Determination of the dimerization equilibrium constants of omeprazole and Pirkle's alcohol through optical-rotation measurements

A new procedure to measure the equilibrium constants for the dimerization (homochiral and heterochiral) reactions of enantiomers in solution was applied to two different compounds, namely omeprazole and Pirkle's alcohol, both in CHCl3. This procedure is based on the measurement of the optical rotation of the solution as a function of its composition, which exhibits a nonlinear dependence on the enantiomer enrichment. Such nonlinearity depends on the extent of dimerization, and it is a strong effect in the case of omeprazole, whose equilibrium constants are 14.0 +/- 3.5 l/mol and 25.2 +/- 4.0 l/mol for the formation of homochiral and of heterochiral dimers, respectively. Pirkle's alcohol exhibits a weaker effect that allows only to estimate the order of magnitude of these constants, i.e., ca. 0.10 and 0.34 l/mol, respectively.