Hydrogen halides charge separation

Rafaeloff, R., Tricot, Y.-M., Nome, F., and Fendler, J.H., Colloidal catalyst coated semiconductors in surfactant vesicles In situ generation of rhodium-coated cadmium sulfide particles in diocta-decyldimethylammonium halide surfactant vesicles and their utilization for photosensitized charge separation and hydrogen generation, J. Phys. Chem., 89, 533,1985. 

Dipo/e Moment The evidence comes from an examination of the dielectric constant of the hydrogen halides. In an electric field, say between the plates of a condenser, molecules that have a charge separation within them will tend to orient themselves with the electric field. Such molecules behave like electric dipoles (Fig. 4.3) and are called polar molecules. The extent of orientation is reflected by a change in the dielectric... 

Apart from the free-radical mode of decomposition, many halogen compounds decompose by unimolecular mechanisms, the most common of these being the direct unimolecular elimination of hydrogen halide. There is evidence that these types of unimolecular reactions involve charge separation of the carbon-halogen bond in the transition state, and they have received considerable attention in recent years. 

Here the transition state, for which the degree of charge separation is estimated to be 0.7 [15], is well on the way to the fully ionised species in which the halide ion is strongly stabilised by hydrogen bonding and the cation is stabilised to some extent by coordination to the lone pair of the solvent. More recent studies by Creary [16] emphasise the dramatic effect of solvent on the rates of ionisation processes. He studied the solvolysis of a benzylic mesylate, known to occur via an ionisation mechanism (equation 12.7). 

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