Dielectric studies of H-bonded complexes of formamide and acetamide with substituted phenols

The hydrogen-bonded complexes formed by formamide and acetamide with phenols of varying pK(a) in 1,4-dioxan were studied. The following phenols (pK(a) in parentheses) were used: p-Cresol (10.17), p-chlorophenol (9.38), 2,4-dichlorophenol (7.85) and p-nitrophenol (7.15). The dipole moment of the complex was determined using the Huyskens method. Our results show that the lone pair electrons of the carbonyl group of the amides is the single acceptor site forming a linear hydrogen bond. The excess dipole moments of the complexes were obtained from bond angle data. Our results show that the proton acceptor abilities of the two amides are almost the same and the polarity of the complexes depends more on DeltapK(a). Increasing the acidity of phenols, the dipolar increments also increase. The results are discussed in the light of polarization and tautomeric equilibrium. (C) 2003 Elsevier B.V. All rights reserved.