HYDRIDE AFFINITIES OF HALOGEN SUBSTITUTED CARBOCATIONS - THE C2H4X+ (X = F, CL OR BR) CATIONS

Relative hydride affinities have been calculated for C2H4X+ (X = F, Cl or Br) at the MP2/6-31G**//MP2/6-31G** level for fluorine and chlorine and the MP2/LANL1DZ*//MP2/ LANL1DZ level for bromine. These calculations show that the order of stability for the bridged isomers is Br > Cl much greater than F. The relative stabilities of the alpha-halo isomers are Br > Cl greater-than-or-equal-to F. The latter order is inconsistent with superacid studies which find the alpha-fluoro cation most stable. In fact, fluorine is generally thought to be superior to other halogens at stabilizing an adjacent carbocation due to more efficient overlap between the empty carbon 2p and filled fluorine 2p atomic orbitals. In alpha-chloro or bromo substituted cations there is a mismatch between the filled halogen 3p or 4p and empty carbon 2p atomic orbitals. Thus it is often argued that in spite of its greater electronegativity, fluorine is a better pi-donor than chlorine or bromine. These calculations demonstrate that any advantage in orbital overlap enjoyed by fluorine is not enough to overcome its greater electronegativity versus the higher halogens. The greater stability of the alpha-fluoro cation in solution is more likely due to differential solvation. The positive charge is computed to be much more concentrated in the alpha-fluoro cation as compared to the alpha-chloro or bromo cations. This is consistent with the fluoro isomer being more readily solvated.