resolution enantiomers

resolution enantiomers - Isolation of Enantiomers via...

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Unformatted text preview: Isolation of Enantiomers via Diastereomer Crystallisation Parathy Anandamanoharan 12 March 2010 A thesis submitted for the degree of Doctor of Philosophy of the University of London Department of Chemical Engineering University College London Torrington Place London WC1E 7JE i ABSTRACT Enantiomer separation remains an important technique for obtaining optically active materials. Even though the enantiomers have identical physical properties, the difference in their biological activities make it important to separate them, in order to use single enantiomer products in the pharmaceutical and fine chemical industries. In this project, the separations of three pairs of diastereomer salts ( Fig1 ) by crystallisation are studied, as examples of the ‘classical’ resolution of enantiomers via conversion to diastereomers. The lattice energies of these diastereomer compounds are calculated computationally (based on realistic potentials for the dominant electrostatic interactions and ab initio conformational energies). Then the experimental data are compared with the theoretical data to study the efficiency of the resolving agent. CH 3 NH 3 +- OOC R Phenylethylammonium R= CH 3 2-phenylpropanoate R= C 2 H 5 2-phenylbutanoate R= OH mandelate Fig1 Three pairs of diastereomer salts studied All three fractional crystallisations occurred relatively slowly, and appeared to be thermodynamically controlled. Separabilities by crystallisation have been compared with measured phase equilibrium data for the three systems studied. All crystallisations appear to be consistent with ternary phase diagrams. In the case of R = CH 3 , where the salt-solvent ternaries exhibited eutonic behaviour, the direction of isomeric enrichment changed abruptly on passing through the eutonic composition. In another example, R = OH, the ternaries indicated near-ideal solubility behaviour of the salt mixtures, and the separation by crystallisation again corresponded. ii Further, new polymorphic structures and generally better structure predictions have been obtained through out this study. In the case of R = CH 3 , an improved structure of the p-salt has been determined. In the case of R = C 2 H 5 , new polymorphic forms of the n-salts, II and III, have been both discovered and predicted. This work also demonstrates that chemically related organic molecules can exhibit different patterns of the relative energies of the theoretical low energy crystal structures, along with differences in the experimental polymorphic behaviour. This joint experimental and computational investigation provides a stringent test of the reliability of lattice modelling to explain the origins of chiral resolution via diastereomer formation....
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This note was uploaded on 10/27/2010 for the course CHEM 337 taught by Professor Kim during the Fall '10 term at CSU Sacramento.

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resolution enantiomers - Isolation of Enantiomers via...

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