Preferential crystallisation in the system mandelic acid / water

Generally, in chemical synthesis of chiral systems a racemic mixture is produced, that is a 50:50 mixture of both enantiomers. The resolution of racemic mixtures are difficult, because the chemical and physical properties of the enantiomers are identical except for the interaction with linear polarized light and other chiral substances. An effective and comparatively cheap technology for enantioseparation is preferential crystallisation (or resolution by entrainment) [1]. Unfortunately, its application is limited up-to-now to conglomerate forming systems, which cover only 5 to 10 % of the chiral substances [2]. However, the major part of chiral systems belong to compound forming systems. The aim of this work is to demonstrate the applicability of the preferential crystallisation principle to compound forming systems for example mandelic acid. The motivation of this work is base on the fact that the ternary solubility diagram of a compound forming system exhibits two three-phase regions, where it should be possible to perform preferential crystallisation. Analogous to the conglomerate applications, the start solution should have an enantiomeric composition similar to the eutectic composition in the chiral system. As products alternatively the desired enantiomer or the racemate will be obtained. In Figure 1 the application of preferential crystallisation in cyclic operation mode is shown for a racemic compound forming systems. Such a preferential crystallization step could be integrated in a hybrid process, where at first an established separation technique is applied to reach a certain enantiomeric enrichment and subsequently preferential crystallisation is used to produce finally the desired pure enantiomer and the racemate as by-product. The racemate could be recycled to the enrichment step again. In the presentation the applicability of preferential crystallisation for separation of mandelic acid enantiomers will be demonstrated. Online monitoring of this separation process is performed with help of a combination of polarimetry and density measurement. The experiments were planned ton the basis of available ternary solubility and metastable zone width data. The feasibility of preferential crystallisation and the influence of different experimental conditions like initial supersaturation, initial enantiomeric composition, seed amount and scale will be shown. Further, experimental results of several cycles producing alternatively the pure enantiomer and the racemic compound are presented. Crystallisation trajectories, the product purity, the product mass gained and the productivity will be discussed. In order to derive general conclusions, the separation of further chiral substances will be studied in future work. [1] J. Jacques, A. Collet, S. H. Wilen: Enantiomers, Racemates and Resolutions, Krieger Publishing Company Malabar, Florida, 1994. [2] A. Collet: Separation and purification of enantiomers by crystallisation methods, Enantiomer 1999, 4 (3-4), 157 – 172