Benzene orbital overlap model

Orbital overlap model of the bonding in benzene, (a) The carbon, hydrogen framework. The six 2p orbitals, each with one electron, are shown uncombined, (b) The overlap of parallel 2p orbitals forms a continuous pi cloud, shown by one torus above the plane of the ring and a second below the plane of the ring. 

Facile dehydrogenation is consistent with kinetic models derived from catalytic conversion studies of cyclohexane to benzene. These models predict an ensemble size for the active site of only one atom. On the other hand, surface science studies propose a model where several metal atoms, on the order of seven, are required, and suggest that specific orientation with respect to subsurface metal atoms is needed. Theoretical studies suggest that the key is to bring cyclohexane sufficiently close to the metal such that strong orbital overlap will occur. Small clusters may be even more effective than surfaces. Further experiments are needed to identify the chemical state of the products of the cluster reactions in order to connect the results with the surface science and catalysis results. 

An alternative but equivalent model for describing benzene (and other resonance-stabilized structures) is molecular orbital theory. We have already seen how this theory can explain the formation of molecular structures such as methane, ethene and others. In localized molecules like ethene, C2H4, two unhybridized p orbitals overlap to form a Jt molecular orbital in which a pair of electrons is shared between tbe nuclei of two carbon atoms. In molecular orbital theory, resonance-stabilized structures are described in terms of delocalized Jt orbitals where the Jt electron clouds extend over three or more atoms. 

FIGURE 113 (a) The framework of bonds shown in the tube model of benzene are cr bonds (b) Each carbon is sp hybridized and has a 2p orbital perpendicular to the cr framework Overlap of the 2p orbitals generates a tt system encompass mg the entire ring (c) Electrostatic potential map of benzene The red area in the center corresponds to the region above and below the plane of the ring where the tt electrons are concentrated... 

We found that these more sophisticated spin-coupled calculations, which used larger basis sets with polarization functions on all of the atoms and which allowed the a orbitals to relax, produced a picture of bonding in the 7t-electron system of benzene which is practically identical to that described earlier. As before, we found six equivalent spin-coupled orbitals which are transformed into one another by successive C6 rotations. The overlaps between the orbitals, ordered cpa to cp6 around the ring, are reported in Table 1. In this case, the electron correlation effects incorporated in the spin-coupled model provide an energy improvement over the SCF description of 170 kJ mol - with a further lowering of 20 kJ mol -1 on including spin-coupled ionic structures. 

Fukui and Fujimoto (1966a) suggest that an overlap or bond-order criterion is another way in which secondary interactions may be examined. If benzene is assumed to be a crude model for the transition state of the [3,3] sigmatropic reaction, it is easy to show that P for the three orbitals is 2(p z +pz + p ) = — 0 33. The negative bond order indicates repulsion. Therefore, the concerted [3,3] sigmatropic reaction prefers the chair transition state in which atoms 2 and 5 are as far apart as possible. 

You will probably be surprised to find cyclooctatetraene (COT for short), unlike benzene, is not planar. Also none of the double bonds are conjugated—there are indeed alternate double and single bonds in the structure but conj ugation is possible only if the p orbitals of the double bonds can overlap here they do not. Since there is no conjugation, there are two C-C bond lengths in cyclooctatetraene—146.2 and 133.4 pm—which are typical for single and double C-C bonds. If possible, make a model of cyclooctatetraene for yourself—you will find the compound naturally adopts the shape below. This shape is often called a tub . 

The following molecular model of a dimethyl-substituted biphenyl represents the lowest-energy conformation of the molecule. Why are the two benzene rings not in the same plane so that their p orbitals can overlap Why doesn t complete rotation around the single bond joining the two rings occur ... 

However, they do provide an explanation for the preferred endo mode of cycloaddition for those alkenes giving meta adducts. Most c/.v-disubstitutcd alkenes like cyclopentene1159 give substantially more endo adduct 8.134 than exo 8.135. Because of the good HOMO/ HOMO and LUMO/ LUMO match with simple alkenes, there will be two sets of secondary orbital interactions to consider. One 8.136 will be between the HOMO of the doubly X-substituted alkene, crudely modelled by i >3 of butadiene, and ip 2 of benzene, and the other 8.137 will be between the LUMO of the alkene, modelled by ip4 of butadiene, and ip5 of benzene. In both cases the secondary overlap is in favour of the endo mode of cycloaddition.1157... 

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