Isotope effects on chemical equilibria

A modified synthesis of phenylacetic-∝,∝ -d₂, 4-methoxyphenylacetic-∝,∝ -d₂, and 4-nitrophenylacetic -∝,∝ -d₂ acids is described. The pKₐ's of these substances relative to that of the protium analogues have been determined. The ratio of the ionization constants of phenylacetic acid and phenylacetic-∝,∝ -d₂ acid in aqueous solution at 25° was found to be K(H)/K(D) = 1.007 ± 0.002. The new value differs considerably from that originally reported by Halevi but the redetermined ratio is shown to be more consistent in some respects with a modified Halevi-Streitwieser-Taft treatment of secondary isotope, effects, based on a greater inductive electron release from deuterium relative to hydrogen. The K(H)/K(D) ratios appropriate to the acids studied show an approximate linear correlation with the pKₐ of the related protium acid. The evaluation of K(H)/K(D) ratios from conductance data appropriate to the pairs of isotopically substituted weak electrolytes is considered in detail. The most precise method for determining the K(H)/K(D) ratio does not require determination of the absolute pKₐ values of the carboxylic acids but can be made by a comparison of the slopes of a least squares fit to the appropriate conductance equations.