Hydrocarbons radical halogenation, mechanism

Free-radical halogenation of hydrocarbons induced thermally or photochemically can be performed with all four halogens, each exhibiting certain specificities. Because of the thermodynamics of the process, however, only chlorination (and bromination) are of practical importance.31,106-108 Fluorination with elemental fluorine is also possible. This reaction, as discussed above (see Section 10.1.1), follows an electrophilic mechanism in the solution phase.109,110 Under specific conditions, however, free-radical fluorination can be performed. 

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 ... 

The hydrocarbon portion of an aliphatic acid can undergo the free-radical halogenation characteristic of alkanes, but because of the random nature of the substitution it is seldom used. The presence of a small amount of phosphorus, however, causes halogenation (by an ionic mechanism) to take place exclusively at the alpha position. T his reaction is known as the Hell-Volhard-Zelinsky reaction, and it is of great value in synthesis. 

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... 

In the first step, hydrogen halides inactivate the reactive OH radicals, then the halide is regenerated, producing slightly reactive R hydrocarbon radicals from the polymer. Due to this self-regenerating mechanism, even a relatively small amount of halogenated compound may exert effective flame-retardance. 

Since carbon-halogen bond cleavage is not a favorable process, and since both carbon and oxygen atoms have an even number of electrons, the oxidation proceeds either via free radicals or via carbenes and triplets. As a result, the oxidation mechanisms are considerably simpler than with hydrocarbons. Section III of this review treats free radical oxidation, whereas Section IV treats oxidation by processes not involving free radicals. In this discussion a free radical is considered to be a species with an odd number of electrons carbenes and triplets are called biradicals and are not included in this category. 

The initial step in the oxidation of a hydrocarbon is substitution. For saturated hydrocarbons such as open- and closed-chain paraffins and the alkyl groups of other hydrocarbons, the substitution reaction moves by a free radical mechanism, and sets up a chain reaction. Such a reaction Ls best illustrated by halogenation. 

Thermal decomposition occurs more rapidly under ultraviolet irradiation or in a halogenated hydrocarbon such as chloroform or carbon tetrachloride. Even the thermostable tetraphenyldibismuthine does decompose in chloroform at 25°C [830M1859]. These interesting findings are explained best by a free radical mechanism. 

Some reactions, such as the halogenation of aliphatic hydrocarbons, proceed by radical mechanisms. Radicals possess an unpaired electron resulting from bond homolysis. They are generally uncharged. 

Halogenated Hydrocarbons - The destruction of cytochrome P-450 by CCl, first attributed to lipid peroxidation, has been shown to occur even under conditions where lipid peroxidation is not detectable.one possible explanation for this inactivation is that the trlchloromethyl radical or a related species obtained by reduction of the halocarbon reacts with the heme moiety or the apoprotein. The ill-defined radio-labeled porphyrins reported in Incubations of labeled CCI4 with hepatic microsomes would provide support for a heme alkylation mechanism were it not for the conflicting report that fluorescent N-alkylated porphyrins similar to those obtained with AIA are not isolated from CCl -incubated microsomes by procedures that result in isolation of the AIA adducts. ... 

Many halogenated hydrocarbons are known as environmental pollutants, for example, polychlorinated biphenyl (PCB), polychlorinated phenols (PCP), polychlorinated benzenes (PCBz), chloro-fluoro aliphatic carbons (CFC), and bromo-fiuoro aliphatic carbons (halone). As described in the previous section, solvated electrons that will attach to the solute whose electron affinity is positive, such as halogenated hydrocarbons, are effectively produced in polar solutions, such as an alcohol or ester. In this section, the dehalogenation mechanism in an alkaline 2-propanol solution (Mucka et al. 1997, Nakagawa 2003, Nakagawa and Shimokawa 2002, Sawai et al. 1974,1975, Shimokawa and Sawai 1977) is presented. The reaction mechanisms are summarized as follows in which R and R+ denote the radical and cation of 2-propanol, respectively, e soiv is the solvated electron, and M-X is the halogenated carbons (Reaction Mechanism 14.4). 

Vinyl polymerization using metallocomplexes commonly proceeds by a radical pathway and rarely involves an ionic mechanism. For instance, metal chelates in combination with promoters (usually halogenated hydrocarbons) are known as initiators of homo- and copolymerization of vinylacetate. Similar polymer-bound systems are also known [3]. The polymerization mechanism is not well understood, but it is believed to be not exclusively radical or cationic (as polymerization proceeds in water). The macrochelate of Cu with a polymeric ether of acetoacetic acid effectively catalyzes acrylonitrile polymerization. Meanwhile, this monomer is used as an indicator for the radical mechanism of polymerization. Mixed-ligand manganese complexes bound to carboxylated (co)polymers have been used for emulsion polymerization of a series of vinyl monomers. Macromolecular complexes of Cu(N03)2 and Fe(N03)3 with diaminocellulose in combination with CCI4 are active in polymerization of MMA, etc. 

The mechanisms by which many solvents exert their toxicity are unclear and may vary from one solvent to another. Halogenated hydrocarbons such as carbon tetrachloride may generate free radicals. Simple aromatic compounds such as benzene may disrupt polyribosomes, whereas some solvents are thought to affect lipid membranes and to penetrate tissues such as the brain. ... 

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