P orbitals of benzene

FIGURE 2.1 The six p orbitals of benzene overlap to form three bonding orbitals, (a), (b), a.nd (c). The three orbitals superimposed are shown in (d). 

This picture of the p orbitals of benzene show resonance overlap with the p orbital of the carbocation. The chloride nucleophile can form a bond to the positive carbon from either the top or the bottom. 

The p orbitals of benzene overlap continuously around the ring. 

Depicted in this diagram are the interactions of the six atomic p orbitals of benzene, which form six molecular orbitals. When orbitals of like phase combine, a bonding interaction results. When orbitals of opposite phase combine, a destabilizing node results. 

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... 

Treatment of the A -phospholenium salt (99) with aqueous alkali gave predominantly benzene and the oxide (100). It is suggested that here ring constraint leads to poor overlap of the 7r-bond in the ring with the p-orbital of the incipient carbanion which would lead to ring opening. Pseudorotation of the initial adduct followed by loss of the phenyl from the apical position becomes competitive. ... 

Given the zwitterionic natnre of single carbenes, the possibility exists for coordinating solvents such as ethers or aromatic compounds to associate weakly with the empty p-orbital of the carbene. Several experimental stndies have revealed dramatic effects of dioxane or aromatic solvents on prodnct distribntions of carbene reactions. Computational evidence has also been reported for carbene-benzene complexes. Indeed, picosecond optical grating calorimetry stndies have indicated that singlet methylene and benzene form a weak complex with a dissociation energy of 8.7kcal/mol. ... 

The electronic and photoelectron spectra of these types of molecules can only be rationalized in terms of appreciable transannular interactions. The transannular resonance integral has been estimated to be about 40% of that between adjacent p orbitals in benzene. It would seem, therefore, that [130] is more correctly referred to as homonaphthalene and [131] as homoanthracene , etc. It is of interest to note that our probes for homoaromaticity correctly discern the importance of homoconjugation in [130] (Williams et al., 1988). Similar significant transannular interactions are also evident in homoazulene [133] (see Scott et al., 1985 Scott, 1986). 

A deeper study of the electron spectrum and the ESR spectrum as well as of the photochemical behavior of 2 was made by Gleiter 27>. He showed that a a-n separation in the [2.2]paracyclophane system is not indicated, since the 1,2 and 9,10 bonds in 2 are parallel to the p orbitals of the benzene rings and favorably situated for a-n interaction. 

In five-membered heterocycles, formally derived from benzene by the replacement of a CH=CH unit by a heteroatom, aromaticity is achieved by sharing four p-electrons, one from each ring carbon, with two electrons from the heteroatom. Thus in pyrrole, where the heteroatom is N, all the ring atoms are sp hybridized, and one sp orbital on each is bonded to hydrogen. To complete the six 7i-electron system the non-hybridized p-orbital of N contributes two electrons (Box 1.9). It follows that the nitrogen atom of pyrrole no longer possesses a lone pair of electrons, and the compound cannot function as a base without losing its aromatic character. 

Fig. 20. (a) One component of the bonding et metal-benzene molecular orbital set. Diagrams (b) and (c) illustrate the effect of tilting the p orbitals of the benzene and cyclopropenyl rings. 

Part 0 shows how the p orbitals of the pi bonds of benzene overlap in a complete cycle around the ring. The hydrogens have been omitted for clarity. Another version is shown in part . 

For the double bonds to be completely conjugated, the annulene must be planar so the p orbitals of the pi bonds can overlap. As long as an annulene is assumed to be planar, we can draw two Kekule-like structures that seem to show a benzene-like resonance. Figure 16-3 shows proposed benzene-like resonance forms for cyclobutadiene and cyclooctatetraene. Although these resonance structures suggest that the [4] and [8]annulenes should be unusually stable (like benzene), experiments have shown that cyclobutadiene and cyclooctatetraene are not unusually stable. These results imply that the simple resonance picture is incorrect. 

Benzene has a planar ring of six sp2 hybrid carbon atoms, each with an unhybridized p orbital that overlaps with the p orbitals of its neighbors to form a continuous... 

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 difference between the amount of energy we expect to get out on hydrogenation (360 kj mol-1) and what is observed (208 kj mol-1) is about 150 kj mol-1. This represents a crude measure of just how extra stable benzene really is relative to what it would be like with three localized double bonds. In order to understand the origin of this stabilization, we must look at the molecular orbitals. We can think of the Jt molecular orbitals of benzene as resulting from the combination of the six p orbitals. We have already encountered the molecular orbital lowest in energy with all the orbitals combining in-phase. 

We have also employed ETS to study the effect of fluorine substitution on the ir orbitals of benzene and ethylene (10). Here we briefly discuss the results for the fluoroethylenes. Fluorine substitution is known to cause only small shifts in tv ionization potentials (IP) of unsaturated hydrocarbons (1 1). For example, the vertical iv IP s of ethylene and perfluoroethylene agree to within 0.1 eV. The reason that has been most often forwarded to explain this is that the electron withdrawing inductive effect, which stabilizes the occupied orbitals, is nearly cancelled by the destabilizing resonance mixing of the fluorine p orbitals with the ir orbitals of the ethylenic double bond. 

---