SITE SELECTIVITY IN THE PROTON-TRANSFER FROM ALKANE RADICAL CATIONS TO ALKANE MOLECULES - DONOR AND ACCEPTOR SITE SELECTIVITY IN THE PROTON-TRANSFER FROM HEPTANE RADICAL CATIONS TO OCTANE MOLECULES IN GAMMA-IRRADIATED N-C7H16/N-C8D18 AND N-C7H16/N-C8H18/1-C6H13CL CRYSTALS AT 77-K

Heptyl radicals formed by gamma-irradiation of n-C7H16, both in neat form and with various concentrations of n-C8D18 added before irradiation, are investigated by ESR spectroscopy. It is observed that the presence of deuterated octane in protiated heptane results in a substantial increase in the relative contribution of protiated l-heptyl radicals to the radical spectrum. Their formation is attributed to the proton-transfer reaction (n-C7H16.+ + n-C8D18 --> 1-C7H15. + n-C8D18H+) which, in accordance with an earlier and experimentally verified postulate stating that a strict relation exists between the radical site in alkyl radicals formed by proton transfer from alkane radical cations to alkane molecules and the structure of the semioccupied molecular orbital of the parent cation, is expected to yield 1-heptyl radicals selectively. The increase in the relative importance of 1-heptyl vs other isomeric heptyl radicals as a result of the presence of octane in heptane is confirmed by analysis of the tetradecanes formed by irradiation of heptane/1-chlorohexane (1 mol%) and heptane/1-chlorohexane (1 mol%)/octane (9 mol%) systems. As to the acceptor site, a study is made by gas chromatography of the yield and isomeric composition of chlorooctanes formed by gamma-irradiation of n-C7H16, containing 1 mol% 1-C6H13Cl and various concentrations of n-C8H18. Formation of chlorooctanes in such systems is largely due to neutralization of protonated octane molecules by chloride ions (C8H19+ + Cl- --> C8H17Cl + H-2) The isomeric composition yields direct information on the proton-acceptor site in the octane molecule. It is observed that mainly 2-chlorooctane is formed, with much smaller yields of 1- and 3-chlorooctane; the formation of 4-chlorooctane is still less pronounced. The selective formation of 2-chlorooctane, which can be rationalized on the basis of structural considerations and of the fact that the proton affinity for primary C-H protonation is smaller than for secondary C-H protonation, confirms that proton transfer to octane molecules takes place from the chain-end position in extended heptane radical cations and indicates that octane solute molecules are packed quite regularly and with conservation of the layer structure in the heptane crystal. Various alternative mechanisms for chlorooctane formation are discussed and are are discarded with respect to the selective formation of 2-chlorooctane on various grounds. The results obtained further support the postulate stating that a strict relation exists between the proton-donor site in the proton transfer from alkane radical cations to alkane molecules and the structure of the semi-occupied molecular orbital of the parent cation.