Khan, Arjuman; (1999) Exchange Reactions of Urea Inclusion Compounds. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Urea inclusion compounds (UICs) consist of a urea 'host' framework that forms hexagonal channels within which 'guest' molecules (organic molecules) can be included. They have a typical chiral space group of P6122 with lattice parameters a = b = 8.23 Å, and c = 11.017 Å. This project investigates exchange reactions of UICs of the type shown below: In these exchange reactions, the guest species A and B are CnH2n+2, = 10-15), CH16Br2, C10H20Cl2, chiral alcohols and chiral bromoalkanes. It has been shown from 1H NMR analysis, that with a polycrystalline sample of a urea inclusion compound, exchange between alkanes and halogenoalkanes occurs with a preference for longer chain molecules. Detailed single crystal X-ray diffraction experiments with single crystal UICs fully immersed in liquid guest provided concrete and more detailed evidence for the exchange occurring within a single crystal. Exchange reactions with partially immersed single crystals were also investigated. Liquid was shown to be expelled out from the top (exposed) end of the crystal. X-ray diffraction data showed two sets of layer lines characteristic of two guest species simultaneously present in the UIC. Solid solution formation for two guest species was attempted by cocrystallisation. DSC and powder X-ray diffraction results showed that the potential for solid solution formation depends crucially on the crystal structure of the UIC. The main results obtained from this study is that guest species are able to enter and leave the urea channels via an exchange process with the preference for long chain alkanes displacing shorter chain ones. In most urea inclusion compounds solid solution formation (homogeneous mixture) by cocrystallisation or via exchange is impossible. There is, however, firm evidence from the exchange reactions that two guest species can be present in the same urea inclusion compound in the form of macroscopic domains (heterogeneous mixture).

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