Radical cation of alkanes

In most of the cases the electron transfer from functional donors to parent radical cations of alkanes and alkyl chlorides is highly exer-gonic. Taking phenol as electron donor and 1-chlorobutane radical... 

The unrestricted and free electron transfer (FET) from donor molecules to solvent radical cations of alkanes and alkyl chlorides has been studied by electron pulse radiolysis in the nanosecond time range. In the presence of arenes with hetero-atom-centered substituents, such as phenols, aromatic amines, benzylsilanes, and aromatic sulfides as electron donors, this electron transfer leads to the practically simultaneous formation of two distinguishable products, namely donor radical cations and fragment radicals, in comparable amounts. 

In the next section we discuss the radical cations generated upon electron transfer from simple -alkanes to holes generated in various matrices upon radiolysis. The ESR spectra observed in these matrices reveal many interesting structural details. We will begin with the special case of the long-elusive methane radical cation, followed by the radical cations of -alkanes and those of branched alkanes. 

Except for conjugated systems such as aromatics, chemically important radical cations of alkanes, alkenes and other small organic and inorganic molecules have never been observed by ESR spectroscopy until the development of new... 

In Summary Fragmentation patterns can be interpreted for structural elucidation. For example, the radical cations of alkanes cleave to form the most stable positively charged fragments, haloalkanes fragment by rupture of the carbon-halogen bond, alcohols readily dehydrate and undergo a cleavage, and alkenes break an allylic bond to form a resonance-stabilized carbocation. 

As in the case of hexafluorobenzene solvent anion, EPR and ODMR spectroscopies suggests that no dimerization of monomer radical anions of benzene and toluene occur in liquid benzene and/or in alkane solutions of benzene (whereas the radical cation of benzene is known to dimerize rapidly). The conductivity studies also indicate that there is no volume change associated with the dimerization [45]. 

The third class of organic donor molecules are a-donors, viz., alkanes and cycloalkanes. These substrates have inherently high ionization and oxidation potentials. Therefore, their radical cations are not readily available by photoinduced electron transfer, but typically require radiolysis and electron impact in the condensed phases or the gas phase, respectively. Thus, radical cations of simple alkanes (methane [206], ethane [207]) or unstrained cycloalkanes (cyclopentane, cyclohexane) [208] were identified and characterized following radiolysis in frozen matrices. In contrast, strained ring compounds have significantly lower oxidation potentials so that the radical cations of appropriate derivatives can be generated by photoinduced electron transfer. 

Anodic oxidation of alkanes is a viable alternative means of generating ej-radical cations from alkanes [23], In contrast with oxidations with short-lived photoexcited species, electrooxidation of the substrate absorbed on the anode involves two consecutive ET steps - oxidation of the alkane then deprotonation to an alkyl radical (Eq. 8) and further oxidation of the alkyl radical to a carbocation (Eq. 9). 

C-H transformation of alkanes by SET is still a developing area of preparative organic chemistry. Generation of cr-radical cations from alkanes in solution requires strong oxidants, and is achieved by photochemical and electrochemical oxidation. Under these conditions even unstrained strained alkanes may be functionalized readily. The C-H substitution is selective if the hydrocarbon forms a radical cation with a definite structure and/or deprotonation from a certain C-H position of the radical cation dominates. Overoxidations are the most typical side reactions that lead to disubstituted alkanes. This can usually be avoided by running the reactions at low alkane conversions. 

Mehnert et al. studied the formation of cationic intermediates in alkane solutions containing a small amount of electron scavengers such as carbon tetrachloride [29.30]. The radical cations of styrene are formed by the following mechanism ... 

In non-polar solvents such as alkanes and alkyl chlorides (RX), ionizing irradiation produces metastable parent radical cations of high reactivity [Eq. (1)]. Usually, these species decay by neutralization, deprotonation and fragmentation. At room temperature such parent radical cations exhibit lifetimes up to 200 nanoseconds. In the presence... 

Radical cations of derivatives of 64 + are accessible also by irradiation of azobicycloalkanes, 70. For a detailed description of the rich chemistry of 64 and its derivatives, and of a large family of azobicyclo[2.2.1]alkanes the reader is referred to a recent review [199]. 

As expected from simple MO considerations, the radical cations of five-membered heterocycles, e.g. the blue species formed from furan, pyrrole, thiophene, and their alkyl derivatives, are n ions. The semi-occupied orbital is the n orbital with the heteroatom in the nodal plane (ai), see Scheme 2, structure 1. In radical cations of a,ca-bis-(l-pyrrolyl) alkanes the charge remains localized on a single ring, rather than being delocalized over both units [5, 10, 11]. 

Radical cations of saturated hydrocarbons have strong electronic absorptions in the visible and near-infrared region of the spectrum. The strongly colored nature of alkane radical cations is in striking contrast to neutral alkanes that absorb electronically only in the vacuum UV. The electronic absorption of alkane radical cations has been studied in the solid phase by matrix isolation using y-irradiation [1-3] and in the gas phase by ion cyclotron resonance (ICR) photodissociation in either the steady-state or pulsed mode of operation [4]. Both methods have their specific merits and drawbacks. A major concern in matrix isolation spectroscopy is spectral purity (because of the possible presence of other absorbing species) and... 

Proton transfer from alkane radical cations to alkane molecules results in the transformation of these cations into neutral alkyl radicals (the conjugate bases). The nature of these radicals is determined by the site of proton donation in the alkane radical cation. Information on the site of proton donation in the proton transfer from alkane radical cations to alkane molecules can thus be derived from EPR spectral analysis of the neutral alkyl radicals formed. To aid the reader in appreciating the results that are presented on this matter below and in understanding related spectra from the literature, a section on the characterization of neutral alkyl radicals by EPR spectroscopy in solid systems is included at this point. 

Radiolysis of cryogenic trichlorofluoromethane containing a suitable n-alkane as solute has proven very suitable for the study of symmetric proton transfer from alkane radical cations to alkane molecules. At low concentration of the alkane solute (RH) in the binary CCljF/alkane system, absorption of ionizing radiation mainly occurs by trichlorofluoromethane resulting in its excitation and ionization. 

In cyclohexane and decalins, reaction (1) is endothermic by 0.1-0.4 eV [60] and it seems reasonable that the excitation of the hole may facilitate the proton transfer. Fragmentation of matrix-isolated hydrocarbon radical cations upon excitation with 2-4 eV photons was observed by EPR (see review [61]). For cycloalkanes, the main photoreaction is reaction (3). For radical cations of methyl-branched alkanes, the loss of CH4 was also observed, while the radical... 

Pulse radiolysis has been used to generate alkene radical cations for kinetic studies. " In this case ionizing radiation produces radical cations of the solvent which, in the case of alkanes and chloroalkanes, have lifetimes that are sufficient to be scavenged by mM concentrations of an appropriate donor. The method has been used to generate styrene radical cations in nonpolar solvents, as indicated in Eq. 13 (RH = cyclohex-ane). . . It has also been used in polar solvents, in which case the alkene is oxidized by a strong transient oxidant such as SO "orTP produced by pulse radiolysis. ... 

For larger alkenes, hyperconjugation with an alkyl group a to an olefinic carbon atom eliminates the need for rotation, so the radical cations of almost all alkenes other than ethene are planar. One-electron oxidation of alkanes leads to a-radical cations. Such ionization removes an electron from an orbital associated with o bonding among carbon atoms (Scheme 2.44). 

---