Halogenation radical processes

With more electrophilic halogenated olefins, additions of tnfluoromethyl hypofluonte appear to go by radical processes [139] and give oligomeric products [140] or one-to-one adducts [141,142] (equation 4) depending on reaction conditions... 

Additions of elemental halogens to unsaturated compounds are among the most common preparations of halogenated fluoroorganics. The transformations are usually fairly clean and proceed in good yields. Besides the numerous examples of halogen addition tofluoroalkenes and fluoroalkyl-substituted alkenes, additions to perfliioropropyl vinyl ether [2] and fluormated styrenes [7, 4] have been reported. Both ionic and free-radical processes occur (equations 1 and 2)... 

Halogenation reactions of alkanes provide good examples of radical processes, and may also be used to illustrate the steps constituting a radical chain reaction. Alkanes react with chlorine in the presence of light to give alkyl chlorides, e.g. for cyclohexane the product is cyclohexyl chloride. 

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... 

With the exception of ring-synthetic procedures, an effort has been made to include all methods of introduction of halogen atoms into a heterocyclic ring. Thus electrophilic, nucleophilic, and radical processes are covered as well as some rearrangements. 

Most of the side reactions have already been discussed in Section 26.1.3.3. (dediazoniation in organic solvents). Chlorinated aliphatic solvents, such as 1,2-dichloroethane and dichloro-methane, lead to an extensive formation of chloroaromatics, and aromatic solvents, even halogenated examples, can be arylated to some extent by arenediazonium tetrafluoroborates (vide supra). For example, during dediazoniation of benzenediazonium tetrafluoroborate in toluene, chlorobenzene, bromobenzene or anisole, 3-5 % of substituted biphenyls Ph-C6H4-X (X = Me, Cl, Br, OMe) are formed together with 0.5-0.8% of fluorobiphenyls.5 Fluorobiphenyls are formed through an ionic pathway (only 2- and 4-isomers are formed) whereas chlorobiphenyls result from a radical process (X = Cl, 2-/3-/4-isomer 26 47 27).243... 

A third transition of less significance is the n-+o transition this is observed in the photolysis of halogenated compounds.7 The a energy level is unusually unstable and the molecule undergoes bond cleavage with the formation of free-radical species. Many photoreactions can, in fact, be thought of in radical terms, and close analogies exist between certain photochemical reactions and free-radical processes. 

The halogenation of saturated aliphatic hydrocarbons is usually achieved by free radical processes.523 Ionic halogenation of alkanes has been reported under superacid catalysis. Olah and co-workers524,525 have carried out chlorination and chlorolysis of... 

The proposed mechanism of oxidative fluorination of unsaturated compounds by halogen fluorides [84-86] and VF5 [33] includes electron transfer from substrate to halogen fluoride or VF5 as a first step, followed by addition of F" to a radical-cation, leading to formation of a radical and its further oxidation to carbocation (see Eq. 12, pathways A,B). It should be pointed out that this is not the only direction, and the actual mechanism may depend strongly on the substrates and reaction conditions. Other mechanisms, such as a radical process (pathway C), cannot be ruled out. 

Halogenated hydrocarbons often bring about substitution at silicon (see Section III,C), but in the case shown in entry 4, Si-Ti (but not Si-Si) bonds are cleaved, and rearrangement to a cyclic Si5 derivative occurs. A radical process is postulated, similar to that observed in Ti-Ge (422) and H-Mn-CO (72) systems. Silicon-zirconium bonds are cleaved in an analogous way (entry 5). 

The mechanisms of methane-halogen reactions are chain radical processes. Carbon tetrachloride Is also prepared from carbon disulfide and chlorine. Sulfur is formed in this reaction, but it is recovered and used for the regeneration of CS3. Carbon disulfide is obtained from the elements, but is also formed from methane and sulfur (see Section 1,1.5.4). 

The mechanisms of these reactions are probably complex but radical processes seem likely. The final products of insertion into the metal metal bond are actually trinuclear species. Insertion into terminal metal hydride or metal alkyl bonds could, in principle, generate mixed-metal dimers. In practice, aside from insertion into the metal halogen bond, such as that shown in equation (65), few such reactions have been reported. 

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