SELECTIVITY OF C-H BOND RUPTURE BY GAMMA RADIOLYSIS AND HOT-RADICAL ATTACK IN SINGLY BRANCHED ALKANE GLASSES

Trapped free radicals are produced in singly branched alkane glasses at 77 °K by γ irradiation, hot-radical attack, or photosensitization by aromatic solutes. In contrast to similar activation in the gas and liquid phases, there is a high selectivity for rupture of a particular type of secondary C-H bond, as indicated by the esr spectra of the resultant free radicals. All three methods of activation show the same selectivity. For molecules with branching in the 3 or 4 position, the free radicals formed result predominantly from loss of a hydrogen atom from the secondary carbon atom nearest the end of the longest carbon chain. When branching is in the 2 position, detectable amounts of the radical resulting from loss of a tertiary hydrogen are also found. Identification of the trapped radical formed from 3-methylpentane glass has been made by comparing its esr spectrum with the spectra of the four possible 3-methylpentyl radicals. The latter have been produced by radiolysis of the corresponding iodides and the chlorides. Photolysis of 3-methylpentyl iodides in various organic glasses at 77 °K produces, with low quantum yield, radicals formed by abstracting hydrogen atoms from matrix molecules, but no radicals formed by rupture of the carbon-iodine bond of the iodide, illustrating the importance of the cage effect in these systems. © 1969, American Chemical Society. All rights reserved.