Canonical structures benzene

In valence bond terms the pyrazine ring may be represented as a resonance hybrid of a number of canonical structures (e.g. 1-4), with charge separated structures such as (3) contributing significantly, as evidenced by the polar character of the C=N bond in a number of reactions. The fusion of one or two benzene rings in quinoxaline (5) and phenazine (6) clearly increases the number of resonance structures which are available to these systems. 

The benzene molecule can now be treated very simply by the Slater method, with the help of the rules formulated by one of us4 for finding the matrix elements occurring in the secular equation. The bonds between the six eigenfunctions can be drawn so as to give the independent canonical structures shown in Fig. 1. Any other... 

Fig. 1. The five canonical structures contributing to the normal state of the benzene molecule.

A possibly more accurate value for the double bond character of the bonds in benzene (0.46) id obtained by considering all five canonical structures with weights equal to the squares of their coefficients in the wave function. There is some uncertainty aS to the significance of thfa, however, because of- the noii -orthogOnality of the wave functions for the canonical structures, and foF chemical purposes it fa sufficiently accurate to follow the simple procedure adopted above. 

The electronic structure of the nitrone group, except for the main A structure, includes four canonical B-E structures. In the case of aromatic derivatives, it is necessary to consider the conjugation with the benzene ring (structures F and G)... 

The magnitude of the above-mentioned shifts of the ortho absorptions by various substituents (ortho shift) lies between the relatively large shifts of the hydrogen atoms cis to the substituents in vinyl compounds and the smaller ortho shifts of the usual benzene derivatives. The ortho shifts in the [2.2]paracyclophane system are thus attributed to an increased double-bond character in the deformed benzene rings, where canonical structures such as 77 could possibly contribute to stabilization of the molecule. 

Comparison of the bond lengths with those of benzene, naphthalene and pyridine suggests that whilst there is certainly aromatic character some bond fixation occurs in the naphthyridines. It is probable that canonical structure (1) (of, for example, 1,5-naphthyridine) and to a lesser extent structures (7) and (8) are the major contributors to the ground state, but the charged structures (9) and (10) must also be included in order to account for the chemical properties of the naphthyridines. 

In this they somewhat resemble the curly arrows used to show resonance. in benzene, where the arrows show where to draw the new bonds, and which ones not to draw in the canonical structure but in this case there is neither a sense of direction nor even an actual movement. The analogy between the resonance of benzene and the electron shift in the Diels-Alder reaction is not far fetched, but it is as well to be clear that one is a reaction, with starting materials and a product, and the other is not. 

Most organic chemists are familiar with two very different and conflicting descriptions of the 7r-electron system in benzene molecular orbital (MO) theory with delocalized orthogonal orbitals and valence bond (VB) theory with resonance between various canonical structures. An attitude fostered by many text books, especially at the undergraduate level, is that the VB description is much easier to understand and simpler to use, but that MO theory is in some sense more fundamental . 

Sometimes it is not possible to assign a single electronic structure to a molecule that may account for all its properties. In such a case the molecule is represented by two or more electronic structures but none of these represents the actual structure of the molecule. The actual structure of the molecule lies somewhere in between these structures but can not be expressed on paper. Such a molecule is said to exhibit resonance. The various structures assigned to the molecule are called contributing structures or canonical structures where as the intermediate structures is called the resonance hybrid. For example benzene (C6H6) maybe assigned the following two structures ... 

Benzene played an important role in my development of freeon dynamics. I knew, by the Rumer rule, that benzene had five canonical structures (i. e., five linearly independent wave functions). These are the two Kekule structures and the three Dewar structures. From this I surmised that the singlet IRS for N = M = 6 should be of five dimensions. On consulting the symmetric group textbooks I... 

Figure 10. Schematic illustration of a tendency of each benzene fragment in naphthalene to retain its aromaticity by producing cis 1,3-butadiene partial localization in its twin-ring as described by the resonance structures (7a) and (7b) yielding the resulting predominant canonical structure (7c). This intuitive argument is supported by the (HF/6-31G ) bond distances and the corresponding 7r-bond orders given within parentheses.

The arrangement of the valency bonds in the various canonical structures is determined by the same rule as for benzene. The atoms are distributed symmetrically in a circle and joined by all possible ways without the crossing of the valence bonds . The full number of independent structures, corresponding to the total number of canonical structures, is then obtained. 

In benzene there are six tt electrons and the number of independent canonical structures is five. If we denote the carbon atoms, by the letters a, b, Cj dy e and /, these structures will be represented as follows ... 

As in benzene there will be five canonical structures... 

Pyrazine may be represented as a resonance hybrid of the canonical structure illustrated (21a -<+ 21d). The molecule is planar, and Pauling (114) states that it is stabilized by about 40kcal/mol as in benzene and pyridine, but resonance energies derived by different methods show considerable variation. Some of these resonance energies together with values for benzene and related heterocycles are summarized in Table 1.2 (115-117). More recent measurements of heats of hydrogenation are given in Section IV. 1B. 

In the time-averaged molecule, every carbon/carbon bond has both single and double bond character, and each bond is the same length in this symmetrical structure. This cyclic system is called an aromatic ring. The representation of benzene that shows each bond as an individual single or double is called a canonical structure and represents a hypothetical structure that does not exist in practice. The representation that indicates every bond as half-way between a... 

One of the arguments for retaining valence bond theory has been the ease with which things like the nucleophilicity of the allyl anion at C-l and C-3 are explained by drawing the canonical structures. Even as simple a version of MO theory as the one presented here does the job just as well. The drawings chemists use for their structures will inevitably be crude representations we shall always have to make some kind of localized drawing, whether it be of a benzene ring,... 

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