Benzene delocalization

In the case of 1,3-butadiene, RAMSES combines the two double bonds to form a single, delocalized r-electron system containing four electrons over all four atoms (Figure 2-50a). The same concept is applied to benzene. As shown in Figure 2-50b, the three double bonds of the Kekule representation form one electron system with six atoms and six electrons. 

The delocalization energy of benzene is 2p (verify this). From information in Exereise 7-6 ealeulate yet another value for the size of the unit p based on the thermodynamic values of the enthalpy of fomiation of benzene. Does this value agree with the themiodynamic values in Problem 14 Does it agree with the spectroscopic value ... 

Carbon atoms can also form cyclic compounds. Aromatic hydrocarbons (arenes), of which benzene is the parent, consist of a cyclic arrangement of formally unsaturated carbons, which, however, give a stabilized (in contrast to their hypothetical cyclopolyenes), delocalized system. 

In addition to its three sp hybrid orbitals each carbon has a half filled 2p orbital that can participate m tt bonding Figure >b shows the continuous rr system that encompasses all of the carbons that result from overlap of these 2p orbitals The six tt electrons of benzene are delocalized over all six carbons... 

The pattern of orbital energies is different for benzene than it would be if the six tt electrons were confined to three noninteracting double bonds The delocalization provided by cyclic conjugation in benzene causes its tt electrons to be held more strongly than they would be in the absence of cyclic conjugation Stronger binding of its tt electrons is the factor most responsible for the special stability—the aromaticity—of benzene... 

During our discussion of benzene and its derivatives it may have occurred to you that cyclobutadiene and cyclooctatetraene might be stabilized by cyclic rr electron delocal ization m a manner analogous to that of benzene... 

C 1 IS more reactive because the intermediate formed by electrophilic attack there IS a relatively stable carbocation A benzene type pattern of bonds is retained m one nng and the positive charge is delocalized by allylic resonance... 

A common example of the Peieds distortion is the linear polyene, polyacetylene. A simple molecular orbital approach would predict S hybddization at each carbon and metallic behavior as a result of a half-filled delocalized TT-orbital along the chain. Uniform bond lengths would be expected (as in benzene) as a result of the delocalization. However, a Peieds distortion leads to alternating single and double bonds (Fig. 3) and the opening up of a band gap. As a result, undoped polyacetylene is a semiconductor. 

Pyrrole has a planar, pentagonal (C2 ) stmcture and is aromatic in that it has a sextet of electrons. It is isoelectronic with the cyclopentadienyl anion. The TT-electrons are delocalized throughout the ring system, thus pyrrole is best characterized as a resonance hybrid, with contributing stmctures (1 5). These stmctures explain its lack of basicity (which is less than that of pyridine), its unexpectedly high acidity, and its pronounced aromatic character. The resonance energy which has been estimated at about 100 kj/mol (23.9 kcal/mol) is intermediate between that of furan and thiophene, or about two-thirds that of benzene (5). 

Semiconductor materials are rather unique and exceptional substances (see Semiconductors). The entire semiconductor crystal is one giant covalent molecule. In benzene molecules, the electron wave functions that describe probabiUty density ate spread over the six ting-carbon atoms in a large dye molecule, an electron might be delocalized over a series of rings, but in semiconductors, the electron wave-functions are delocalized, in principle, over an entire macroscopic crystal. Because of the size of these wave functions, no single atom can have much effect on the electron energies, ie, the electronic excitations in semiconductors are delocalized. 

The most impressive example of resonance stabilization is benzene, in which the delocalization is responsible for a stabilization of 30-36 kcal/mol, the resonance energy of benzene. 

DE, since the 7c-electron energy is 4a + 4/3, the same as that for two independent double bonds. Thus, at this level of approximation, HMO theory prediets no stabilization for eyelobutadiene from delocalization and furthermore predicts that the molecule will have unpaired electrons, which would lead to very high reactivity. In addition, cyclobutadiene would suffer angle strain, whieh is not present in benzene. The extreme instability of eyelobutadiene is then understandable. Higher-level MO ealculations modify this picture somewhat and predict that eyelobutadiene will be a rectangular molecule, as will be diseussed in Chapter 9. These ealculations, nevertheless, agree with simple HMO theory in... 

Several methods of quantitative description of molecular structure based on the concepts of valence bond theory have been developed. These methods employ orbitals similar to localized valence bond orbitals, but permitting modest delocalization. These orbitals allow many fewer structures to be considered and remove the need for incorporating many ionic structures, in agreement with chemical intuition. To date, these methods have not been as widely applied in organic chemistry as MO calculations. They have, however, been successfully applied to fundamental structural issues. For example, successful quantitative treatments of the structure and energy of benzene and its heterocyclic analogs have been developed. It remains to be seen whether computations based on DFT and modem valence bond theory will come to rival the widely used MO programs in analysis and interpretation of stmcture and reactivity. 

The meaning of the word aromaticity has evolved as understanding of the special properties of benzene and other aromatic molecules has deepened. Originally, aromaticity was associated with a special chemical reactivity. The aromatic hydrocarbons were considered to be those unsaturated systems that underwent substitution reactions in preference to addition. Later, the idea of special stability became more important. Benzene can be shown to be much lower in enthalpy than predicted by summation of the normal bond energies for the C=C, C—C, and C—H bonds in the Kekule representation of benzene. Aromaticity is now generally associated with this property of special stability of certain completely conjugated cyclic molecules. A major contribution to the stability of aromatic systems results from the delocalization of electrons in these molecules. 

The picture of benzene as a planar- framework of a bonds with six electrons in a delocalized TT orbital is a useful, but superficial, one. Six electrons cannot simultaneously occupy any one orbital, be it an atomic orbital or a molecular- orbital. We can fix this with the more accurate molecular- orbital picture shown in Figure 11.4. We learned in Section 2.4 that when atomic orbitals (AOs) combine to give molecular- orbitals (MOs), the final number of MOs must equal the original number of AOs. Thus, the six 2p AOs of six 5/r -hybridized carbons combine to give six tt MOs of benzene. 

Pyrrole, furan, and thiophene, on the other hand, have electron-rich aromatic rings and are extremely reactive toward electrophilic aromatic substitution— rnore like phenol and aniline than benzene. Like benzene they have six tt electrons, but these tt electrons are delocalized over five atoms, not six, and ar e not held as strongly as those of benzene. Even when the ring atom is as electronegative as oxygen, substitution takes place readily. 

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 ... 

Aeeording to your Lewis structure(s) and to the actual geometry of the molecule, is the bonding in planar corannulene fully delocalized (as in benzene), or are some CC bonds long and some short Do your results support the notion that planar corrannulene is resonance stabilized Explain. 

---