Fragmentation reactions of radicals

Fitchett M, Gilbert BC, Wilson RL (1988) Fragmentation reactions of radicals formed from sugar phosphates and the hydroxyl radical an investigation by electron spin resonance spectroscopy and pulse radiolysis. J Chem Soc Perkin Trans 2 673-689... 

Fragmentation reactions of radical cations are well documented [Additional Reading 1]. Ionization occurs by the removal of an electron from either a sigma (0),... 

SECTION 12.7. REARRANGEMENT AND FRAGMENTATION REACTIONS OF FREE RADICALS... 

Rearrangement and Fragmentation Reactions of Free Radicals 12.7.1. Rearrangement Reactions... 

The primary products of a PET reaction are generally radicals or radical ions. These reactive species can react in different ways to produce new molecules. The most common reactions of radicals and radical ions are the mutual reverse reactions addition mA fragmentation. In contrast to radicals, there are two different ways for radical ions to fragment. One way is the fragmentation into a radical ion and a neutral molecule. The second way leads to two different reactive intermediates a radical and an ion (Scheme 3). 

Though the fragmentation is one of the basic reactions of radical ions, this destructive reaction pathway seems to be synthetically useless at first sight. Nevertheless, the electron-transfer-induced bond cleavage can be specific and, for this reason, synthetically useful (e.g., for ring enlargement reactions). 

A large number of homolytic fragmentation reactions of carbon radicals with p-leaving groups are known from studies in the gas and condensed phase. 

Despite great advances in our understanding of electron transfer (ET) and subsequent reactions of radical ions over the last 20 years, work in this area continues to provide challenges and surprises. While it was recognized many years ago that the addition or removal of an electron can be used to activate molecules towards addition or fragmentation it is only recently that the kinetics of a number of these reactions have been quantified systematically to allow comparison with and improvement of ET theories. Normally one... 

There are a number of non-electrochemical techniques that have proven invaluable in combination with electrochemical results in understanding the chemistry and the kinetics. Laser flash photolysis (LFP) is a well-established technique for the study of the transient spectroscopy and kinetics of reactive intermediates. The technique is valuable for the studying of the kinetics of the reactions of radical anions, particularly those that undergo rapid stepwise dissociative processes. The kinetics of fragmentation of radical anions can be determined using this method if (i) the radical anion of interest can be formed in a process initiated by a laser pulse, (ii) it has a characteristic absorption spectrum with a suitable extinction coefficient, and (iii) the rate of decay of the absorption of the radical anion falls within the kinetic window of the LFP technique typically this is in the order of 1 x 10" s to 1 X 10 s . 

Scheme 14.—Proposed Fragmentation Reactions of the N-Centered Radical from 2-Amino-2-deoxy-D-glucose.34...

It is noted that the 1,2-H shift [reaction (21)] always competes with the (3-fragmentation reaction of oxyl-radicals [reaction (25)]. In the main chain of polymers, P-fragmentation causes a chain break. The ratio of the importance of reaction (21) vs. reaction (25) is determined by the stabilization of the radical that is released in the P-fragmentation process (25) (Grollmann and Schnabel 1980 Schuchmann and von Sonntag 1982). 

In the presence of O2, it is converted into the corresponding peroxyl radical [reaction (266)]. The bimolecular decay of this peroxyl radical with the other peroxyl radicals present is this system leads to erythrose [reaction (267)] in 15% yield, i.e., the p-fragmentation reaction of a short-lived oxyl radical intermediate is of minor importance. 

The first process involves electron ionization to form radical M-1"1 molecular ions. This process has been observed primarily for nonpolar molecules. The proposed mechanisms are charge-exchange transitions between sputtered ions and the neutral organic molecules or electron attachment of low-energy secondary electrons to neutral molecules. The fragmentation reactions of the M ions usually follow the dissociation pathways for odd-electron gas-phase ions. 

The concept that free radical reactions are responsible for the initial fragmentation reactions of large coal molecules and for the essential hydrogen transfer reactions that occur during coal dissolution reactions is quite widely accepted (1). The in-... 

The difference in the extent of exchange at the benzylic and aliphatic positions of Tetralin is measurably less when the iner-tinites rather than the vitrinites from PSOC-106 and PSOC-828, are used to promote the reaction. Indeed, the inertinite obtained from PSOC-828 enhances the exchange reactions at the ben-zylic and aliphatic positions of Tetralin to a comparable degree. This decrease in selectivity suggests that the radicals formed in the initial fragmentation reactions of these semifusinite-rich macerals differ significantly in their behavior from the radicals produced from the other macerals. 

Let us briefly overview the results of relevant studies. The mechanism of the photolysis of hydrocarbon peroxide radicals of different types was studied [119], and the decomposition of the electron-excited peroxide fragment with an 0-0 bond cleavage was found to be the primary act of the process. When the reaction is not complicated by the adsorption of molecules on the solid surface, its kinetic parameters can be determined. The rate constants of the reactions of radicals =Si, =Si-0-0, =Si-CH, =Si-0-CH, =Si-CH2-CH, and =Si-0- C = O with H2(D2), CH4, C2H6, and C3H8 were measured in Ref. [16]. 

Fig. 2.4. Reactions of radicals generated from a ligand bound to a receptor. X-Y or X = Y is the photoactivatable ligand Y is radiolabeled but X is not Z is an atom in the ligand binding site (e.g. H) S represents termination of the reaction by H, by another radical from the buffer, or by X. Elimination of a radical from the receptor could also terminate the reaction. If termination occurred at stage c, a covalent adduct with a radiolabeled fragment of XY would not be formed but the receptor might be inactivated (see e.g. Ogez et al., 1977 Smith...

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