Orbital picture of benzene

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. 

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.)... 

We have just examined the atomic orbital picture of benzene. Now let us look at the molecular orbital picture, comparing the six tt molecular orbitals of benzene with those of 1,3,5-hexatriene, the open-chain analog. Both sets are the result of the contiguous overlap of six p orbitals, yet the cyclic system differs considerably from the acyclic one. A comparison of the energies of the bonding orbitals in these two compounds shows that cyclic conjugation of three double bonds is better than acyclic conjugation. 

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... 

Because all six carbon atoms and all six p orbitals in benzene are equivalent, it s impossible lo define three localized tt 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 -tt electrons are completely delocalized around the ring. In resonance terms (Sections 2.4 and 2.5), benzene is a hybrid of two equivalent forms. Neither form... 

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

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 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. 

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.)... 

So, our picture of benzene has now been elaborated somewhat to show the orbitals. The 2p orbitals on the six carbons, extending above and below the plane containing the carbons and their attached hydrogens, overlap to form a circular cloud of electron density above and below the plane of the ring. 

The molecular orbital picture of the tropylium ion closely resembles that of benzene. 

The valence-bond picture of benzene with six electrons in a delocalized ir orbital is a useful, but superficial, one. Only two electrons can occupy a single orbital, be it an atomic orbital or a molecular orbital. The molecular orbital picture shown in Figure 11.4 does not suffer from this defect. We learned in Section 2.4 that when AOs combine to give MOs, the final nunAer of MOs must equal the original number of AOs. Thus, the six 2p AOs of benzene combine to give six it MOs. 

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