An investigation of the deuteration of mandelic acid, with an additional paper on the measurement of acidity at temperatures over 100°

The deuteration of optically active mandelic acid was investigated by shaking it in a sealed, evacuated tube under various conditions of temperature, and with different catalytic agents. The effect of acid, base, and platinum catalysts was examined, and in each case the degree of exchange occurring in the alpha position of the mandelic acid was correlated with the simultaneous change in its optical activity. The results indicated that with a basic catalyst present, the reaction occurred by a racemisation mechanism, an attacked molecule having an equal chance of retaining its configuration or undergoing an inversion. The platinum-catalysed exchange in acid solution was more complex, with both racemisation and retention mechanisms playing a part. With no catalyst present, racemisation of the acid occured, but without a corresponding exchange. In presence of platinum, the benzene ring, as well as the alpha hydrogen atom, was deuterated. These conclusions were confirmed by resolutions of the deuterated mandelic acid, when the observed deuterium contents of the resolved fractions were found to be in good agreement with the figures calculated assuming the mechanisms postulated for the first part of the experiment. The resolution also showed that alpha hydrogen exchange and nuclear deuteration were quite independent. Mechanisms for the various reactions have been suggested, and involve an Initial ionisation of the alpha hydrogen atom, either on the catalyst surface, or under the influence of a basic catalyst. The racemisation without exchange is considered to occur by a mechanism involving an initial fission of the bond between the alpha carbon atom and the carboxyl group.