hypothesis, because the charge distribution from that experiment had to be assumed to be positive at least at 3-, 5-, 7-positions. According to him, such reactions could be explained if the seven-membered ring had the structure

Br–

(4)

H

Research into the fundamental structure of the ASMR compounds proceeded from a natural compound by way of tropolone and tropone to tropylium cation (Fig. 16) (59). It should be underscored here that methodologically this process moved in the direction of eliminating impurities step by step from the ring. This was also the process comparing the various hypotheses regarding the ASMR compounds. O

OH

O

Natural Compound

Figure 16. Process toward the fundamental structure of ASMR compounds.

As a result of such progress (together with the subsequent successful application of Hückel’s rule), the aromatic sextet rule and Bamberger’s axiom were revived. Furthermore, Hückel’s rule began to be regarded as effective, and the MO theory got considerable additional support in chemistry because Hückel’s rule was derived from the MO theory (60). In addition, the structure and


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stability of benzene began to be explained generally in terms of both Hückel’s rule and Penney’s study. Tetsuo Nozoe and His Early Study of Hinokitiol By 1949, the Japanese organic chemist Tetsuo Nozoe had independently published three probable hypotheses for the fundamental structure of hinokitiol (β-thujaplicin) (61). At the time, he thought that the mesomeric state of his three structures might be possible (Fig. 17). His type A hypothesis was the equivalent of the tropolone hypothesis and type C of the vinylogue hypothesis, but type B differed slightly from the rule of six hypothesis because of his ignorance of it at the time. He sent papers describing his hypotheses to leading European and American chemists in 1949. Thus it is possible that Nozoe’s hypotheses were the initial stimulus for the revival of the rule of six, which was well known to Western chemists but was not considered effective in those days (62). δ–

1 —

δ+ O

2

O

O H

H

O

δ–

H

O

O

1 — 2

Type A

Type B

δ+

δ– Type C

Figure 17. Nozoe’s three hypotheses for the fundamental structure of hinokitiol.

Concluding Remarks Since the 1960s, new theories of aromaticity have been proposed. Hückel’s rule is restricted to the monocyclic, coplanar π-electron systems. Once a broader theory has been established, a more complete history will appear. This, however, is beyond the scope of the present paper. Acknowledgments My heartfelt thanks are due to Tetsuo Nozoe and to the members of “Tuesday Seminar” organized by Masakatsu Yamazaki and Tadaaki Kimoto at Tokyo Institute of Technology, for their great kindness and useful advice. The present paper is a summary of the author’s Ph.D. thesis, Tokyo Institute of Technology, 1991. Literature Cited 1. Hückel, E. Z. Phys. 1931, 70, 204–286. Autobiography: Hückel, E. Ein Gelehrtenleben; Verlag Chemie: Weinheim, 1975. 2. Kekulé Centennial; Benfey, O. T. Ed.; American Chemical Society: Washington, DC, 1966; Theoretical Organic Chemistry, Papers presented to the Kekulé Symposium organized by the Chemical Society; International Union of Pure and Applied Chemistry, Section of Organic Chemistry; Butterworths: London, 1959. 3. Breslow, R. Chem. Eng. News 1965, 43(26), 90–99; Dictionary of the History of Science; Bynum, W. F.; Brown, E. J.; Porter, R., Eds.; Macmillan: London, 1981; pp 28–29; Clar, E. The Aromatic Sextet; Wiley: London, 1972; Garratt, P. J. Aromaticity; Wiley-Interscience: New York, 1986; pp 3–8, 11, 19; Garratt, P. J. Endeavour (New Ser.) 1987, 11, 36–42; Klein, D. J. J. Chem. Educ. 1992, 69, 691–694; Koeppel, T. A. Ph.D. Thesis, University of Pennsylvania, 1973; Kolb, D. J. Chem. Educ. 1979, 56, 334–337; Russell, C. A. The History of Valency; Leicester Univ.: New York, 1971; Snyder, J. P. Nonbenzenoid Aromatics; Academic: New York, 1969; Vol. 1, Chapter 1. 4. Bamberger, E. Ber. Dtsch. Chem. Ges. 1891, 24, 1758–1764; Liebigs Ann. Chem. 1893, 273, 373–379. 5. Armit, J. W.; Robinson, R. J. Chem. Soc. 1925, 127, 1604–1618. 6. Armstrong, H. E. J. Chem. Soc. 1887, 51, 258–268; Tilden, W. A. J. Chem. Soc. 1888, 53, 879–888. 7. Armstrong, H. E. Phil. Mag. 1887, 23, 73–109.

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