Free radicals hydrogen halides

The hydrogenolyaia of cyclopropane rings (C—C bond cleavage) has been described on p, 105. In syntheses of complex molecules reductive cleavage of alcohols, epoxides, and enol ethers of 5-keto esters are the most important examples, and some selectivity rules will be given. Primary alcohols are converted into tosylates much faster than secondary alcohols. The tosylate group is substituted by hydrogen upon treatment with LiAlH (W. Zorbach, 1961). Epoxides are also easily opened by LiAlH. The hydride ion attacks the less hindered carbon atom of the epoxide (H.B. Henhest, 1956). The reduction of sterically hindered enol ethers of 9-keto esters with lithium in ammonia leads to the a,/S-unsaturated ester and subsequently to the saturated ester in reasonable yields (R.M. Coates, 1970). Tributyltin hydride reduces halides to hydrocarbons stereoselectively in a free-radical chain reaction (L.W. Menapace, 1964) and reacts only slowly with C 0 and C—C double bonds (W.T. Brady, 1970 H.G. Kuivila, 1968). 

Among the hydrogen halides only hydrogen bromide reacts with alkenes by both electrophilic and free radical addition mechanisms Hydrogen iodide and hydrogen chlo ride always add to alkenes by electrophilic addition and follow Markovmkov s rule Hydrogen bromide normally reacts by electrophilic addition but if peroxides are pres ent or if the reaction is initiated photochemically the free radical mechanism is followed... 

Hydrogen bromide is unique among the hydrogen halides m that it can add to alkenes either by electrophilic or free radical addition Under photochemical conditions or m the presence of peroxides free radical addition is observed and HBr adds to the double bond with a regio selectivity opposite to that of Markovmkov s rule... 

Free-Radical Addition Reactions 12.4.1. Addition of Hydrogen Halides... 

Addition of hydrogen halide across fluoroalkenes and fluoroalkylalkenes is an important route to halogen-containing fluoroorganics Both lontc and free radical... 

The addition of hydrogen halides to simple alkenes, in the absence of peroxides, takes place by an electrophilic mechanism, and the orientation is in accord with Markovnikov s rule. " When peroxides are added, the addition of HBr occurs by a free-radical mechanism and the orientation is anti-Markovnikov (p. 985). It must be emphasized that this is true only for HBr. Free-radical addition of HF and HI has never been observed, even in the presence of peroxides, and of HCl only rarely. In the rare cases where free-radieal addition of HCl was noted, the orientation was still Markovnikov, presumably beeause the more stable product was formed. Free-radical addition of HF, HI, and HCl is energetically unfavorable (see the discussions on pp. 900, 910). It has often been found that anti-Markovnikov addition of HBr takes place even when peroxides have not been added. This happens because the substrate alkenes absorb oxygen from the air, forming small amounts of peroxides... 

The catalytic hydrogenation of alkyl halides (RX) probably also proceeds via the intermediate formation of free radicals, which are formed in this case by the abstraction of a halogen atom (see Section III,B). 

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... 

There is a discussion of some of the sources of radicals for mechanistic studies in Section 11.1.4 of Part A. Some of the reactions discussed there, particularly the use of azo compounds and peroxides as initiators, are also important in synthetic chemistry. One of the most useful sources of free radicals in preparative chemistry is the reaction of halides with stannyl radicals. Stannanes undergo hydrogen abstraction reactions and the stannyl radical can then abstract halogen from the alkyl group. For example, net addition of an alkyl group to a reactive double bond can follow halogen abstraction by a stannyl radical. 

The photoadditions of halogens, hydrogen halides, and alkylhalides to olefins have been extensively documented.<107) Photohalogenation reactions occur by absorption of light by the halogen, leading to excitation of a non-bonded p electron to an antibonding a excited level, followed by decomposition of the molecule into free radicals ... 

Hydrogen halides will easily add to unsaturated compounds under radiolysis or photolysis. The free-radical chain reaction process is initiated by the dissociation of the halide or by the radiolytic production of radicals from the halide or the organic compound. Thus, for the radiolysis of a mixture of HBr and ethene the postulated initiation is... 

The addition of chlorinated waxes will trap free radicals formed during the basic combustion process, by combination with a hydrogen halide, usually HC1. 

Other than the iodine compound, the hydrogen halides can be obtained by reactions of the elements, but the reactions can be explosive. A mixture of H2 and Cl2 will explode if the reaction is initiated with a burst of light, which separates some Cl2 molecules to produce C1-. The reaction between H2 and Cl2 is a classic case of a free radical reaction. A vast number of hydrolysis reactions yield hydrogen halides. 

This technique was quickly adopted by others and it was soon found by F.O. Rice and co-workers that the pyrolysis of many organic compounds at 800 to 1000°C removed metallic mirrors, implicating the formation of free radicals. The cleavage of larger free radicals into smaller radicals and olefins under these conditions, was also proposed (equation 22), as well as chain reactions in which radicals abstract hydrogen from alkanes. Reactions of alkyl halides with metal atoms in the gas phase were also found by M. Polanyi and co-workers to yield alkyl radicals (equation 23). 

Other methods for the preparation of alkyl halides are electrophilic addition of hydrogen halides (HX) to alkenes (see Section 5.3.1) and free radical halogenation of alkanes (see Section 5.2). 

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