Ring–chain tautomerism. Part V. 8-Aroyl- and 8-acyl-1-naphthoic acids

Abstract

The equilibrium constants for ring–chain tautomerism in a series of 8-aroyl-1-naphthoic acids in dioxan have been determined by an i.r. spectroscopic method and have been correlated with the substituent constants σ₁ and σ_R⁰ using a modified Hammett equation. The observed pK_a values, which have been measured in 80%(w/w) 2-methoxyethanol–water, have been corrected for these tautomeric equilibria to give the true pK_a^T value. The effect of substitution on the latter values have been assessed using the Hammett equation. The equilibrium constants for ring–chain tautomerism in a series of 8-acyl-1-naphthoic acids have been determined by using the observed pK_a and estimated true pK_a^T values in 80%(w/w) 2-methoxyethanol–water. These equilibria constants have been correlated with steric substituent constants E_s. The ‘cis’ and ‘trans’ conformers of methyl 8-aroyl- and 8-acyl-1-naphthoates has been detected by i.r. spectroscopy and the carbonyl stretching frequencies of the ester carbonyl groups have been assigned. The results are related to the polar, resonance, and steric effects of the substituents.