Acid‐catalyzed hydrolysis of phenyldiazoacetic acid. Effect of an α‐diazo group on carboxylic acid acidity

Rates of acid‐catalysed hydrolysis of phenyldiazoacetic acid were measured in aqueous solution at 25°C across the entire acidity range pC = 1–13. The reaction was found to occur through both the non‐ionized and inonized forms of the substrate, with the ionized form reacting faster by a factor of 650. Appreciable solvent isotope effects in the normal direction (kH/kD > 1) and the occurrence of general acid analysis show that hydrolysis occurs by rate‐determining proton transfer to the diazocarbon atom of the substrate. Analysis of the rate profile for the reaction provides an estimate of the acidity constant of phenyldiazoacetic acid, pKa = 3·70, which is apparently the first determination of the acid strength of an α‐diazocarboxylic acid. Comparison of this result with the pKa of phenylacetic acid shows the diazo group to have a mild (4×) acid‐strengthening effect, whose modest magnitude is attributed to opposing resonance and polar effects.