Carbon-halogen bond scission

Electrochemical reduction of monohalobenzenes is generally believed to involve irreversible two-electron scission of the carbon-halogen bond followed by rapid protonation of the phenyl carbanion to give benzene. However, publications by Saveant and coworkers [8,177] have provided quantitative evidence that in nonaqueous media a more intricate mechanistic scheme for the reduction cleavage of these compounds must be considered. Accordingly, one can postulate a one-electron transfer to give a radical anion... 

Besides scission of the /3-carbon-hydrogen bond, scission of the /3-carbon-halogen bond is also possible. In photolyses of a-chloronitroso compound 188 nitrone 190 is obtained177. Its formation is discussed via an intermediate aminyloxide 189 which, however, has not been detected by ESR spectroscopy. For the formation of chloro-... 

Dehydrohalogenation which results from the breakage of a carbon-halogen bond and the subsequent liberation of HCl, as in PVC, is an example of a non-chain scission reaction. PVC dehydrochlorination leads to the formation of polyenes, cyclic structures... 

The limitations of this reagent are several. It caimot be used to replace a single unactivated halogen atom with the exception of the chloromethyl ether (eq. 5) to form difluoromethyl fluoromethyl ether [461 -63-2]. It also caimot be used to replace a halogen attached to a carbon—carbon double bond. Fluorination of functional group compounds, eg, esters, sulfides, ketones, acids, and aldehydes, produces decomposition products caused by scission of the carbon chains. 

The autoxidation of isobutane is now mainly carried out to obtain terf-butyl hydroperoxide [36]. Halogenated metalloporphyrin complexes are reported to be efficient catalysts for the aerobic oxidation of isobutane [18,37]. It was found that the oxidation of isobutane by air (lOatm) catalyzed by NHPI and Co(OAc)2 in benzoni-trile at 100 °C produced tert-butyl alcohol in high yield (81%) along with acetone (14%) (Eq. (6.3)) [38]. 2-Methylbutane was converted into the carbonacetic acid, rather than the alcohols, as prindpal products. These cleaved products seem to be formed via P-sdssion of an alkoxy radical derived from the decomposition of a hydroperoxide by Co ions. The extent of the P Scission is known to depend on the stability of the radicals released from the alkoxy radicals [39]. It is thought that the 3-scission of a terf-butoxy radical to acetone and a methyl radical occurs with more difficulty than that of a 2-methylbutoxy radical to acetone and an ethyl radical. As a result, isobutane produces terf-butyl alcohol as the principal product, while 2-methylbutane affords mainly acetone and acetic acid. 

Chain transfer reactions mostly proceed by abstraction of a monovalent atom such as hydrogen or a halogen. The scission of a bond carbon - oligovalent (e.g., H) atom is of interest for the introduction of endgroups into a polymer produced in a free radical reaction. Radically induced vinyl monomer polymerization with the possibility of chain transfer to a polymer of different chemical structure present in the reaction mixture leads to graft copolymers if bond scission occurs outside the main chain, no matter whether a single atom or a grouping is abstracted. Quite a different result is obtained if a radical attack involves a bond in the main chain of the polymer, if this bond scission occurs at a monovalent atom, which must be at the chain end, there is block copolymer formation. If bond scission occurs inside the polymer backbone, either block or random copolymers are produced [63]. 

Because the bonds between carbon and halogen atoms are relatively weak (except for C-F), halogenated compounds have some potential as photoinitiators. Halogenated acetophenon derivatives, such as a,a,a,-tri-chloro-4-terr-butylacetophenone, were shown to undergo both P- and a-scission the first process was found to dominate [28]. 

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