Benzene, cyclic orbital interaction

Benzene (Scheme 11) serves as a simple model to illnstrate the cyclic orbital interaction in the cyclic systems. 

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... 

The orbital phase theory can be applied to the thermodynamic stability of the disubstituted benzene isomers. The cyclic orbital interaction in the benzene substituted with two EDGs is shown in Scheme 21. The orbital phase is continuous in the meta isomer and discontinuous in the ortho and para isomers (Scheme 22, cf. Scheme 4). 

Scheme 21 Cyclic orbital interaction in benzenes with two electron-donating groups...

Benzene. In the molecular orbital approach for benzene, each carbon in the ring is considered to use sp2 hybrid orbitals. These orbitals are involved in carbon-carbon (t bonding (from overlap of sp2 hybrids on adjacent carbons) and carbon-hydrogen a bonding (from overlap of an sp2 hybrid on each carbon with the Is orbital of hydrogen). This leaves on each carbon a p orbital not participating in the hybrids and available to participate in a cyclic n system. When these six p orbitals interact, six n molecular orbitals are formed, as illustrated in Figure 2-12. 

The orientation of the reactions of EDG-substituted benzenes is determined by delocalization from EDG to E via B. The delocalization contains the cyclic interactions of the electron-donating orbital (edg) of EDG, electron-accepting orbital (e ) of the electrophile, and the HOMO (b) and the LUMO (b ) of the benzene ring (Scheme 17). For the ortho and para orientations, the delocalization is favored by the orbital phase... 

We can reach a similar conclusion from an interaction diagram, by looking at the effect of changing butadiene 1.24 into cyclobutadiene 1.25 (Fig. 1.47). This time there is one drop in n energy and one rise, and no net stabilisation from the cyclic conjugation. As with benzene, we can see that the drop is actually less (from overlap of orbitals with a small coefficient) than the rise (from overlap of orbitals with a large coefficient). Thus cyclobutadiene is less stabilised than butadiene. 

In this chapter, we will uncover the sources of these differences in stability between cyclic polyenes and begin to see the chemical and physical consequences of this special kind of conjugative interaction of 2p orbitals. We will encounter the molecule benzene (Fig. 13.2), in which the overlap of six carbon 2p orbitals in a ring has great consequences for both structure and reactivity. We will also see a generalization of the properties that make benzene so stable, and will learn how to predict which cyclic polyenes should share benzene s stability and which should not. 

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