Benzene electron delocalization

The cyclic conjugation and orbital phase are both continuous in benzene. Electrons delocalize in a cyclic manner. The cyclic conjugation is discontinuous in borazine in Scheme 33). Electrons cannot delocalize in a cyclic manner, but only between the neighboring pairs of donors and acceptors. There arises a fundamental question how electrons delocalize in the isoelectronic molecules where C=C bonds are replaced with N-B bonds. 

In addition to its three sp hybrid orbitals each carbon has a half filled 2p orbital that can participate m tt bonding Figure >b shows the continuous rr system that encompasses all of the carbons that result from overlap of these 2p orbitals The six tt electrons of benzene are delocalized over all six carbons... 

During our discussion of benzene and its derivatives it may have occurred to you that cyclobutadiene and cyclooctatetraene might be stabilized by cyclic rr electron delocal ization m a manner analogous to that of benzene... 

There are assnmed to be three n bonds. A, B, and C, in benzene. Here we consider the electron delocalization from A to C. The electron delocalization via B is the same as that in the linear conjngate hexatriene (Schemes 2 and 3) used as a model of non-cyclic conjngate systems. The cyclic orbital interaction has been shown to be favored by the phase continnity (Scheme 5a). There is an additional path for the delocalization in cyclic geometry, which is the direct path from A to C or from a to c. The path gives rise to the cyclic a-b-c and a-b -c interactions. The cyclic orbital interactions satisfy the orbital phase continnity conditions... 

Resonance structures result from a phenomenon known as electron delocalization. The electron pairs in the three double bonds in a benzene ring are delocalized. These are electrons that belong to no particular atom or bond. As a consequence, no ordinary double bonds exist in a benzene ring. The electrons are in an orbital that extends across adjacent atoms. This smear of electrons is usually represented as a circle within the ring. 

This conclusion, nevertheless, should not be considered categorical but it points to the necessity of careful consideration of the correlation between the AEdis value and the part of it that relates to cyclic electron delocalization. It has been shown by use of TRE calculations of aromatic benzene and antiaromatic cyclobutadiene molecules that in the case of benzene the distortion into a Kekule-type structure is characterized by a change of the aromatic cyclic Tr-electron delocalization energy in an opposite direction... 

Magnetic susceptibility anisotropy has been used for the estimation of relative aromaticity of some azines in comparison with benzene (77JCS(P2)897). If the extent of ir-electron delocalization for benzene is taken as 1.0, the corresponding values for azines are pyridine 0.7, pyridazine 0.7, pyrimidine 0.5, and 1,3,5-triazine 0.3. 

Atomic-orbital models, like that shown for benzene, are useful descriptions of bonding from which to evaluate the potential for electron delocalization. But they are cumbersome to draw routinely. We need a simpler representation of electron delocalization. 

The extra stability of benzene relative to the hypothetical 1,3,5-cyclohexatriene can be called its stabilization energy. Most (but not all) of this stabilization may be ascribed to resonance or electron delocalization. 

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