Der Einfluß der Solvatation auf die thermische cis→trans-Isomerisierung quartärer Stilbazolium-Salze / The Effect of Solvation on the Thermal cis→trans Isomerization of Quarternary Stilbazolium Salts

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Abstract
The solvent, temperature, concentration, pH, substitution as well as anion dependency were measured of the reaction rate constants for thermal cis→trans isomerization of quarternary stilbazolium salts (1). From the ionic conductance of 1 in the different solvents used the dissociation constants as well as the degree of dissociation were calculated according to the method by Shedlovsky. The rate of the cis→trans isomerization is extremely sensitive to the solvent. In protic solvents the reaction rate decreases linearly with increasing Z-values indicated by Kosower. In dipolar aprotic and non-polar solvents the reaction rate is independent of the solvent parameters. It is concluded that the thermal cis→trans isomerization of stilbazolium salts (1) in dilute solutions occurs within the cation independently of the anion. The enormous increase of the rate of isomerization from stilbenes and stilbazoles to stilbazolium salts as well as their strong dependency on the solvent is due to the positive charge in 1 and the associated specific solvation phenomena. A possible polar transition state is discussed.

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