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Historical Overview

Source: T. M. Krygowski, H. Szatylowicz, ChemTexts 2015, 1, 12.

"Classification and theory are not ends in themselves. If they generate new experimental work, new compounds, new processes, new methods - they are good; if they are sterile - they are bad." (E. D. Bergman, in Aromaticity, Pseudo-aromaticity, Anti-aromaticity, Proceedings of an International Symposium held in Jerusalem 31 March - 3 April 1970 [Eds. E. D. Bergmann, B. Pullman], Israel Academy of Science and Humanities, Jerusalem, 1971, pp. 392-392)

Aromaticity belongs to one of the four most commonly used terms in organic chemistry and related fields, along with conformation, H-bonding, and polymerization [1]. As it can be seen from the histogram in Figure 1.1, the topic of aromaticity appears on average more than 40 times per day in 2020. So great interest in research related to aromaticity is associated with a large number of works on the definition of the term, as well as on experimental and theoretical studies on the physicochemical characteristics attributed to the aromatic character of chemical species in their ground, excited, and transition states [2].

Figure 1.1 The number of scientific papers, appearing per day, in which the indicated term appears in title, abstract or keywords (asterisk denotes any adequate ending, i.e., s, ed, ing, ity, or nothing) [1, 2].

The term aromatic applied for the classification of organic species was used for the first time in 1856 by Hoffman in his work on monocarboxylic derivatives of benzene [3]. However, discovery of the archetypic aromatic compound benzene and estimation of its empirical formula as well as its physical properties had been done already 30 years earlier in 1825 by Faraday [4]. Then for almost half of century, most efforts were devoted to the investigation of the structure of benzene and chemical properties of its derivatives. Two aspects were taken into account at that time. Kekulé regarded as aromatic compounds those that are structurally similar to benzene [5, 6], whereas Erlenmeyer [7] considered as aromatic compounds those having alike chemical properties (reactivity) as benzene. Later Kekulé also accepted the importance of a similarity to benzene in chemical properties, and in his Lehrbuch der organischen Chemie [8], classified alcohols, aldehydes, and acids into either aromatic or aliphatic ones. From the stoichiometry, it was known that benzene is built up of single and double bonds, but troubles appeared in trying to understand its structure. The first cyclic structure of benzene as a hexagon was suggested by Laurent in 1855 and presented by Loschmidt [9], but much closer to the actual viewpoint is a structural formula by Kekulé [8], as shown in Figure 1.2.

In the Kekulé formula, the carbons are arranged in a hexagon with alternating?double?and?single bonds between?them. In 1869, Ladenburg [10] questioned Kekulé's structure, indicating that there should be four isomers for disubstituted benzene derivatives (Figure 1.3), whereas only three of them were known. Moreover, he suggested another view of its structure, i.e. by presenting it as - actually named - prismane. Figure 1.4 presents relations between isomers of benzene and relates them to isomers of prismane.

Figure 1.2 The Loschmidt (a) and the Kekule (b) cyclic formulas of benzene.

Figure 1.3 Assumed isomers by Ladenburg for disubstituted benzene derivatives.

In defense of his cyclic structure, Kekulé accepted some assumption that the double and single bonds are in a permanent exchange - again an ingenious intuition? The concept of prismane-like structure of benzene was questioned and rejected by Baeyer [13] since 1,2-disubstituted benzene derivatives did not exhibit optical activity.

At the same time as benzene, another classically aromatic hydrocarbon, naphthalene, was found in 1821 as a result of distillation of tar coal [14]. Its structure as two benzenes fused was proposed in 1866 by Erlenmeyer [7]. Other important aromatic compounds were those containing heteroatoms. They were discovered at the end of the 19th century. Anderson in 1849 [15] obtained the first heteroaromatic molecule named pyridine through his studies on the distillation of bone-oil and other animal matter. In 1870, Limpricht obtained furan [16], whereas in 1890, Hantzsch presented a method of synthesis of pyrrole derivatives [17]. Thiophene accompanying benzene in tar coal was discovered by Meyer in 1883 [18]. It exhibited chemical properties similar to benzene. In 1891, Bamberger [19] proposed a cyclic structure for all these compounds.

Figure 1.4 Relationships between substituted benzene and prismane (as a structure of benzene) derivatives [11, 12].

At the end of 19th century, quite a number of aromatic compounds were known, and it was known that these compounds are less reactive than their olefinic analogues. The question that was posed at that time was: Is the resistance of aromatics, thus formally unsaturated compounds, due to their cyclic structure? The question was answered by the work of Willstätter [20], who obtained a derivative of cyclooctatetraene and found that its chemical behavior is similar to a typical olefinic molecule.

A new impetus came from physics. Discovery of the electron and then its role in atoms, and later in molecules, gave a new view on molecular structure. At that time, structural formulae were presented basing on an octet-rule introduced by Lewis [21] and then developed by Langmuir [22]. A modern understanding of the chemical bond was established suggesting that a?chemical bond?is a pair of?electrons?shared by two atoms. In 1922, Crocker [23] presented a new version of the benzene structure built up in agreement with the Lewis-Langmuir theory. It is shown in Figure 1.5.

Figure 1.5 Crocker's electronic structure of benzene.

Crocker's structure took into account six equivalent carbon atoms with six electrons in between. This idea was, for a few years, not noticed, until about 1925 when Armit and Robinson [24] made use of it and introduced the concept of electron sextet, and noted that "the group of six electrons.resists disruption," describing for the first time the concept of aromatic sextet. They also introduced an assignment of the aromatic sextet by a circle inside the six membered rings. All these works inspired Clar [25] to formulate the p-sextet rule as an extension of the Hückel's 4n?+?2p-electron rule from monocyclic species to benzenoid systems. Moreover, in 1938, Evans and Warhurst [26] already noted the analogy between the p-electrons of benzene and the six delocalized electrons in the cyclic transition state of the Diels-Alder reaction between butadiene and ethylene, and suggested for the first time the existence of aromatic transition states.

The next important step in description and understanding aromaticity has come as a result of development of quantum mechanics [27] and its applications to chemical problems [28-30]. Then an important contribution by Hückel has to be stressed. He introduced the concept of s and p orbitals, describing electrons differing in their properties (symmetry), and then worked out a simplified molecular orbital theory, nowadays known as Hückel Molecular Orbital (HMO) theory [28]. Application of the HMO theory allows interpretation of the nonaromatic properties of cyclobutadiene and cyclooctatetraene, giving a solution for the older question posed in the time of Willstätter. A famous "magic" rule from these works resulted: 4n?+?2 saying that cyclic p-electron systems containing 4n?+?2 electrons are stable, whereas those containing 4n electrons are less stable. This rule was formulated by Doering et al. [31], who noted that heptatrienyl cation is more stable than cyclopentadienyl cation since the first one possesses 4n?+?2p-electrons whereas the latter has 4n. A wide and a strong documentation of this rule has been given by Roberts, Streitwieser, and Regan [32], and the rule may be more generally stated as follows: "those monocyclic coplanar systems of trigonally hybridized atoms which contain 4n?+?2p electrons will possess relative electronic stability" [33].

The determination in 1959 of the structure of the closo borane B10H102- ion by Lipscomb [34], and the synthesis three years later of the first derivatives of closo-dodecaborate and closo-decaborate by Muetterties's group [35], released the concept of aromaticity from the two dimensions and moved it to three dimensions. In 1972, Baird [36] predicted the existence of the lowest-lying triplet state aromaticity for...