Delocalization energy benzene

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 ... 

Thermochemical and X-ray studies on the simplest 7r-trishomobenzene, cisxisxis-1,4,7-cyclononatriene [8], indicate that this molecule is not homoaromatic (Roth et al., 1964). However, photoelectron spectroscopy clearly demonstrates that there are considerable homoconjugative interactions within [8] (Bischof et al., 1970 Martin and Mayer, 1983). These seemingly contradictory conclusions can be reconciled when it is realized that, using the PES data, the delocalization energy in [8] is estimated to be only about 5% of that in benzene. The manifestations of such a small delocalization energy would be very difficult to detect experimentally by means of NMR spectroscopy. X-ray crystallography, or thermochemistry. 

This conclusion, nevertheless, should not be considered categorical but it points to the necessity of careful consideration of the correlation between the AEdis value and the part of it that relates to cyclic electron delocalization. It has been shown by use of TRE calculations of aromatic benzene and antiaromatic cyclobutadiene molecules that in the case of benzene the distortion into a Kekule-type structure is characterized by a change of the aromatic cyclic Tr-electron delocalization energy in an opposite direction... 

Although the Hiickel An+ 2 rule is rigorously derived for monocyclic systems, it is also applied in an approximate way to fused-ring compounds. Since two fused rings must share a pair of ir electrons, the aromaticity and the delocalization energy per ring is less than that of benzene itself. Decreased aromaticity of polynuclear aromatics is also revealed by the different C—C bond lengths. 

Ignoring O (see eqn (10-5.1)), the 7r-electron energy for the ground state will be 6a+80 which, when compared with the 71-electron energy of three ethylene molecules (6 a+60), shows that the delocalization energy of benzene is 2/ . 

Remembering that p is negative, we see that n bonding has stabilized the molecule by 8p. Energies expressed in units of / are not very informative, however, unless we can estimate the value of p. We shall next turn to the calculation of the delocalization energy of benzene in units of /i. Since the delocalization energy may be estimated experimentally, we shall then be able to evaluate p. It is not practicable to do this by computation. 

Experimentally, the delocalization energy of benzene is estimated in the following way. The actual enthalpy of formation of benzene can be determined by thermochemical measurements. The energy of the hypothetical molecule cyclohexatriene can be estimated by using the bond energies for C—C, C=C, and C—H found in other molecules such as ethane and ethylene. The difference between these energies is the experimental value of the delocalization energy. We then evaluate / , since... 

The MO delocalization energy (DE) calculated using a simple LCAO approach indicated that pyran-4-one (DE, 2.868/8) possesses greater aromaticity than benzene (DE 2/8) (62CCC1242). Subsequent more refined calculations gave substantially lower values, but still indicated considerable aromaticity for pyran-4-one. Values of DE, DEsp and aromaticity indices for pyran-4-one and chromone which were obtained using various MO methods are presented in Table 9. 

The calculated delocalization energy for benzene is the difference between these quantities, or (6a + 8/3) — (6a + 6/3) = 2/3. That is to say, the calculated delocalization energy is the difference between the energy of... 

It was shown in Section 21-3 that benzene is 36-38 kcal more stable than the hypothetical molecule 1,3,5-cyclohexatriene on the basis of the differences between experimental heats of combustion, or hydrogenation, and heats calculated from bond energies. We call this energy difference the stabilization energy (SE) of benzene. We have associated most of this energy difference with 7r-electron delocalization, which is the delocalization energy (DE). The difference between SE and DE will be small only if our bond-energy tables are reliable and steric and strain effects are small. 

For each we can write two equivalent planar VB structures, and the qualitative VB method would suggest that both compounds, like benzene, have substantial electron-delocalization energies. However, the planar structures would have abnormal C—C=C angles, and consequently at least some degree of destabilization associated with these bond angles (Section 12-7). Nonetheless, estimation of the strain energies show that while they are substantial, they are not prohibitive. Should then these molecules be stabilized by resonance in the same sense as benzene is postulated to be ... 

This geometry precludes the possibility of two equivalent VB structures, as for benzene, because, as you will see if you try to make a ball-and-stick model, 25b is highly strained and not energetically equivalent to 25a at all. Thus we can conclude that the delocalization energy of cyclooctatetraene is not large enough to overcome the angle strain that would develop if the molecule were to become planar and allow the tt electrons to form equivalent tt bonds between all of the pairs of adjacent carbons. 

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