Liquid crystalline catalysis. Kinetics of the rearrangement of allyl p-(dimethylamino)benzenesulfonate in a smectic B-solvent.

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Allyl p-(dimethylamino)benzenesulfonate (ASE) was found to promptly rearrange to the corresponding quaternary zwitterion (AZWI) in a smectic, liquid crystalline solvent (OS 35) 4-pentylcyclohexyl 4-propylcyclohexanecarboxylate. On the contrary, this reaction does not occur in isotropic solvents. In the present study we investigate the effect of temperature and of reactant concentration on the reaction rate. A plot of logk vs. 1⁄T shows a strog upward curvature. In addition, the rate constant, k, strongly depends on ASE concentration: relevant rate depression occurs beyond a threshold value of solute concentration, which in turns depends on temperature. These results are interpreted in terms of reaction mechanism and solute–solvent interaction. From these results we can conclude that smectic B phase has a strong catalytic effect on this particular kind of nucleophilic displacement. Furthermore, catalysis does not depend on solute expulsion from the bulk of the phase; finaly, the detailed mechanism of the catalytic effect may depend on the different location of the solute in distinct solubilization sites existing within the smectic mesophase.
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