Toluene resonance energy

Forster (1968) points out that R0 is independent of donor radiative lifetime it only depends on the quantum efficiency of its emission. Thus, transfer from the donor triplet state is not forbidden. The slow rate of transfer is partially offset by its long lifetime. The importance of Eq. (4.4) is that it allows calculation in terms of experimentally measured quantities. For a large class of donor-acceptor pairs in inert solvents, Forster reports Rg values in the range 50-100 A. On the other hand, for scintillators such as PPO (diphenyl-2,5-oxazole), pT (p-terphenyl), and DPH (diphenyl hexatriene) in the solvents benzene, toluene, and p-xylene, Voltz et al. (1966) have reported Rg values in the range 15-20 A. Whatever the value of R0 is, it is clear that a moderate red shift of the acceptor spectrum with respect to that of the donor is favorable for resonant energy transfer. 

Admitting the elsewhere enunciated problems with the enthalpy of formation of isopropyl fluoride, we conclude that the resonance energies of the halobenzenes decrease in the order F, 7.1 2.2 Cl, -12.3 2.1 Br, -20.2 4.9 I, - 20.6 6.1 kJ mol"1. The resonance energies of halobenzenes roughly track the earlier order with maximum stabilization for fluorine, although we must admit our surprise that only fluorobenzene is stabilized by this definition. Alternatively, since isopropylmethane and methylbenzene (i.e. isobutane and toluene) are common to all of the above reactions, it suffices to look merely at the differences of the enthalpies of formation of the correspondingly substituted benzene and propane (equation 31). 

A further deviation from additivity which should be taken account of is the substitution effect, or increase of stability on substitution in hydrocarbons, mentioned in Section 10,6.6. This, for example, causes the observed resonance energy to be greater in toluene and the xylenes than in benzene. 

In normal Diels-Alder reactions benzene and naphthalene usually prove to be quite inert as dienes anthracene adds reactive dienophiles at the central ring. These results are entirely as expected on the basis of the aromaticity of the systems. By contrast, tetrazine diester and 3,6-bistrifluoro-methyltetrazine, due to their low-lying LUMOs and lower resonance energy, will react even with aromatic compounds as dienophiles (Scheme 41) benzene, toluene, anisole, Af,A -dimethylaniline, and methylthiobenzene (226) react at 140°C <81CZ342, 87AG(E)332>. In monosubstituted benzene... 

Draw resonance structures for the possible radicals resulting from hydrogen atom abstraction from toluene. Which would you anticipate to be the most stable Why Compare energies for the different radicals (radical A, radical B,. ..). Is the lowest-energy radical that which you anticipated Are any of the alternatives significantly better than any of the others Explain your reasoning. 

Many, but not all, endothermic compounds have been involved in violent decompositions, reactions or explosions, and in general, compounds with significantly positive values of standard heat of formation may be considered suspect on stability grounds. Notable exceptions are benzene and toluene (AH°f +82.2, 50.0 kJ/mol 1.04, 0.54 kJ/g, respectively), where there is the resonance stabilising effect of aromaticity. Values of thermodynamic constants for elements and compounds are tabulated conveniently [1], but it should be noted that endothermicity may change to exothermicity with increase in temperature [2], There is a more extended account of the implications of endothermic compounds and energy release in the context of fire and explosion hazards [3], Many examples of endothermic compounds will be found in the groups ... 

Contribution from the three structures, V-VII, stabilizes the radical in a way that is not possible for the molecule. Resonance thus lowers the energy content of the benzyl radical more than it lowers the energy content of toluene. This extra stabilization of the radical evidently amounts to 19 kcal/mole (Fig. 12.1). 

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