Weak Acidity of Vinyl CH Bonds Enhanced by Halogen Substitution

Abstract

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.

Publisher

American Chemical Society

Publication Date

1-1-2014

Publication Title

Journal of Organic Chemistry

Department

Chemistry and Biochemistry

Document Type

Article

DOI

https://dx.doi.org/10.1021/jo402703v

Keywords

Resolution infrared-spectra, Vibrational assignments, Deuterated modifications, Microwave spectroscopy, Potential constants, CIS-Trans

Language

English

Format

text

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