Hans Güsten

Die photolytische α-spaltung von benzoinalkyläthern in sauerstoffhaltigem methanol

The primary step in the photolytical α-cleavage of benzoin alkyl ether in oxygen saturated methanol at room temperature is the formation of a benzoyl (1) and an α-alkoxybenzyl radical (2), which react via their peroxi radicals 3 und 4 to the final products yielding perbenzoic acid (5), alkyl benzoate (7), benzaldehyde (11), benzaldehyde dimethylacetal (10) and benzil to 100%. Product analysis of the final products of the photolysis of specifically deuterated benzoin methyl ethers shows that methyl benzoate, benzaldehyde and benzaldehyde dimethylacetal are formed via the α-methoxybenzyl radical (2) while the perbenzoic acid results from the benzoyl radical (1). Both the peroxi radicals 3 and 4 have an independent reaction pathway to the final products. Hydrogen abstraction of 3 and 4 from the solvent methanol give rise to hydroxy methyl radicals which yields formic acid, hydrogen peroxide, performic acid and formaldehyde via their hydroxy methyl peroxi radicals. The reaction pathway of the primary radicals is discussed.

Notizen: Zur Photochromie des N-Isopropyl-4- nitrothiobenzimidsäure-methylesters/The Photochromic Reaction of Methyl N-Isopropyl-4-nitrothiobenzimidate

Methyl N-isopropyl-4-nitrothiobenzimidate (1) exhibits reversible photochromism to a dark blue coloured species in the crystalline state only. Experimental evidence for a topochemically controlled nitro-aci-nitro tautomerism of 1 in the electronic excited state is presented.

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

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.