Radical reactions nonchain

Radical initiators are molecules bearing one or several weak bonds with a BDE of about 100-200 kJ mol-1. When the temperature of the reaction is sufficiently high, the initiator decomposes with homolysis of the weakest bond and produces free radicals, which initiate a chain or nonchain free radical reaction. 

Most radicals are highly reactive, and there are few examples where one would produce a stable radical product in a reaction. Reference to a radical reaction in synthesis or in Nature, almost always concerns a sequence of elementary reactions that give a composite reaction. Multistep radical sequences are discussed in general terms in this section so that the elementary radical reactions presented later can be viewed in the context of real conversions. The sequences can be either radical chain reactions or radical nonchain reactions. Most synthetic apphcations involve radical chain reactions, and these comprise the bulk of organic synthetic sequences and commercial applications. Nonchain reaction sequences are largely involved in radical reactions in biology. Some synthetic radical conversions are nonchain processes, and some recent advances in commercial polymerization reactions involve nonchain sequences. 

Reversible redox reactions can initiate radical chemistry without a follow-up reduction or oxidation reaction. In successful reactions of this type, the redox step that produces the radical is thermodynamically disfavored. For example, Cu(I) complexes react reversibly with alkyl hahdes to give Cu(II) hahde complexes and an alkyl radical. The alkyl radical can react in, for example, an addition reaction, and the product radical will react with the Cu(II) hahde to give a new alkyl halide. This type of reaction sequence, which has been apphed in living radical polymerizations, is in the general family of nonchain radical reactions discussed earlier. ... 

Unless a substrate or reagent contains an acidic or basic site, the conditions for most radical reactions are neutral. Thus, ionic side reactions such as base-catalyzed epimerization are rarely a problem. While radical reactions are typically conducted at temperatures above ambient, this is often solely for experimental convenience most commercially available initiators require heating to generate radicals. Many radical reactions should succeed at lower temperatures provided that the chain is maintained (in chain methods) or that the rate of generation of radicals is sufficiently rapid (in nonchain methods). Low temperature initiators are available.30,31... 

Although the synthetic transformations that can be accomplished by radical reactions are virtually limitless, precious few general methods exist by which radical reactions can be conducted. These methods can be classified in two main groups chain and nonchain. 

Although chain methods have been the foundation of the recent resurgence of radical reactions in synthesis, many of the earliest preparatively useful radical addition reactions were based on nonchain processes. Of late, such nonchain processes have regained importance. 

A free-radical reaction may proceed by a chain or a nonchain mechanism. There are many experimental methods for determining whether a chain or a nonchain mechanism is operative in a reaction, but these methods don t help much in pen-cil-and-paper problems such as the ones in this book. Luckily, the reagents or reaction conditions will usually indicate which type of mechanism is operative. 

The major classes of nonchain free-radical reactions are photochemical reactions, reductions and oxidations with metals, and cycloaromatizations. 

The present second edition of this book corrects two major errors (the mechanisms of substitution of arenediazonium ions and why Wittig reactions proceed) and some minor ones in the first edition. Free-radical reactions in Chapter 5 are reorganized into chain and nonchain processes. The separate treatment of transition-metal-mediated and -catalyzed reactions in Chapter 6 is eliminated, and more in-text problems are added. Some material has been added to various chapters. Finally, the use of italics, especially in Common Error Alerts, has been curtailed. 

Many hydroperoxides have been prepared by autoxidation of suitable substrates with molecular oxygen (45,52,55). These reactions can be free-radical chain or nonchain processes, depending on whether triplet or singlet oxygen is involved. The free-radical process consists of three stages ... 

The radiation yield depends on the temperature of oxidation and the initiation rate, i.e., the intensity of radiation IT [233], Radoxidation occurs as an initiated chain reaction at an elevated temperature when peroxyl radicals react more rapidly with hydrocarbon RH than disproportionate, kp(2kt) [RH]2 > (see Chapter 2)]. Radoxidation proceeds as a nonchain reaction at low temperatures when peroxyl radicals disproportionate more rapidly than react with hydrocarbon. The temperature boundary Tv between these two regimes of oxidation depends on the value of radiation intensity 7r. The values of Tv for irradiated heptane oxidation is as follows [233] ... 

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