Halobenzenes resonance energies

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). 

In the absence of direct calorimetric data for the enthalpy of formation of tert-butyl fluoride, what happens if we use the value of-351.1 kJ mol"1 derived elsewhere in this study. So doing, we conclude the resonance energies of the halobenzenes are all negative and near-... 

Here, the more positive the value of < 33(X), the less resonance energy for the haloben-zene. The differences are found to be nearly comparable F, 235.1 Cl, 234.2 2.6 Br, 237.5 4.5 I, 236.9 6.8 kJmoT1 We close with a question do we really believe that all four halobenzenes are equally destabilized ... 

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. 

By analogy to our halobenzene discussion, we note there are alternative definitions for the resonance energy of acyl halides. We choose only one in the name of brevity and only this one to be consistent with earlier practice. 

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