Solvent effects in ultraviolet spectral studies of hydrogen bonding between phenol and N,N-dimethylacetamide

Ultraviolet spectral studies of hydrogen bonding between phenol and N,N-dimethylacetamide (DMA) in several media are reported. The equilibrium constant for the formation of the phenol–DMA complex is strongly solvent dependent, varying from 295 1/mole in cyclohexane to 130 in CCl4 and 16 in CHCl3, all at 28°. The greatly reduced value in CHCl3 indicates that the measured equilibrium constant is only an apparent one which does not take into account the decrease in free DMA concentration resulting from hydrogen-bonded complex formation with the solvent acting as hydrogen donor. In CCl4/CHCl3 mixed solvent, in the range of [chloroform] = 0 to 1.227 M, the measured equilibrium constant, K′, varies linearly with K′ [chloroform]. The slope of the line corresponds to the equilibrium constant for the formation of the hydrogen-bonded complex between CHCl3 and DMA in CCl4. The value, 0.9 1/mole, agrees with that obtained from a proton magnetic resonance method. The agreement is particularly noteworthy when we consider that the concentrations of phenol used in the proton magnetic resonance and ultraviolet spectral methods differ by a factor of 200, which leads definitely to the conclusion that the hydrogen-bonded CHCl3–DMA complex formed is 1:1. In cyclohexane/CHCl3 mixed solvent, similar results are obtained.