Effect of fluorine substitution on the carbon acidity of methane, methyl isocyanide, acetonitrile, acetaldehyde, and nitromethane

The effect of fluorine substitution on the acidity of the title compounds has been examined via ab initio molecular orbital theory. Multiple fluorine substitution has a synergetic effect on the acidity of methane, In the case of substituted methanes, the acidity of methyl isocyanide increases linearly with the fluorine substitution while a smaller effect is observed with acetonitrile, and little effect is found with acetaldehyde and nitromethane. The acetaldehydes are unique in that the anion of difluoroacetaldehyde remains planar, whereas all of the other difluoroanions become markedly pyramidal with large inversion barriers. The ionization process was examined in terms of the changes in atomic charges and bond orders. Fluorine has a larger effect on the acidity of compounds with pyramidal anions than those with planar ones. The stabilization appears to be a consequence of the competition for p character by the fluorine atoms which tends to place the anionic charge into an orbital with high s character.