Fluorine-19 magnetic resonance in measurement of the thermodynamics of the reaction of o-alkylphenols with hexafluoroacetone.

The reaction of several o-alkylphenols with hexafluoroacetone at 0-68 degrees was followed by measuring the fluorine magnetic resonance. A knowledge of the equilibrium of the hemiacetal formation is useful in devising schemes for determination of these compounds. The enthalpy and entropy changes for the equilibrium were found to become less negative with increasing size of the ortho-substituents, indicating the importance of steric interaction. The equilibrium data correlate well with Taft's constant and sigma(ortho) from pK(a), values, but deviate from sigma(-) derived from measurements of phenolic proton chemical shifts.