Alkanes charge neutralization

How are those radicals formed In a liquid that is exposed to ionizing radiation the formation of radicals is preceded by a number of rather complex steps. Basically, in the primary steps that follow the absorption of ionizing radiation the molecules of the absorbent become ionized or electronically excited. In nonpolar liquids such as alkanes, the charge neutralization processes are fast and therefore the lifetime of the charged species, molecular cations (RH ") and electrons, is very short. The directly formed excited molecules (RH ) or those that are created in the neutralization processes (RH ) can lose their energy in processes such as radiative and nonradiative conversion to the ground state, collisional deactivation, etc. These processes do not result in a chemical change in the system. Alternatively, these excited molecules can dissociate into molecular products and free radicals. This latter chain of events, that leads to the formation of radicals, is summarized by reactions 2-11. 

The formation of Lewis-base adducts of BH3 is well estabfished, and being charge neutral, L-BH3 systems perhaps more closely resemble alkanes in their potential figating properties, than does the borohydride... 

A very different neutrally charged complex for alkane activation has been reported recently and is shown in Scheme 34(A). The compound is a hydridoplatinum(II) complex bearing an anionic ligand based on the familiar nacnac-type, but with a pendant olefin moiety (97).This complex is extremely soluble in arenes and alkanes and activates C-H bonds in both types of hydrocarbons. This is indicated by deuterium incorporation from deuterated hydrocarbon into the substituents on the arene of the ligand and into the Pt hydride position (A A-d27, Scheme 34). The open site needed for hydrocarbon coordination at Pt(II) is created by olefin insertion instead of anion or solvent substitution (97). 

Johnson and Willson interpreted the main feature of the observations on solid polyethylene doped with aromatic solutes in terms of an ionic mechanism it was analogous to that proposed for irradiated frozen glassy-alkane-systems in which ionization occurred with G = 3 — 4 [96], The produced charged species, electron and positive hole, were both mobile as indicated by the radiation-induced conductivity. The production of excited states of aromatic solutes was caused mainly by ion-electron neutralization. The ion-ion recombination was relatively slow but it might contribute to the delayed fluorescence observed. On the basis of Debye-Simoluchovski equation, they evaluated the diffusion coefficients of the radical anion of naphthalene and pyrene as approximately 4 x 10 12 and 1 x 10 12 m2 s 1 respectively the values were about three orders of magnitude less than those found in typical liquid systems. 

If a hydrogen atom is abstracted from an alkane by an alkyl radical, both the initial and final state of the reaction involve neutral species and it is only the transition state where some limited charge separation can be assumed. In the case of a homolytic O—H bond fission, however, the initial state possesses a certain polarity and possible changes in polarity during the reaction depend on both the lifetime of the transition state and the nature of the attacking radical. If the unpaired electron is localized mainly on oxygen in the reactant radical, the polarity of the final state will be close to that of the initial state and any solvent effect will primarily depend on the solvation of the transition state. Solvent effects can then be expected since the electron and proton transfers are not synchronous. 

This index is provided so that the reader might locate information about a particular complex of interest. It is organized as follows The first section contains the neutral binary complexes, followed by charged dimers, and then by larger complexes in the last section. Within each section, the complexes are ordered by the type of molecules contained. The order is as follows XH, YH2, ZH3, carbonyl, carboxyl, imine, amide, nitrile, alkyne, alkene, alkane, others. (X refers to any halogen F, Cl, Br, I Y to O, S, etc. and Z to N, P, etc.) Any alkylated or similar substitutions follow immediately after their parent group. 

Three major types of cationic species that can be derived from saturated hydrocarbons are alkyl carbenium ions (R+), alkane radical cations (RH +) and alkyl carbo-nium ions (RH2+). The term carbocations is usually reserved to denote alkyl carbenium and carbonium ions only. Pentacoordinated alkyl carbonium ions (proton-ated alkanes) are the species that result from protonation of alkane molecules they are of paramount importance as reactive intermediates/transition states in the initiation of (Br0nsted) acid-catalyzed conversions of saturated hydrocarbons. Upon dissociation of alkyl carbonium ions, trivalent alkyl carbenium ions are formed and these are responsible for the further progression of acid-catalyzed conversions of alkanes. Alkyl carbenium ions may also be formed by ionization of neutral alkyl radicals and by proton addition to olefins. In both carbenium and carbonium ions, the positive charge is very much located on a particular part of the cation. 

It is, however, only the average charge distribution over the alkane molecule that is neutral. Electrons are moving continuously, so at any instant the electron density on one side of the molecule can be slightly higher than that on the other side, giving the molecule a temporary dipole. 

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