Substituent effects.: 7.: Phenyl derivatives.: When is a fluorine a π-donor?

The effect of substituents on the energies of phenyl derivatives was examined with the aid of group-transfer reactions with ethane to give toluene and a substituted methane. The energies of these reactions were Linearly related with a slope of 1 to those found for a corresponding set of vinyl derivatives. A comparison of the bond dissociation energies of the phenyl and vinyl derivatives gave the same result, indicating that substituents interact with benzene and ethylene in the same fashion and to the same degree. Further information was provided by rotational barriers and electron-density difference plots. The protonated forms of benzene, toluene, fluorobenzene, and chlorobenzene were examined, and the MP3/6-311++G** calculated proton affinities were in good agreement with the experimental data that found the halobenzenes to be slightly more basic than benzene in the gas phase. The para positions were the more basic, and the m-positions were less basic by 8-9 kcal/mol. The question of when fluorine is a pi-donor was examined.