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

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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.