Mechanism of free-radical reactions

The fundamental mechanisms of free radical reactions were considered in Chapter 11 of Part A. Several mechanistic issues are crucial in development of free radical reactions for synthetic applications.285 Free radical reactions are usually chain processes, and the lifetimes of the intermediate radicals are very short. To meet the synthetic requirements of high selectivity and efficiency, all steps in a desired sequence must be fast in comparison with competing reactions. Owing to the requirement that all the steps be fast, only steps that are exothermic or very slightly endothermic can participate in chain processes. Comparison between addition of a radical to a carbon-carbon double bond and addition to a carbonyl group can illustrate this point. 

Mechanisms of Free Radical Reactions in Tumor Cells... 

Moreover, the Boys procedure may also be used for localizing the singly occupied molecular orbitals obtained by the UHF approach (Pee-ters and Leroy, 1977). Thus, as shown in Sections II and IV, the electronic structure of open-shell systems and the mechanism of free-radical reactions can also be described in terms of the corresponding centroids of charge. 

Dneprovkii AS, Miltsov SA (1986) Mechanisms of free-radical reactions. XIX. Selectivity of the free-radical chlorination of 1-chloroalkanes by N-chloropiperidine. Zh Org Khim (Russ) 22(2) 265-269... 

Inclusion of the ionic structures in the three-electron model made it possible to take into account the effects of polarization upon the mechanism of free-radical reactions and to arrive at the following conclusion addition of a free radical to a bond attacked by polar groups occurs in such a position for which the new bond that forms in the transition state possesses the most ionic character. If the reaction occurs under thermodynamic control, then the polarization effects stabilize the product thereby facilitating the development of reaction. Otherwise, products may form that are not thermodynamically the most stable. These qualitative conclusions have been supported [7] by ab initio calculations of the relative stability of reactants and products as well as the activation barriers for a series of the reactions ... 

The time dependent mechanism of free radical reactions poses serious concern for radiation degradation of polymers, particularly in the presence of oxygen due to its high diffuslonal mobility and reactivity. Parkinson [18] has outlined the free radical reactions in radlatlon-Induced polymer oxidation as follows ... 

The use of gamma radiation for the Initiation of free radical chain reactions In liquids offers several advantages over the more conventional methods of initiation. With this technique radical reactions that are not readily accessible by other methods can be studied. Because of the wide temperature range within which radiolytic initiation can be applied this method allows an accurate determination of Arrhenius parameters and therefore can bring better understanding and deeper insight to the factors that control the rates and mechanism of free radical reactions. 

The free radical mechanism is confirmed by the fact that if a substituted aromatic hydrocarbon is used in this reaction, the incoming group (derived from the diazotate) may not necessarily occupy the position in the benzene ring normally determined by the substituent present—a characteristic of free radical reactions. 

The three-step mechanism for free-radical polymerization represented by reactions (6.A)-(6.C) does not tell the whole story. Another type of free-radical reaction, called chain transfer, may also occur. This is unfortunate in the sense that it complicates the neat picture presented until now. On the other hand, this additional reaction can be turned into an asset in actual polymer practice. One of the consequences of chain transfer reactions is a lowering of the kinetic chain length and hence the molecular weight of the polymer without necessarily affecting the rate of polymerization. 

Certain kinetic aspects of free-radical reactions are unique in comparison with the kinetic characteristics of other reaction types that have been considered to this point. The underlying difference is that many free-radical reactions are chain reactions that is, the reaction mechanism consists of a cycle of repetitive steps which form many product molecules for each initiation event. The hypothetical mechanism below illustrates a chain reaction. 

In this chapter, we discuss free-radical substitution reactions. Free-radical additions to unsaturated compounds and rearrangements are discussed in Chapters 15 and 18, respectively. In addition, many of the oxidation-reduction reactions considered in Chapter 19 involve free-radical mechanisms. Several important types of free-radical reactions do not usually lead to reasonable yields of pure products and are not generally treated in this book. Among these are polymerizations and high-temperature pyrolyses. 

The mechanism of free-radical addition follows the pattern discussed in Chapter 14 (pp. 894-895). The method of principal component analysis has been used to analyze polar and enthalpic effect in radical addition reactions. A radical is generated by... 

Still another, and chains, long or short, may be built up. This is the mechanism of free-radical polymerization. Short polymeric molecules (called telomers), formed in this manner, are often troublesome side products in free-radical addition reactions. 

The study of the detailed mechanism of free radical initiation (rate constant k ) and ozone decay (rate constant d) by the reaction with cyclohexane, cumene, and aldehydes gave the following results (7 = 298 K) ... 

The chain generation by the reaction with dioxygen was studied for esters of different structures. Four mechanisms of free radical formation were evidenced. 

Other very convincing evidences for free radical-mediated mechanism of decomposition and reactions of peroxynitrite and nitrosoperoxocarboxylate were demonstrated by Lehnig [140] with the use of CIDNP technique. This technique is based on the effects observed exclusively for the products of free radical reactions their NMR spectra exhibit emission characterizing a radical pathway of their formation. Lehnig has found the enhanced emission in the 15N NMR spectra of N03- formed during the decomposition of both peroxynitrite and nitrosoperoxocarboxylate. This fact indicates that N03- was formed from radical pairs [ N02, H0 ] and [ N02, C03 ]. Emission was also observed in the reaction of both nitrogen compounds with tyrosine supposedly due to the formation of radical pair [ N02, tyrosyl ]. 

The formation of hydroxyl or hydroxyl-like radicals in the reaction of ferrous ions with hydrogen peroxide (the Fenton reaction) is usually considered as a main mechanism of free radical damage. However, Qian and Buettner [172] have recently proposed that at high [02]/ [H202] ratios the formation of reactive oxygen species such as perferryl ion at the oxidation of ferrous ions by dioxygen (Reaction 46) may compete with the Fenton reaction (2) ... 

The mechanism of free radical formation in the reactions catalyzed by MPO and other heme peroxidase may be presented as follows [180] ... 

MECHANISMS OF SECONDARY REACTIONS. The primary processes involved in absorption of radiation in polymers lead to the expectation of free radical and ionic mechanisms for the secondary chemical reactions. Electron spin resonance (ESR) spectroscopy has proved extremely valuable for observation of free radical reactions in polymers, where various radicals are stabilized in the solid matrix at different temperatures. 

Knowledge of bond dissociation enthalpies (DH) has always been considered fundamental for understanding kinetics and mechanisms of free radicals. DHs offer an interesting window through which to view stability of radicals. Indeed, based on Reaction (2.1) the bond dissociation enthalpy of silanes D/f(R3Si—H) is related to enthalpy of formation of silyl radicals, A//f (RsSi ), by Equation (2.2). 

In 1983 Barton and co-workers reported the use of iron-based reagents in the functionalization of alkanes (equation 72). The elucidation of the mechanisms involved a long dispute, but the role of free radical reactions appears to be well established. ... 

Diacyl peroxides are another important source of free-radicals and, consequently, are also commonly used as initiators of free-radical reactions. There is a vast amount of data pertaining to the kinetics and mechanism of decomposition of these compounds in conventional solvents there are a number of side reactions, both radical and ionic in nature, that complicate the kinetics of their decomposition. Generally, these compounds decompose by initial 0-0 bond cleavage that generates carboxyl radicals (RC02 ), which subsequently decarboxylate yielding R (Scheme 4.7)... 

Hayashi, T. Namiki, M. On the mechanism of free radical formation during browning reaction of sugars with amino compounds. Agric. Biol. Chem. 1981, 45, 933-9. 

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