Carbon−Hydrogen vs Carbon−Carbon Protonation in the Proton Transfer from Alkane Radical Cations to Alkane Molecules. A Study in γ-Irradiated CCl₃F/Decane at 77 K

Abstract

A study is made of the formation of the isomeric chlorodecanes, and a search is made for shorter-chain 1-chloroalkanes (5 ≤ n C ≤ 9) formed in CCl3F/decane by γ-irradiation at 77 K and subsequent melting at various concentrations of decane. In such systems, proton transfer from decane radical cations to decane molecules resulting in the formation of pentacoordinated decane carbonium ions (protonated decanes) is quite extensive at high decane concentration, as is evidenced by the fact that under such conditions (i) the ESR spectrum of the irradiated system at 77 K largely consists of decyl radicals and (ii) the contribution of 1-chlorodecane to chlorodecane formation after melting the sample is quite small. The contribution of decyl radicals to the paramagnetic absorption sharply increases and the contribution of 1-chlorodecane to chlorodecane formation strongly decreases with increasing concentration of decane. It is observed that at high decane concentration formation of secondary chlorodecanes is extensive, with 2-chlorodecane being by far the most prominent, but that shorter-chain 1-chloroalkanes are essentially absent. From the results obtained it is concluded that proton transfer from decane radical cations to decane molecules in γ-irradiated CCl3F/decane results in the selective formation of secondary C−H protonated decanes with a clear preference for the penultimate position and that formation of C−C and primary C−H protonated decanes by this process is negligible.

Properties of C−C Bonds inn-Alkanes: Relevance to Cracking Mechanisms

“…The carbonium mechanism, in particular, is seeing increased research interest, with new experiments [11][12][13][14][15][16][17] but particularly with computational chemistry methods. [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35] Carbonium ions, or protonated alkanes, were first detected in mass spectrometry experiments 36 and have been spectroscopically detected only in the gas phase.…”

Proton Transfer from Alkane Radical Cations to Alkanes

“…Information on the site of proton acceptance in the symmetric proton transfer from alkane radical cations to alkane molecules can be derived from chromatographic analysis of chloroalkanes formed in CCl 3 F/alkanes after c-irradiation at 77 K and subsequent melting [26,27]. Because of the amorphous state of alkane aggregates in CCl 3 F, any acceptor site selectivity observed must be intrinsic in nature.…”

“…When a particular cation is intrinsically unstable, its dissociation energy is intuitively assumed to be (near) zero. This misconception is at the origin of various erroneous representations of the relative order of the energies of C-C and secondary C-H protonation [14,27].…”

Symmetric and asymmetric proton transfer from heptane and octane radical cations to heptane molecules in γ-irradiated n-C7H16–n-C8H18–2-C6H13Cl crystals: structural disorder in mixed alkane crystals

“…This can be attributed to site-selective proton transfer from heptane radical cations in their electronic ground state to heptane molecules, with either the radical cation or the neutral molecule being dislocated in the crystal due to the presence of octane [eqn. ( 8 (7) In this process, proton transfer takes place selectively from planar chain-end C-H bonds in heptane radical cations to penultimate C-H bonds in heptane molecules. The reaction undoubtedly also contributes to the previously observed increase in the prominence of 1-heptyl radicals due to the presence of octane in γ-irradiated n-C 7 H 16 -n-C 8 D 18 .…”

“…5 Information on (intrinsic) acceptor site selectivity in symmetric proton transfer from n-alkane radical cations to n-alkane molecules has been gathered from γ-irradiated frozen CCl 3 F-n-alkane solutions by gas chromatographic analysis of the appropriate chloroalkanes after melting. 6, 7 The proton acceptance appears restricted to C-H bonds at secondary carbon atoms (no proton transfer to C-C bonds nor to C-H bonds at primary carbon atoms), with a preference for the penultimate position and mutually equal (but considerably lower relative to the penultimate site) transfer to the different interior positions. Selective transfer to penultimate C-H bonds has also been observed in asymmetric proton transfer, viz., in the proton transfer from heptane radical cations to decane molecules in γ-irradiated heptane-decane-1-chloroheptane crystals, by chromatographic analysis after melting.…”