First thermodynamic dissociation constants of 5,5-disubstituted barbituric acids in water at 25 °C. Part 1. 5,5-Dialkyl-,5-alkenyl-5-alkyl-,5-alkyl-5-aryl-, 5,5-dialkenyl-, 5,5-diaryl-, and 5,5-dihalogeno-barbituric acids

Abstract

First thermodynamic dissociation constants (K₁)[expressed as _pK₁ values (–logK₁=_pK₁)] have been determined for a series of 14 acids (Ia; R¹= Me; R²= Me, Et, Prⁱ, Ph; R¹= Et; R²= Et, Prⁱ, Ph, 3-NO₂.C₆H₄, 4-NO₂.C₆H₄; R¹= allyl, R²= Prⁱ; R¹= R²= allyl, Ph, Br, Cl) with C(5)-substituents (R¹ and R²) having widely differing polar (R¹= R²= Me, Et, Ph, Br, or Cl) and steric (R¹= R²= Me to R¹= Et and R²= Prⁱ) effects. Thermodynamic dissociation constants, expressed as _pK₁ values, cover the range 8.51–5.55 for the derivatives studied. The R¹= Me; R²= Me, Et, Prⁱ or Ph derivatives are observed to be weaker acids than the corresponding series in which Me is replaced by Et, i.e R¹= Et; R²= Et, Prⁱ or Ph. This reversal of the order of the acid strengths from the order anticipated for polar effects alone for C(5)-substituents is believed to be due to steric effects. The steric effects are acid strengthening.