First theory, benzene structure

Aromatics were thought at one time to be unsaturated because the structure was thought to have double bonds. (See the first-theory benzene structure in Figure 5.38.) The structure appeared to have three double bonds to satisfy the octet rule of bonding. However, in reality, aromatics do not behave like unsaturated compounds. They bum with incomplete combustion. They are unreactive, so it is theorized that instead of three double bonds, they have a unique structure where the six extra electrons are in a state of resonance within the benzene ring. They are not attached to any one of the carbons, but rather go from one to another at a speed faster than the speed of light, much the same way a rotor works inside a distributor in an automobile. The... 

X-ray investigations of the structure of benzene began in 1923 when Broome took the first X-ray powder photographs of the molecule. Later, Cox (1928) determined the cell dimensions and space group and showed that the molecule was at least centrosymmetric. The development of the work on the benzene structure has been reviewed by Cox (1958). A more detailed paper on the crystal structure of benzene at — 3°C (Cox et al., 1958) has established that the benzene molecule does not deviate significantly from the 6jmmm symmetry predicted by chemical theory, the maximum deviation of the carbon atoms from the mean molecular plane being 0-0013 A. 

HMO theory is named after its developer, Erich Huckel (1896-1980), who published his theory in 1930 [9] partly in order to explain the unusual stability of benzene and other aromatic compounds. Given that digital computers had not yet been invented and that all Hiickel s calculations had to be done by hand, HMO theory necessarily includes many approximations. The first is that only the jr-molecular orbitals of the molecule are considered. This implies that the entire molecular structure is planar (because then a plane of symmetry separates the r-orbitals, which are antisymmetric with respect to this plane, from all others). It also means that only one atomic orbital must be considered for each atom in the r-system (the p-orbital that is antisymmetric with respect to the plane of the molecule) and none at all for atoms (such as hydrogen) that are not involved in the r-system. Huckel then used the technique known as linear combination of atomic orbitals (LCAO) to build these atomic orbitals up into molecular orbitals. This is illustrated in Figure 7-18 for ethylene. 

The results of our calculations based on both the static and dynamic theories show that most of the nonbenzenoid cyclic conjugated systems examined exhibit in a greater or lesser degree a marked double-bond fixation. The static theory indicates that even in benzene there exists a hidden tendency to distort into a skewed structure and that such a tendency is actually realized in [4n-f-2] annulenes larger than a certain critical size. In nonalternant hydrocarbons bond distortion is a rather common phenomenon. Fulvenes, fulvalenes and certain peri-condensed nonalternant hydrocarbons undergo a first-order bond distortion, and... 

The resonance theory can be applied successfully to explain the structure of benzene. First of all, let us have a look at the resonance theory. According to this theory... 

We have used the concepts of the resonance methods many times in previous chapters to explain the chemical behavior of compounds and to describe the structures of compounds that cannot be represented satisfactorily by a single valence-bond structure (e.g., benzene, Section 6-5). We shall assume, therefore, that you are familiar with the qualitative ideas of resonance theory, and that you are aware that the so-called resonance and valence-bond methods are in fact synonymous. The further treatment given here emphasizes more directly the quantum-mechanical nature of valence-bond theory. The basis of molecular-orbital theory also is described and compared with valence-bond theory. First, however, we shall discuss general characteristics of simple covalent bonds that we would expect either theory to explain. 

We already have alluded to the difficulties encountered in the interpretation of the structure of benzene in Sections 1-1G and 6-5. Our task here is to see what new insight the VB and MO treatments can give us about benzene, but first we will indicate those properties of benzene that are difficult to explain on the basis of simple structure theory. 

A recent summary of the history and dynamics of the theoretical models of benzene39 cites a view that even though the current molecular orbital (MO) view of benzene seems complete and ultimate while the valence bond (VB) view seems obsolete, the recent findings about zr-distortivity in benzene indicate that the benzene story is likely to take additional twists and turns that will revive the VB viewpoint (see footnote 96 in ref 39). What the present review will show is that the notion of delocalized zr-systems in Scheme 1 is an outcome of both VB and MO theories, and the chemical manifestations are reproduced at all levels. The use of VB theory leads, however, to a more natural appreciation of the zr-distortivity, while the manifestations of this ground state s zr-distortivity in the excited state of delocalized species provides for the first time a physical probe of a Kekule structure .3... 

The concept of electronic delocalization has germinated in the pre-electron period to Kekule s structural theory and its application to benzene as a prototype of a family of compounds so-called aromatics . Kekule had to address two major properties of benzene revealed from substitution experiments. The first was the empirical equivalence of all positions of benzene, what is called today the Dfjh symmetry of both geometry and electronic structure, and second the persistence of the aromatic essence in chemical reactions, what we recognize today as aromatic stability . Thus, Kekule postulated that there is a Ce nucleus and the four valences of the carbons are distributed to give two oscillating structures, which when cast in our contemporary molecular drawings look like part a in Scheme 2.39-44 One of the many alternative hypotheses on the nature of... 

The history of benzene is one of the most intriguing in science. It started in 1825 with the isolation of benzene by Michael Faraday from the condensed phase of pyrolyzed whale oil. Its planar cyclic structure was first proposed in 1861 by the Austrian physicist and physical chemist Johann Josef Loschmidt [1—5]. However, it was only fully understood some 70 years later, around 1930, with the advent of the modem theories of aromaticity, i.e. the theory of molecular orbitals (Hiickel s theory) [6-8] and the theory of resonance [9-12]. 

The carbon chains are meta related on the central ring so for the first time we have a branched polymer, Complexity can rapidly increase as more phenols linked through more formaldehydes can be joined on to this core structure at several points. Each benzene ring could, in theory, form three new C-C bonds. 

Structure oriented design refers to projects aimed at the creation of molecules with unusual structural characteristics not necessarily related to some useful property. This brings to mind a plethora of classical studies such as the previously discussed syntheses of cyclooctatetraene, Dewar benzene, or asterane. More recent examples include the syntheses of the dodecahedrane and tetrahedrane frameworks. The goal of the investigations in this area is first to invent and then to synthesize certain non-trivial molecules having unique structural features. This uniqueness very often refers to a novelty in the general shape of the molecules (as for dodecahedrane or catenanes), which are otherwise constructed in accordance with the classical concepts of structural theory. At the same time, quite a number of unprecedented structures have... 

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