Basicity of the carbonyl group. Part 6. Calorimetric and spectrometric study of complexation of para-substituted N-ammoniobenzamidates by boron trifluoride

Abstract

The complexation of 8 para-substituted N-ammoniobenzamidates by boron trifluoride has been studied by i.r. and ¹H n.m.r. spectroscopy, as well as by calorimetry. The formation of 1 : 1 addition compounds is demonstrated except for the p-cyano-compound (8) which complexes more than 1 mol. equiv. of boron trifluoride; this is attributed to the ability of the cyano-group to complex weakly with boron trifluoride. Spectroscopic evidence is presented which demonstrates that the complexation site is the carbonyl oxygen of the amidate function. The enthalpies of complexation correlate well with the linear free energy parameter σ_p⁰ indicating that through conjugation effects in the free and complexed substrate are similar and rather small; the possible origin of these effects is discussed.