Elementary Rates of Decomposition of the Homologous Series Ethyl through 2-Octyl Radicals

Abstract

The rates of decomposition of the secondary alkyl radicals 2-butyl, 2-pentyl, 2-hexyl, 2-heptyl, and 2-octyl were measured by an internal comparison method at 300°K. Activation to a level around 45 kcal mole−1 was by addition of H atom to the appropriate α-olefin. The validity of an assumption of invariance of thermochemical parameters, including the bond-dissociation energy D[3(—C)—(4—C)] for the higher members of the radical series, was established by measurements made with 5-Me-2-hexyl radicals. No evidence was found for any significant amount of H migration and isomerization of any of these excited radicals. Together with earlier work, measurements now exist for the homologous series ethyl through 2-octyl radicals. Theoretical rate constants agree well with the experimental values. Our earlier experimental verification for relatively smaller molecules that all vibrational and internal rotational degrees of freedom must be treated as active is now extended to the largest member of this series. Within the accuracy of the test provided by this work, intramolecular energy relaxation is also confirmed to be effectively random even for the highest member. The data illustrate the effect on rates of quantum-statistical weight changes due to structural increments in the series.