A General Liquid-Liquid Partitioning Equation and Its Consequences: Learning from the pH Dependent Extraction of a Pharmaceutical Intermediate.

We observed that the product of a Buchwald-Hartwig coupling reaction extracted from the organic phase easily, relative to the starting aryl bromide as the pH was lowered. This was surprising given the similarity of their p Ka's. The product's extraction curve was also significantly steeper than expected. A consideration of the relevant equilibria, including all three of the product's p Ka's, led to a general model for the liquid-liquid extraction behavior of ionizable molecules. This model predicts the observed, useful extraction behavior.