Resonance energy, of aromatic hydrocarbons

Table CXIII. Resonance Energies of Aromatic Hydrocarbons...

Herndon, W.C. (1973b). Resonance Energies of Aromatic Hydrocarbons. A Quantitative Test of Resonance Theory. J.Am.Chem.Soc., 95,2404-2406. 

The nature of the aromatic character Table 2.1 Resonance energies of aromatic hydrocarbons (kJ/mol) ... 

Herndon WC (1973) Resonance energies of aromatic hydrocarbons. A quantitative test of resonance theory. J Am Chem Soc 95 2404—2406... 

To produce more reliable predictions of aromaticity, Hess and Schaad (following a suggestion of Dewar) calculated delocalization (resonance) energies of cyclic hydrocarbons by comparing the compounds Htickel-theory with a value calculated for a hypothetical acyclic conjugated polyene with the same number and kinds of bonds as in a localized structure of the cyclic hydrocarbon. [B. A. Hess and L. J. Schaad, J. Am. Chem. Soc., 93, 305, 2413 (1971) 94, 3068 (1972) 95, 3907 (1973) B. A. Hess, L. J. Schaad, and C. W. Holyoke, Tetrahedron, 28, 3657, 5299 (1972) Schaad and Hess,... 

Swinborne-Sheldrake R, Herndon WC, Gutman I (1975) Kekule stmctures and resonance energies of benzenoid hydrocarbons. Tetrahedron Lett 16 755-758 Carter PC (1949) An empirical equation for the resonance eneigy of polycyclic aromatic hydrocarbons. Trans Faraday Soc 45 597-602... 

Much as benzene and phenyl are the simplest aromatic hydrocarbon or arene and aryl group respectively, methane and methyl are the simplest aliphatic hydrocarbon or alkane and alkyl group respectively. The resonance energy of the various halobenzenes may be identified as the negative of the exothermicity of reaction 34. 

The resonance energies of some aromatic hydrocarbons are given in Table CXIII. Further insight into the resonance of benzene may be obtained from a study of the three successive stages of the hydrogenation reaction. We have... 

Carter, EG. (1949). An Empirical Equation for the Resonance Energy of Polycyclic Aromatic Hydrocarbons. Trans.Faraday Soc., 45, 597-602. 

Aromaticity. VIII). Resonance energies of cata-condensed benzenoid polycyclic hydrocarbons. Rev. Roum. Chim., 15, 1243—1250. 

Benzene peroxide may be more stable in solution since it is protected by the surrounding benzene molecules. The stability of peroxides in the series benzene—naphthalene—anthracene should increase with increasing resonance energy of the aromatic compound. The resonance energy of benzene is 36 kcal mole-1, that of naphthalene is 61 kcal mole-1 and that of anthracene is 83 kcal mole-1. Thus pure anthracene peroxide can be separated and stored for a long time [Refs. 91, 93, 96, 102, 241], whereas benzene peroxide exists only in situ and decomposes in all attempts at separation. The characteristic absorption band of anthracene peroxide also occurs at 2780 A and it may be assumed that the band is characteristic of the peroxides of cyclic hydrocarbons. 

Azulene is a deep-blue hydrocarbon with resonance energy of 205 kJ/mol (49 kcal/mol). Azulene has ten pi electrons, so it might be considered as one large aromatic ring. Its electrostatic potential map shows one ring to be highly electron-rich... 

Another example of a nonbenzenoid aromatic hydrocarbon is the compound azulene. Azulene has a resonance energy of 205 kj moP There is substantial separation of charge between the rings in azulene, as is indicated by the electrostatic potential map for azulene shown in Fig. 14.18. Factors related to aromaticity account for this property of azulene (see Practice Problem 14.12). 

Table 2.1 summarizes the resonance energies of some aromatic hydrocarbons and heterocyclics. 

Several other experimental determinations of the resonance energy of benzene have been performed using different model compounds, and although these determinations differ somewhat in their results, they all agree that the resonance stabilization of benzene is large. Following are resonance energies for several other aromatic hydrocarbons. 

---