Radical chain mechanism halogenation

The elementary steps (1) through (3) describe a free-radical chain mechanism for the reaction of an alkane with a halogen. 

Tri-rc-butylstannane is able to reductively replace halogen by hydrogen. Mechanistic studies indicate a free radical chain mechanism.199 The order of reactivity for the halides is RI > RBr > RC1 > RF, which reflects the relative ease of the halogen atom abstraction.200... 

Michael Faraday reported in 1821 that chlorine addition to alkenes is Stimulated by sunlightand today this is taken to indicate the involvement of a free radical process (equation 26). Free radical chain mechanisms were proposed in 1927 by Berthoud and Beraneck for the isomerization of stilbene catalyzed by Br2 (equation 27), and by Wachholtz for bromine addition to ethyl maleate (equation 28).Later studies showed inhibition of halogen addition by reaction of the intermediate radicals with oxygen, and a free radical chain mechanism for solution and gas phase halogenations as in equation (26) was shown (equation 29). Kinetic and mechanistic... 

A significant observation concerning bromine addition is that it and many of the other reactions listed on page 360 proceed in the dark and are not influenced by radical inhibitors. This is evidence against a radical-chain mechanism of the type involved in the halogenation of alkanes (Section 4-4D). However, it does not preclude the operation of radical-addition reactions under other conditions, and, as we shall see later in this chapter, bromine, chlorine, and many other reagents that commonly add to alkenes by ionic mechanisms also can add by radical mechanisms. 

The silver(I) salts of carboxylic acids react with halogens to give unstable intermediates which readily decarboxylate thermally to yield alkyl halides. The reaction is believed to involve homolysis of the C-C bond and a radical chain mechanism. 

Alkanes are fuels they burn in air if ignited. Complete combustion gives carbon dioxide and water less complete combustion gives carbon monoxide or other less oxidized forms of carbon. Alkanes react with halogens (chlorine or bromine) in a reaction initiated by heat or light. One or more hydrogens can be replaced by halogens. This substitution reaction occurs by a free-radical chain mechanism. 

The reaction of an alkyl halide with tributyltin hydride, using a radical initiator, results in the replacement of the halogen by hydrogen. The reaction follows a radical chain mechanism as outlined in Figure 21.3. Examples are provided in the following equations ... 

Rather than using the halogens themselves, other halo n radical donors are more commonly used in laboratory scale synthesis. One of the simplest of these is CCU, which can chlorinate alkanes by a free radical chain mechanism.The chain lengths are not very long (equations 76-78), because of their slightly endothermic nature and in part because the reaction is also kinetically rather slow. Elevated temperatures are therefore normally required. Nitrosylchloride at 1(X) C has also been used for these reactions. ... 

Apart from the molecular and the radical-chain mechanisms for the decomposition of halogenated hydrocarbons, the study of allyl bromide pyrolysis " revealed a third possibility, namely a radical non-chain mechanism, a type originally proposed by Daniels and Veltman for ethyl bromide pyrolysis. Its application to allyl bromide pyrolysis is shown in the following scheme as proposed by Maccoll ... 

From a mechanistic point of view, two different ionic mechanisms have to be considered (due to the presence of oxygen the radical chain mechanism plays no role in the technical process) first, the uncatalyzed reaction of ethylene and chlorine and second, the metal halide catalyzed reaction. Both routes compete in this process. The uncatalyzed halogenation was studied extensively for the bromina-tion of olefins [14, 15] (Scheme 4). It is commonly accepted that the halogenation of olefins starts with formation of a 1 1 -complex of halogen and alkene followed by formation of a bromonium ion. Subsequent nucleophilic attack of a bromine anion leads to the dibromoalkane. However, when highly hindered olefins (such as tetraneopentylethylene) are used, formation of a 2 1 r-complex, as an intermediate between 1 1 ir-complex and a bromonium ion, is detectable by UV spectroscopy. In the catalyzed reaction the metal halide polarizes the chlorine bond, thus leading to formation of a chloronium or carbonium ion. Subsequent nucleophilic attack of a chloride anion gives the dichloroalkane [12] (Scheme 5). 

Another, related type of reaction is the halodifluoromethylation of nucleophiles by dihalodifluoromethanes (e.g. CF2Br2) [9]. This reaction is always initiated by a single electron transfer from the nucleophile to the CF2XY species (X and Y denote halogens other than fluorine). The subsequent fate of the resulting radical ion pair depends on the ability of the nucleophile to form a stabilized radical, and also on the choice of solvent [10]. For phenoxides [4a, 5, 11] and thiophenoxides [4c, Ila] a reaction pathway via difluorocarbene is usually preferred whereas enamines and ynamines are halodifluoromethylated by a radical chain mechanism (see also Section 2.2.1) [12] (Scheme 2.169). 

Photohalogenation a photoinitiated reaction between a halogen donor, such as a halogen molecule and a substrate proceeds typically via a radical chain mechanism therefore, the reaction quantum yield, closely related to the average chain length, is greater than unity.155... 

There are in principle three possibilities for reaction of halogens with aromatic hydrocarbons, namely, addition, substitution in the nucleus, and substitution in a side chain. The last of these is discussed on pages 152 and 157. Substitution of benzene by chlorine or bromine is an ionic reaction,114 whereas photochemical or peroxide-catalyzed addition of these halogens involves a radical-chain mechanism.115 Substitution in the side chain also proceeds by a radical mechanism,116 addition rather than side-chain substitution being favored by higher chlorine concentrations.115... 

Halogenated systems in the gas phase act by interrupting the radical chain mechanism that sustains the flame, whereby they are highly efficient. The different halogen-free systems, on the other hand, work like this ... 

Halogenation proceeds by the free-radical chain mechanism shown in Mechanism 10.2. 

Reduction by hydrogen-atom donors necessarily involves intermediates with unpaired electrons. Tri-n -butyltin hydride is the most important example of this type of reducing agent. It is able to reductively replace halogen by hydrogen in many types of halogen compounds. Mechanistic studies have indicated a free-radical chain mechanism. The ability of trisubstituted stannanes to function effectively in these... 

Kim and Bunnett also found that the product ratios from reactions of 95c and 96c could be shifted toward the ratios observed with other halogens by adding a radical trap such as tetraphenylhydrazine to the solvent (liquid ammonia) or by carrying out the reaction in a solvent composed of 50% ammonia and 50% diethyl ether. The authors therefore proposed a radical chain mechanism, shown in equations 8.89 through 8.93, for formation of... 

This radical is intermediate in selectivity between chlorine and bromine atoms. The selectivity is also solvent and temperature dependent. A typical ratio, in chlorobenzene as solvent, is tertiary secondary primary = 60 10 1. Scheme 12.4 (p. 690) gives a number of specific halogenation reactions that proceed by radical chain mechanisms. 

Halogen flame retardants are very effective because they interfere with the radical chain mechanism in the gas phase of the combustion process. We have previously seen that during the combustion of polymers, radical chain reactions involving the OH and H radicals occur, as shown in Fig. 4.26. 

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