Weak Acidity of Vinyl CH Bonds Enhanced by Halogen Substitution

As
shown by the rates of proton–deuteron exchange in ethylenes
with halogen substituents, the weak acidity of vinyl CH bonds is enhanced
by halogen substitution. Relative rates of exchange in basic deuterium
oxide reflect the relative acidities. Substitution in the α
position has the strongest effect. Less electronegative halogens such
as bromine increase the acidity more than does fluorine. The vinyl
CH acid strengths correlate closely with the energies of deprotonation
of isolated molecules into isolated anions, as computed with the MP2/cc-pVQZ
model. The smaller deprotonation energies are associated with the
stronger acids. Atomic charges from a natural bond order analysis
done with the MP2/aug-cc-pVQZ model show that the negative charge
becomes more dispersed in the anions of the stronger acids. Results
are given for 13 haloethylenes and for 6 halogen-substituted butadienes,
cyclopropenes, and a cyclobutene.