Abstraction of halogens

One of the older preparative free-radical reactions is the addition of polyhalomethanes to alkenes. Examples of addition of carbon tetrabromide, carbon tetrachloride, and bromoform have been recorded. The reactions are chain processes that depend on facile abstraction of halogen or hydrogen from the halomethane ... 

Primary alkyl chlorides are fairly stable to fluorine displacement. When fluorinated, 1-chloropropane is converted to 1-chloroheptafluoropropane and 1-chloto-2-methylbutane produces 39% l-chlorononafluoro-2-methylbutane and 19% perfluoro-2-methylbutane. Secondary and tertiary alkyl chlorides can undergo 1,2-chlorine shifts to afford perfluonnated primary alkyl chlorides 2-Chloro-2-methylpropane gives l-chlorononafluoro-2-methylpropane, and three products are obtained by the fluorination of 3-chloropentane [7] (equation 1). Aerosol fluorina-tion of dichloromethane produces dichlorodifluoromethane which is isolated in 98% purity [4 (equation 2). If the molecule contains only carbon and halogens, the picture is different. Molecular beam analysis has shown that the reaction of fluorine with carbon tetrachlonde, lodotrichloromethane, or bromotrichloromethane proceeds first by abstraction of halogen to form a trichloromethyl radical [5]... 

The addition of an alkyl halide to the d7 complex [Co(CN)6]3- results in a one-electron oxidation of the metal and proceeds by homolytic abstraction of halogen. For a methyl or benzyl halide, an organocobalt product is formed,... 

Cleavage with cesium fluoride was used in the case of stannylsilanes (equation 34)49. The generated stannyl anions are very effective in synthetic applications, mainly in abstraction of halogen to initiate organic 4 + 2 cycloadditions (equation 35)49, a reaction which constitutes one of its chemical characterizations. 

The oxidation scheme for CX3 radicals (where each X is any of F, Cl or Br) is similar to that of CHX2 radicals. However, abstraction of halogen from a CXsO radical is usually endothermic and is therefore an improbable process under atmospheric conditions. The result of oxidation is the formation of carbonyl halides CX20. 

Figure 3.166 shows the reactivity and potential of these early transition metal pincer carbene complexes taking vanadium as an example. The broad range of compounds becomes possible owing to the rich redox chemistry of vanadium that makes different oxidation states easily accessible. Here, examples for vanadium(ll), vanadium(III) and vanadium(IV) are shown. Abstraction of halogen is facile making the introduction of different TT-donor ligands possible, even weak ones like acetonitrile. 

Metallic copper, Cu(0), can also be employed for controlled radical polymerization in the absence or presence of copper ) or copper halides.126 129 Cu-(0) is most probably converted in situ into an active Cu species via abstraction of halogen from the initiator and the polymer terminal or from added copper ) or copper halides. The use of Cu(0) as an accelerator or reducing agent of Cu(II) species will be discussed later (section II.D). 

Side reactions such as termination and transfer were investigated in the polymerizations of styrene,291 acrylates,292 and methacrylates.293 The occurrence of thermally initiated radical polymerizations was observed in the copper-catalyzed styrene polymerization, while the resulting polymer chain can be converted into the dormant polymer terminal via abstraction of halogens from the persistent metal radical in higher oxidation states.294... 

If organosilicon compounds have an acetoxy group on the carbon adjacent to the silicon atom, a facile 1,2-shift of the acetoxy group from carbon to silicon has been observed1 s, in addition to capture of the initially formed silyl radical by abstraction of halogen from the CC14 solvent (equation 3). Similar behavior was shown by a related benzyldimethylsilyl... 

Cathodic reduction consists of electron supply from the cathode, followed by acquisition of protons by anionically charged substrates. Thus, ready accessibility of the reductive electron transfer processes leads to a variety of reactions, including reductive abstraction of halogens [13, 14], reduction of alcohols to alkanes [15], reduction of imines, oximes, and hydrazones to the corresponding amines [16], and conversion of nitro groups to the corresponding nitroso, hydroxy amino, and amino groups [17]. In... 

It is generally accepted that the main mechanism of flame retardant action of halogenated flame retardants is in the gas phase, and it is primarily the chemical mode of action. The reaction begins with the abstraction of halogen radical from the flame retardant. This halogen immediately abstracts hydrogen from either the flame retardant additive or the polymer. An example of such a sequence of reactions, with the participation of bromine and an aliphatic polymer, is... 

Another criterion of mechanism is the effect of structure on rate. It is reasonable to assume that the slowest step in the reaction chain is the abstraction of halogen atom from the halide, Eq. (43). Menapace has studied the relative reactivities of a number of halides by allowing them to compete in pairs for an insufficient amount of hydride and analyzing the resulting reaction mixture for unreacted starting materials, products, or both 32,56). Pertinent results are listed in Table XIV. It is clear that the relative reactivities are those expected for a reaction leading to the production of a free radical. For example, benzyl bromide is more reactive than t-butyl bromide... 

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