Molecular Orbital Description of Benzene

Having just seen a resonance description of benzene, let s now look at the alternative molecular orbital description. An orbital view of benzene makes clear the cyclic conjugation of the benzene molecule and the equivalence of the. six carbon-carbon bonds. Benzene is a planar molecule with the shape of a regular hexagon. All C-C-C bond angles are 120 , all six carbon atoms are sp -hybridized, and each carbon has a p orbital perpendicular to the plane of the six-membered ring. 

Since all six carbon atoms and all six p orbitals in benzene are equivalent, it s impossible to define three localized ir bonds in which a given p orbital overlaps only one neighboring p orbital. Rather, each p orbital overlaps equally well with both neighboring p orbitals, leading to a picture of benzene in which the six n electrons are completely delocalized around the ring. 

The six benzene ir molecular orbitals. The bonding orbitals / 2 and ip, have the same (flergy and are said to be degenerate as are the antibonding orbitals ip/ and ips. The orbitals ipi and ip have no jt electron density on two carbons because of a node passing through these atoms. 

Problem 1S.4 Pyridine is a flat, hexagonal molecule with bond angles of 120°. It undergoes electrophilic substitution rather than addition and generally behaves like benzene. Draw an orbital picture of pyridine to explain its properties. Check your answer by looking ahead to Section 15.7. 

Key Ideas Let s review what we ve learned thus far about benzene and, by extension, about other benzene-like aromatic molecules  

Erich Hiickel (1896-1980) was bom in Stuttgart, Germany, and received his Ph.D. at the University of Gottingen with Peter Debye. He was professor of physics, first at Stuttgart and later at Marburg (1937-1961). 

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Cyclic conjugation although necessary for aromaticity is not sufficient for it Some other factor or factors must contribute to the special stability of benzene and compounds based on the benzene ring To understand these factors let s return to the molecular orbital description of benzene... 

A molecular orbital description of benzene has three tt orbitals that are bonding and three that are antibonding Each of the bonding orbitals is fully occupied (two electrons each) and the antibonding orbitals are vacant... 

Another important polyatomic molecule is benzene, C6f I6, the parent of the aromatic compounds. In the molecular orbital description of benzene, all thirty C2s-, C2p-, and Hls-orbitals contribute to molecular orbitals spreading over all twelve atoms (six C plus six H). The orbitals in the plane of the ring (the C2s-, C2px, and ( 2/ -orbitals on each carbon atom and all six Hls-orbitals) form delocalized o-orbitals that bind the C atoms together and link the H atoms to the C atoms. The six C2pz-orbitals, which are perpendicular to the ring, contribute to six delocalized tt-orbitals that spread all the way around the ring. However, chemists... 

The molecular orbital description of benzene is much more complex than the two MOs formed in Figure 17.8. Because each of the six carbon atoms of benzene has a p orbital, six atomic p... 

Molecular Orbital Description of Benzene 566 Aromaticity and the Hiickel 4n+2 Rule 567... 

We ll come back to the molecular orbital description of benzene later in this chapter (Section 11.19) to see how other conjugated polyenes compare with benzene. 

This equivalence of the valence bond and molecular orbital descriptions of the bonding in these complexes arises from the alternant1 properties of the metal-butadiene bonding network. A similar equivalence between the two theories occurs for benzene and other polyenes that have alternant 7r-systems (73, 140). 

The molecular orbital description of bonding in benzene derivatives and the importance of the delocalisation of electrons. The concept of resonance stabilisation and the (4n+2) rule. 

Compare atomic charges and electrostatic potential maps for the three cations. For each, is the charge localized or delocalized Is it associated with an empty a-type or Tt-type orbital Examine the lowest-unoccupied molecular orbital (LUMO) of each cation. Draw all of the resonance contributors needed for a complete description of each cation. Assign the hybridization of the C" atom, and describe how each orbital on this atom is utilized (o bond, n bond, empty). How do you explain the benzene ring effects that you observe ... 

Having just seen a resonance description of benzene, let s now look at the alternative molecular orbital description. We can construct -tt molecular orbitals for benzene just as we did for 1,3-butadiene in Section 14.1. If six p atomic orbitals combine in a cyclic manner, six benzene molecular orbitals result, as shown in Figure 15.3. The three low-energy molecular orbitals, denoted bonding combinations, and the three high-energy orbitals are antibonding. 

These simple molecular orbital pictures provide useful descriptions of the structures and spectroscopic properties of planar conjugated molecules such as benzene and naphthalene, and heterocychc species such as pyridine. Heats of combustion or hydrogenation reflect the resonance stabilization of the ground states of these systems. Spectroscopic properties in the visible and near-ultraviolet depend on the nature and distribution of low-lying excited electronic states. The success of the simple molecular orbital description in rationalizing these experimental data speaks for the importance of symmetry in determining the basic characteristics of the molecular energy levels. 

The MO description of benzene, for example, is derived from a linear combination of six atomic 77-orbitals 08. The molecular orbitals ifi are then given by... 

The Kekule description of benzene, as expressed in the classical VB form, appears to be much closer to reality than is a description in terms of delocalized molecular orbitals, and it provides a clear picture of the behavior of correlated electrons in this molecule. The special properties of benzene arise fundamentally from the mode of coupling of the electron spins around the carbon ring framework. Except for small but crucial distortions of the orbitals, the spin-coupled and classical VB descriptions of this molecule are very similar. 

Aromatic systems play a central role in organic chemistry, and a great deal of this has been fruitfully interpreted in terms of molecular orbital theory that is, in terms of electrons moving more-or-less independently of one another in delocalized orbitals. The spin-coupled model provides a clear and simple picture of the motion of correlated electrons in such systems. The spin-coupled and classical VB descriptions of benzene are very similar, except for the small but crucial distortions of the orbitals. The localized character of the orbitals allows the electrons to avoid one another. Nonetheless, the electrons are still able to influence one another directly because of the non-orthogonality of the orbitals. 

Cooper, D. L. Cerratt, J. Raimondi, M. Nature 1986,323,699 reported a spin-coupled valence bond method for calculation of molecular electronic structure and concluded that "our results suggest that the Kekule description of benzene, as expressed in the classic VB form, is in fact much closer in reality than is a description in terms of delocalized molecular orbitals."... 

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