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

Abstract

Heptyl radicals formed by y-irradiation of n-C7Hl6, both in neat form and with various concentrations of n-CsDls 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 1 -heptyl radicals to the radical spectrum. Their formation is attributed to the proton-transfer reaction (n-C7H16*+ + n-CsDl8 -1-C7H15* + n-CsDlgH+) 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 l-heptyl radicals selectively. The increase in the relative importance of l-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/ l-chlorohexane( 1 mol %) and heptane/lchlorohexane( 1 mol%)/octane(9 mol 7%) systems. As to the acceptor site, a study is made by gas chromatography of the yield and isomeric composition of chlorooctanes formed by y-irradiation of n-C.1Hl6, containing 1 mol 5 % l-C6H1&1 and various concentrations of n-CsHls. Formation of chlorooctanes in such systems is largely due to neutralization of protonated octane molecules by chloride ions (CsH19+ + C1--CsH17Cl+ H2). 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.

Proton acceptor site selectivity in the proton transfer from heptane radical cations to decane molecules in γ-irradiated heptane/decane/1-chloroheptane crystals at 77 K

“…Results obtained in heptane/octane/1-chlorohexane 6 firmly indicate that in the proton transfer from alkane radical cations to alkane molecules in γ-irradiated binary n-alkane crystals, proton transfer takes place selectively from planar chainend C-H bonds in the radical cations to directly accessible secondary C-H bonds in adjacent alkane molecules (viz., only penultimate C-H bonds in octane in the case of the heptane/ octane/1-chlorohexane system). In the present heptane/decane/ 1-chloroheptane system 2-chlorodecane is preferentially formed, with considerably smaller yields for the other chlorodecane isomers.…”

“…of this process despite the presence of heptane molecules, which are altogether much more abundant, results from the fact that (i) planar chain-end C-H bonds in heptane radical cations, from which proton donation takes place, can come into contact with secondary C-H bonds in decane molecules, in contrast to the situation for heptane molecules in pure heptane crystals in which case only contact with primary C-H bonds is possible 5, 13 and that (ii) the proton affinity for secondary C-H protonation is considerably larger than for primary C-H protonation. [6][7][8]14 The dislocation of heptane molecules by decane can make secondary C-H bonds in heptane accessible, but such dislocations do not extend far beyond the place of origin. Upon warming the sample, the protonated decanes are neutralized by chloride ions.…”

“…The results obtained provide evidence on intrinsic acceptor site selectivity as well as on structurally determined acceptor site selectivity (i.e., acceptor site selectivity resulting from donor site selectivity in combination with structural factors) and allow extension of the rule on acceptor site selectivity in the proton transfer from alkane radical cations to alkane molecules in irradiated binary alkane crystals. From the selective formation of 2-chlorooctane in irradiated heptane/octane/1-chlorohexane crystals, 6 it was concluded that in the proton transfer from alkane radical cations to alkane molecules in γ-irradiated binary n-alkane crystals, proton transfer takes place selectively from planar chain-end C-H bonds in the radical cations to those secondary C-H bonds in adjacent alkane molecules that are in direct contact with the proton-donating planar C-H bonds (structurally determined acceptor site selectivity). On the basis of the present results and of recently reported data on γ-irradiated CCl 3 F/alkanes 7,8 it may be concluded that, when different secondary C-H bonds in the accepting alkane molecule are equally accessible to the planar chain-end C-H bonds in the radical cation from which proton donation takes place, then transfer occurs preferentially to the penultimate position (intrinsic acceptor site selectivity).…”

“…Such studies can also indirectly yield evidence on the dependence of the site of proton donation on the electronic structure of the alkane radical cation. Structurally-determined acceptor site selectivity has been observed in heptane/octane/1chlorohexane 5,6 crystals; in such crystals, planar chain-end C-H bonds in heptane radical cations are in close contact with chain-end and penultimate C-H bonds in octane molecules only, regardless of the fact that octane is packed in the extended or gauche-at-C2 conformation, and selective transfer to the penultimate position is observed. The selectivity in the proton donation to secondary rather than to primary C-H bonds in alkane molecules has a thermodynamic origin, viz.…”

Proton Transfer from Alkane Radical Cations to Alkanes

“…With respect to the site of proton acceptance in the proton transfer from matrix radical cations to solute molecules, two systems have been investigated exhaustively, viz. c-irradiated heptane/octane/1-chlorohexane [33] and heptane/ decane/1-chloroheptane [34]. A third system, viz.…”