Benzene orbital picture

The picture of benzene as a planar framework of ct bonds with six electrons m a delo cahzed rr orbital is a useful but superficial one Six elecfrons cannof simulfaneously occupy any one orbifal be if an afomic orbifal or a molecular orbifal We can fix fhis wifh the more accurate molecular orbital picture shown m Figure 114 We learned m Section 2 4 that when atomic orbitals (AOs) combine to give molecular orbitals (MOs) the final number of MOs musf equal fhe original number of AOs Thus fhe six 2p AOs of SIX sp hybridized carbons combine fo give six tt MOs of benzene... 

Indole is an aromatic heterocycle that has a benzene ring fused to a pyrrole ring. Draw an orbital picture of indole. 

The molecular orbital picture of benzene proposes that the six jt electrons are no longer associated with particular bonds, but are effectively delocalized over the whole molecule, spread out via orbitals that span all six carbons. This picture allows us to appreciate the enhanced stability of an aromatic ring, and also, in due course, to understand the reactivity of aromatic systems. There is an alternative approach based on Lewis structures that is also of particular value in helping us to understand chemical behaviour. Because this method is simple and easy to apply, it is an approach we shall use frequently. This approach is based on what we term resonance structures. 

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. 

Problem 10.5 How is the structure of benzene explained by (a) resonance, (b) the orbital picture, (c) molecular orbital theory <... 

Benzene s relative lack of reactivity is a consequence of its electronic structure. As shown by the orbital picture in Figure 23.3b, each of the six carbons in benzene is sp2-hybridized and has a p orbital perpendicular to the ring. When these p orbitals overlap to form pi bonds, there are two possibilities, shown in Figure 23.3c. 

On the basis of this resonance picture only, organic chemists initially expected that cyclobutadiene, like benzene, would have a large resonance stabilization and would be especially stable. Yet cyclobutadiene proved to be an extraordinarily elusive compound. Many unsuccessful attempts were made to prepare this compound before it was finally synthesized at very low temperature in 1965. The compound is quite unstable and reacts rapidly at temperatures above 35 K. As we shall see, cyclobutadiene is a member of an unusual group of compounds that are actually destabilized by resonance. To understand why benzene is so stable while cyclobutadiene is so unstable, we must examine a molecular orbital picture for these compounds. 

The facts are consistent with the orbital picture of the benzene molecule. X-ray and electron diffraction show benzene (Fig. 10.4) to be a completely flat,... 

Problem 10.3 The carbon-hydrogen bond dissociation energy for benzene (112 kcal) is considerably larger than for cyclohexane. On the basis of the orbital picture of benzene, what is one factor that ntay be responsible for this What piece of physical evidence tends to support your answer Hint Look at Fig. 10.4 and )see Sec. 5.4.)... 

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. 

As we saw in Chapter 3, the orbitals of the benzyl system can be used as models for the orbitals of Z- and X-substituted benzenes. (The more important frontier orbital for attack by a nucleophile is shown in Fig. 6-7 in bold face, and the more important frontier orbital for attack by an electrophile is shown in italics.) It is then clear how the favoured sites of attack listed at the bottom follow from the orbital picture. In those cases where meta attack is observed, this simple picture predicts both ortho and meta attack. As we saw in Chapter 3, we must not forget that ip2 is very little lower in energy than and tj/6 is very little higher in energy than When the contribution of these orbitals is taken into account, the meta position, rather than the ortho and the meta position, is the favoured site of attack. 

The molecular orbital picture of pyridine is shown in Figure 6.7. There is a difference between this picture and that for benzene. In benzene, the HOMO consists of two filled orbitals of equal energy (hence degenerate), but in pyridine there is only one filled HOMO. Importantly, none of the orbitals are degenerate. 

Draw a molecular orbital picture for the resonance hybrid benzenonium ion shown in eq. 4.16, and describe the hybridization of each ring carbon atom. Hint Examine the molecular orbital structure of benzene in Figure 4.2.)... 

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