Radical chain reactions mechanism example

In the early days of alkene chemistry, some researchers found that the hydrohalogenation of alkenes followed Markovnikov s rule, while others found that the same reaction did not. For example, when freshly distilled but-l-ene was exposed to hydrogen bromide, the major product was 2-bromopropane, as expected by Markovnikov s rule. However, when the same reaction was carried out with a sample of but-l-ene that had been exposed to air, the major product was 1-bromopropane formed by antl-Markovnikov addition. This caused considerable confusion, but the mystery was solved by the American chemist, Morris Kharasch, in the 1930s. He realised that the samples of alkenes that had been stored in the presence of air had formed peroxide radicals. The hydrohalogenation thus proceeded by a radical chain reaction mechanism and not via the mechanism involving carbocation intermediates as when pure alkenes were used. 

For example, the quantum yield for a CdS-, TiOj-, or ZnO-mediated valence isomerization of hexamethyl-dewar benzene to hexamethylbenzene is greater than xmity [106]. A cation radical chain reaction mechanism accounts for this observation (Fig. 2). 

In order to achieve bromination at benzyhc carbons a different set of conditions were used. Specifically, TMS-Br was mixed with sodium bromate in carbon tetrachloride (CCI4) between room temperature and reflux. An example of this reaction, which is be-heved to proceed via a radical chain reaction mechanism, is the selective bromination of phenylacetonitrile (eq 37). ... 

The primary radical yields are often 3. A much higher value (>10) indicates chain reaction. In fact, the chain reaction mechanism for the formation of HC1 from a gaseous mixture of hydrogen and chlorine exposed to radium irradiation is one of the earliest example of this kind, although the detailed chemistry was later shown to involve dissociated atoms rather than electrons and ions, as was originally proposed (see Bansal and Freeman, 1971). 

The rate law obtained from a chain-reaction mechanism is not necessarily of the power-law form obtained in Example 7-2. The following example for the reaction of H2 and Br2 illustrates how a more complex form (with respect to concentrations of reactants and products) can result. This reaction is of historical importance because it helped to establish the reality of the free-radical chain mechanism. Following the experimental determination of the rate law by Bodenstein and Lind (1907), the task was to construct a mechanism consistent with their results. This was solved independently by Christiansen, Herzfeld, and Polanyi in 1919-1920, as indicated in the example. 

Chain-reaction mechanisms differ according to the nature of the reactive intermediate in the propagation steps, such as free radicals, ions, or coordination compounds. These give rise to radical-addition polymerization, ionic-addition (cationic or anionic) polymerization, etc. In Example 7-4 below, we use a simple model for radical-addition polymerization. 

In this section, each method of generating carbon-based fluorinated radicals will be introduced and discussed in terms of mechanism, and it will be seen that virtually all of the useful methods for generating perfluoro and partially fluorinated radicals in a practical manner involve well defined and controlled free radical chain reactions. Some representative examples which demonstrate the preparative use of these methodologies will be presented in the final section of this review. 

The ability of radicals to propagate by abstraction is a key feature of radical chain reactions, which we shall come to later. There is an important difference between homolysis and abstraction as a way of making radicals homolysis is a reaction of a spin-paired molecule that produces two radicals abstraction is a reaction of a radical with a spin-paired molecule that produces one new radical and a new spin-paired molecule. Radical abstractions like this are therefore examples of your first radical reaction mechanism they are in fact substitution reactions at H and can be compared with proton removal or even with an Sfj2 reaction. 

This type of reaction is important industrially since it is one of the few that allows compounds containing functional groups to be made from alkanes. As you might guess, since it needs light for initiation, the process is another example of a radical chain reaction. As with the radical addition of HBr to alkenes, we can identify initiation, propagation, and termination steps in the mechanism. 

Scheme 2.106 Examples of the addition of alkyl radicals to highly fluorinated olefins (Rfh = CF2CHFCF3) (beloiv), and the mechanism ofthe radical chain reaction (above) [32a].

The chain reaction mechanism is frequently referred to as autocatalytic because it starts slowly, but the rate becomes faster as the reaction proceeds. Not many examples of drug substances that decompose by a free radical chain mechanism are known because the process requires participation of a very reactive (i.e., unstable) compound. This usually means a compound susceptible to oxidation and is illustrated by the photo-oxidation of benzaldehyde, as shown in Scheme 2.2 (Moore, 1976) ... 

The products can be formed either by the reaction of a radical (produced within the photo-process) with the initial product or in a radical chain reaction. These two possible mechanisms are discussed in the following examples. 

A few further examples of nucleophilic displacement of halogen from halogenothiophenes by a radical chain SrN, mechanism have been reported. The photostimulated reaction of 2-halo or 3-halothiophene with excess of tetrabutylammonium benzenethiolate in MeCN takes place by an SrN mechanism <87JOC5382>. The 2- or 3-(phenylthio)thiophene is formed in modest yields. The initiation and propagation steps are shown in Scheme 119. The first step may be the photostimulated electron transfer from thiolate anion to the halothiophene (ThX). The reaction does not take place in the dark. 

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