Superacidity and Superelectrophilicity of BF₃-Carbonyl Complexes

Ren, J.; Cramer, C. J.; Squires, R. R.
J. Am. Chem. Soc. 1999, 121, 2633.

Measurements of the gas-phase acidities and hydride affinities of BF₃ complexes of some simple carbonyl compounds have been carried out by the flowing afterglow-triple quadrupole technique. Dramatic enhancements in the alpha-CH acidities of acetaldehyde, acetone and 1,1,1-trifluoroacetone are found, amounting to 44-50 kcal/mol relative to the uncomplexed compounds. The measured gas-phase acidities are: delta-H_acid[CH₃CRO-dot-BF₃] = 316, 324, and 306 kcal/mol for R = H, CH₃, and CF₃, which ranks these compounds among the superacids. The gas-phase results for [CH₃CHO-dot-BF₃] are combined with solvation energy calculations to estimate a 33 kcal/mol (24 pK_a unit) acidity enhancement for this complex in solution, indicating a pK_a for [CH₃CHO-dot-BF₃] of about -7 in water, and an acid strength comparable to that of HCl. The gas-phase hydride affinity (HIA) of [CH₃CHO-dot-BF₃] has been measured to be 66 kcal/mol greater than that of uncomplexed acetaldehyde (39.5 kcal/mol), making it a superelectrophile--comparable in its gas-phase hydride binding energy to BH₃CN and o-quinone.

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