Reaction of Cation Radicals

The simplest type of process undergone by radical cations in the mass spectrometer is cleavage into a radical and a cation. Thus the mass spectrum of an n-paraffin, C H2 +2 normally shows no peak at the mass number M = 14n + 2) corresponding to the initially formed cation radical C H2 +2- The radical ion is unstable to dissociation into alkyl cation and a hydrogen atom or alkyl radical, namely 

Here m can have any value from unity to n. Thus the mass spectrum shows a series of peaks at mass number M — 1, M — 15, M — 29, etc., corresponding to the various alkyl cations derived from the parent radical cation by breaking a CH or CC bond. 

In the case of the paraffins, the initially formed radical cation is unstable with respect to dissociation. One can, however, also observe similar cleavage of bonds in cases where the cleavage is endothermic. This is because the radical cation is generated by bombardment with very energetic electrons and so may be formed initially in an electronically excited state. Internal conversion will then lead to a vibrationally excited form of the radical cation in which the excess vibrational energy may be enough to counter the endo-thermicity of bond rupture. 

The amount of energy made available in tliis way can indeed be very large, more than enough to break any bond. One might therefore expect a chaotic mixture of products to be formed by random bond cleavage. If this 

Similarly, the effects of substituents on such cleavage can be well interpreted in terms of the principles deduced in Chapter 5. Thus compounds of type ArCH2R, where ArH is an even AH, readily undergo the following cleavage, assisted by stabilization of the resulting carbonium ion by the E aryl group  

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Mechanisms of the reactions of cation radicals with nucleophiles. 68... 

The question of dimerization mechanism has also surfaced in the study of reactions of cation radicals. The anodic oxidation of 4,4 -dimethoxystilbene is accompanied by the formation of dimeric products, the nature of which depends upon the nucleophile involved in the reaction as indicated in Scheme 5 (Parker and Eberson, 1969 Eberson and Parker, 1970 Steckhan, 1978 Burgbacher and Schafer, 1979). The reaction was studied by spectroelectro-chemistry (Steckhan, 1978) in acetonitrile containing methanol. Competing mechanisms were proposed involving the reactions of the radical cation (D7) with methanol (134) and by dimerization (135) giving rise to rate law (136). Theoretical working curves were used to find the best fit of the data... 

Given that single-electron oxidation is mechanistically important in these reactions, the thermodynamic criteria discussed earlier can be used, together with the known reactions of cation radicals, to predict new organic oxidation products. For example, there is a wide variety of oxidations that proceed by photoelectrochemical methods. Alcohols, for example, can be oxidized to the corresponding carbonyl compounds in high yield [106-118] (Eq. 13). 

Three studies on radical cations discuss the characterization of polynuclear aromatic radical cation salts as organic metals (8), the reactions of cation radicals with neutral radicals (9), and the magnetic-electrical properties of perfluoroaromatic radical-cation salts (10). Chapters on polynuclear aromatic compounds in nonvolatile petroleum products (II) and in coal-based materials (12) present reviews of the subject and new findings. The remaining chapters in this book discuss the thermal conversion of polynuclear aromatic compounds to carbon (13), the nitration of pyrene by mixtures of N02 and N204 (14), the spectra, structures, and chromatographic retention times of large polycyclic aromatic hydrocarbons (15), the desulfurization of polynuclear thiophenes correlated with tt electron densities (16) and simple theoretical methods to predict and correlate polynuclear benzenoid aromatic hydrocarbon reactivities (IT). 

THIS CHAPTER IS CONCERNED WITH A REACTION of aromatic and hetero-cyclic cation radicals about which only little is so far known their ability to react with neutral radicals. The reaction is expressed simply for the coupling of an aromatic cation radical (ArH +) with a radical (R-) in equation 1. This simple equation, presently only poorly documented, is nevertheless part of current thinking in two reactions of wide scope electrophilic aromatic substitution and reactions of cation radicals with nucleophiles. The product of equation 1 is a a complex, (ArHR)+, which is structurally the same as that... 

The main emphasis of this chapter will be on our attempts to find evidence for radical coupling in reactions of the thianthrene cation radical (Th +) with nucleophiles, that is, for the occurrence of steps such as equations 7 and 8. However, because discussions of reactions of cation radicals with radicals are relatively sparse, we will refer first to other works that principally deal with aromatic nitration. 

In regard to the reactions of cation radicals with stable radicals, such as l,l-diphenyl-2-picrylhydrazyl, and with well-known spin traps, virtually nothing can be found in the literature. The pyridine cation radical has been reported to add to the spin trap phenyl-N-tert-butyl nitrone (37). 

Formation, Properties and Reactions of Cation Radicals in Solution... 

Many electrochemical studies involve reactions of the electrogenerated cation radicals with added nucleophile or in coupling reactions a number of these are discussed in the sections on reactions of cation radicals. Added nucleophile causes the disappearance of the CV cathodic (reversal) peak current and usually an increase in the forward (anodic) peak current because the RNu species formed in the reaction of cation radical and nucleophile is more easily oxidized than the parent [see reactions (73)—(75)]. Moreover, the oxidation peak is shifted towards negative potentials [see... 

Disproportionation is an important concept in cation radical reactions. Many reactions of cation radicals with nucleophiles have the stoichiometry of eqn (99) (or a variant of it). Without the aid of... 

The simplest electron transfer reactions of cation radicals are those involving electron transfer with parent molecules as shown in (106). 

Chemiluminescent and Electrogenerated Chemiluminescent Reactions of Cation Radicals... 

The question of disproportionation is quite controversial in reactions of cation radicals with water. Murata and Shine (1969) found that the reaction of thianthrene cation radical (Th +) with water gave quantitatively equal amounts of thianthrene (Th) and thianthrene... 

Reaction of cation radicals with chloride and bromide ions is not too well defined in the literature. Phenothiazine cation radical perchlorate in acetonitrile solution containing halide ion gave 70-80% yields of 3-chloro- and 3-bromophenothiazine according to (130). Small amounts (4%) of 3,7-dihalogenophenothiazine were also... 

Little is to be found in the literature on the reaction of cation radicals with alkylamines. Shang and Blount (1974) found that DPA + reacts with triethylamine in an ECE sequence to give the... 

The reactions of cation radicals described briefly above must be distinguished from amination reactions, in which amine cation radicals react with aromatics and olefins. These are described later. 

As far as we are aware, reactions of cation radicals with carbonyl compounds were until recently very rare. Cyclization of the 6-/3-ketopropionic ester during oxidation of the metalloporphyrin [52] by iodine is thought by Cox et al., (1969) to occur in the... 

Some vinyl compounds can function as donor molecules because they possess a low ionization potential. The acceptors can be neutral molecules, like quinones, anhydrides, nitrile compounds, etc. They can also be ionic intermediates, such as metal ions, ionized acids, and carbon cations. An interaction of an acceptor with a donor is followed by a subsequent collapse of the charge transfer complex. This can result in formation of cation radicals capable of initiating cationic polymerizations. The exact mechanism of the reaction of cation radicals with olefins is still not completely determined. 

Allyl resonance is important in reactions of cations, radicals, and anions. Be sure you can manipulate the allyl system easily. 

Laser flash photolysis techniques have been employed to evaluate the dynamics of decarboxylation reactions of cation radicals derived from a-aminocarboxylates. > In one report, variously substituted aminium radicals 47 were generated by laser flash excitation of anilinocarboxylates 46 (Scheme 21) in MeCN solutions containing the acceptor, 1,4-dicyanobenzene. These transients undergo fast, first-order decay by a pathway involving loss of carbon dioxide. The rate constants for decarboxylation were found to be in the range of 8 x 10 to 4 x 10 s . In addition, the rates show the same dependence on nitrogen, a-alkyl and a-phenyl substituents, as do the related a-CH deprotonation, a-desilylation, and retro-Aldol cleavage reactions. 

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