Bond Rupture in the Radiolysis of n-Alkanes: An Application of Gel Permeation Chromatography to Studies of Radical Scavenging by Iodine

摘要

A study of the distribution of radicals produced in the radiolysis of lquid normal alkanes from C_5 to C_(12) is presented as an illustrative example of the use of gel permeation chromatography(GPC) in iodine scavenging studies. Separation of the alkyl iodides by GPC offers particularly attractive possibilities because elutions are carried ut at room temperature, minimizing decomposition of the product iodides. Chromatograms recorded spectrophotometrically in the characteristic absorption bands of the alkyl iodides at approx260 nm show that the only radicals produced in appreciably yield from the normal alkanes are those expected from simple bond rupture. Contour plots of three-dimensional chromatographic data obtained with a diode array detector show little interference at 260 nm from other products. The absence of any significant yield of iodine-containing products other than those expected is conclusively demonstrated by complementary radiotracer studies using ~(125)I_2. Both monitoring approaches show that scission of the carbon skeleton accounts for approx25% of the radicals formed and occurs most frequently in the region of the central carbon-carbon bonds. Elimination of a terminal methyl group is relatively infrequent. An expression that describes the yields of radicals produced by rupture of the carbon backbone is derived from the observed fragmentation patterns. The present studies are particularly important in that the yields of primary and secondary radicals produced by H elimination from the normal alkanes are readily determinable since their respective iodides are well-resolved chromatographically. For the normal alkanes it is found that H-atoms are lost approximately three times more rapidly from a secondary position than from a primary position and that loss from the different secondary positions is essentially statistical. These studies provide considerably greater detail on the competing processes involved in bond rupture than has heretofore been available and serve as the basis for modeling the overall processes that produce radicals in the radiolysis of normal alkanes.