PHOTO-INITIATED REACTIONS OF THIOLS AND OLEFINS: I. THE THIYL RADICAL CATALYZED ISOMERIZATION OF BUTENE-2 AND 1,2-ETHYLENE-d2
Kinetic studies have been made of the isomerization of butene-2 and 1,2-ethylene-d2 catalyzed by thiyl radicals produced from the photolysis of methanethiol. The rate of isomerization was found to be first order with respect to both the olefin and [Formula: see text] concentrations. The lack of influence of pressure on the reaction rate, at pressures above about 4 mm, leads to a simple mechanism in which isomerization is considered to occur as a result of thermal decomposition of the collisionally stabilized adduct radical produced in the reaction [Formula: see text]. The rate constants for this attack step were found to be 2 × 107 and 4.8 × 106 l mole−1 s−1 for butene-2 and ethylene-d2, respectively. In both cases the activation energy for isomerization was found to be close to zero. From a kinetic study of the isomerization of cis-butene-2 in the presence of butadiene-1,3, which acts as a retarder, the attack constant for butadiene at 25 °C was found to be 4.5 × 108 l mole−1 s−1.