H. Kelm

The effect of pressure on the [2 + 2 → 4] cycloaddition of diphenylketen to enol ethers

The volume profiles for the [2 + 2] cycloaddition reactions between diphenylketen and butyl vinyl ether, 2-methoxypropene, and 2,3-dihydro-4H-pyran, respectively, have been determined. For the first example a wide range of solvents was studied. The Kirkwood theory seems to be not fully adequate for the description of the solvent effects found for the rate constant and especially for the partial molar volumes. Solvent cohesive energy densities offer an alternative description. In most solvents the volumes of reaction and the volumes of activation are similar in magnitude and support a concerted mechanism for the cycloaddition.

Reaktionskinetische Untersuchungen zur Bildung von Carbeniumionen des Triphenylmethyl-Typs / Kinetic Investigations of the Formation of Carbenium Ions of the Triphenylmethyl Type

The rates of formation of three mono- and three dicarbenium ions of the triphenylmethyl type from the corresponding carbinols in trifluoro acetic acid - toluene mixtures were investigated using stopped flow techniques. The reaction mechanisms are discussed in view of the obtained dependencies of the rate constants on acid concentration and temperature.

Untersuchung der cis-trans-Isomerie des WITTIGschen Kohlenwasserstoffs / Investigation of the cis-trans-Isomerism of the WITTIG Hydrocarbon

The cis-trans-isomerism of the WITTIG hydrocarbon was investigated in solid state and solution by means of fluorescence spectroscopy. The fluorescence behavior of both isomers in 2-methyltetrahydrofurane was determined as a function of concentration, temperature, and wavelength of exciting radiation. Furthermore, irradiation experiments were undertaken with light of various wavelengths. The results obtained are in agreement with the assumption that the WITTIG hydrocarbon behaves with regard to the cis-trans-isomerism like a 1,3-butadien derivative, i.e. a thermal but no photochemical cis-trans-isomerisation can be detected. The enthalpy difference between the two isomers was estimated to ΔΗ = 250 ± 50 cal/mole. It could be shown that the fluorescence of the cis-isomer is quenched by the trans-isomer. This quenching occurs probably according to the resonance energy transfer mechanism.