Chlorine atoms abstraction

Each chlorine atom formed m the initiation step has seven valence electrons and IS very reactive Once formed a chlorine atom abstracts a hydrogen atom from methane as shown m step 2 m Figure 4 21 Hydrogen chloride one of the isolated products from... 

This behavior stems from the greater stability of secondary compared with primary free radicals The transition state for the step m which a chlorine atom abstracts a hydro gen from carbon has free radical character at carbon... 

Most solvents for hydroperoxides are not completely inert to radical attack and, consequendy, react with radicals from the hydroperoxide to form solvent-derived radicals, either by addition to unsaturated sites or by hydrogen- or chlorine-atom abstraction. In equation 15, S—H represents solvent and S is a solvent radical. 

Important differences are seen when the reactions of the other halogens are compared to bromination. In the case of chlorination, although the same chain mechanism is operative as for bromination, there is a key difference in the greatly diminished selectivity of the chlorination. For example, the pri sec selectivity in 2,3-dimethylbutane for chlorination is 1 3.6 in typical solvents. Because of the greater reactivity of the chlorine atom, abstractions of primary, secondary, and tertiary hydrogens are all exothermic. As a result of this exothermicity, the stability of the product radical has less influence on the activation energy. In terms of Hammond s postulate (Section 4.4.2), the transition state would be expected to be more reactant-like. As an example of the low selectivity, ethylbenzene is chlorinated at both the methyl and the methylene positions, despite the much greater stability of the benzyl radical ... 

Acyl radicals can fragment with toss of carbon monoxide. Decarbonylation is slower than decarboxylation, but the rate also depends on the stability of the radical that is formed. For example, when reaction of isobutyraldehyde with carbon tetrachloride is initiated by t-butyl peroxide, both isopropyl chloride and isobutyroyl chloride are formed. Decarbonylation is competitive with the chlorine-atom abstraction. 

A chlorine atom abstracts a hydrogen This step produces a molecule of atom from a methane molucule. hydrogen chloride and a methyl radical. 

For tertiary, secondary, and primary chlorides the reduction becomes increasingly difficult due to shorter chain lengths. On the other hand, the replacement of a chlorine atom by hydrogen in polychlorinated substrates is much easier. Table 4.2 shows the rate constants for the reaction of (TMS)3Si radical with some chlorides [32]. The comparison with the analogous data of Table 4.1 shows that for benzyl and tertiary alkyl substituents the chlorine atom abstraction is 2-3 orders of magnitude slower than for the analogous bromides. 

The silaylides formed from coordination of silylenes to chlorocarbons are believed to undergo competitive rearrangement (to the formal Cl—C insertion product) and fragmentation (to the product of formal HC1 abstraction by the silylene)5. In addition, these silaylides are believed to undergo dissociation into the radical pairs that would result from direct chlorine atom abstraction by the silylene111. 

Decomposition of benzoyl peroxide in hexamethyldisilane at 80° C gives, as major products, benzene, benzoic acid, l,2-bis(pentamethyldisilanyl)-ethane and benzylpentamethyldisilane (151). The reaction of hexamethyldisilane in carbon tetrachloride with benzoyl peroxide (at reflux temperature) and with di-tert-butyl peroxide (in a sealed tube at 129° C) gives (chloro-methyl)pentamethyldisilane as the main product arising from the silane (150). In no case are rearrangement products formed. Therefore, in solution at relatively low temperature, the pentamethyldisilanylmethyl radical does not undergo rearrangement as in the thermolysis. The main fate of this free radical is dimerization in the absence of solvent or chlorine atom abstraction when carbon tetrachloride is present. 

The EPR spectra of the NHC boryl radicals that were generated through HAT to the ferf-butoxyl radical clearly show the delocalized 7i-type nature of these intermediates postulated to be essential by calculations [10, 12]. It was also demonstrated that the decay of the EPR signals could be fitted to a second-order decay having 2kt = 9 x 106 M-1 s-1. In agreement with this kinetic analysis, the NHC boryl radicals ultimately dimerize to give bis-NHC diborane derivatives. With the aid of EPR spectroscopy it was also established that the NHC boryl radicals readily abstract bromine atoms from primary, secondary, and tertiary alkyl bromides. However, chlorine atom abstraction is much slower and useful only for benzyl chloride. 

A chlorine atom abstracts a hydrogen atom from methane in the first propagation step. Then the methyl radical that is formed abstracts a chlorine atom from Cl2. The chlorine atom that is produced in the second propagation step reacts again as in the first propagation step. This cycle of two propagation steps is repeated many times in a chain reaction. 

Yang H, Snee PT, Kotz KT, Payne CK, Frei H, Harris CB. Femtosecond infrared studies of a prototypical one-electron oxidative-addition reaction chlorine atom abstraction by the Re(CO)5 radical. J Am Chem Soc 1999 121(39) 9227-9228. 

T. Agapie, P. L. Diaconescu, and C. C Cummins, Methine (CH) Transfer via a Chlorine Atom Abstraction/Benzene-Elimination Strategy Molybdenum Methylidyne Synthesis and Elaboration to a Phosphaisocyanide Complex, Angew. Chem. Int. Ed. 45, 862-870 (2006). 

The radical reaction of carbon tetrachloride with aliphatic double bonds involves addition of the trichloromethyl radical to the double bond, followed by chlorine atom abstraction from carbon tetrachloride by the intermediate radical to give the product. After the addition of the trichloromethyl radical to /3-pinene, a fragmentation occurs prior to formation of the product. 

Collision of a chlorine atom with a methane molecule is both probable and productive, 1 Q chlorine atom abstracts a hydrogen atom, with one electron, to form a molecule of hydrogen chloride ... 

In the first step of the reaction, a chlorine atom abstracts hydrogen to yield hydrogen chloride and a j c-butyl free radical. The carbon that carries the odd electron in the free radical is jp. j ybridized trigonal, Sec. 2.21), and hence a part of the molecule is flat, the trigonal carbon and the three atoms attached to it lying in the same plane. In the second step, the free radical abstracts chlorine from a chlorine molecule to yield jec-butyl chloride. But chlorine may become attached to either face of the flat radical, and, depending upon which face, yield either of two products R or S (see Fig. 7.1). Since the chance of attachment to one face is exactly the same as for attachment to the other face, the enantiomers are obtained in exactly equal amounts. The product is the racemic modification. 

Another example of this type of selectivity, more muted but still easily measurable, is the different selectivities shown by methyl radicals and chlorine atoms for the methylene and methyl groups of propionic acid 7.14. Methyl radicals abstract the hydrogen atoms on C-2 5.2 times faster than the hydrogen atoms on the methyl group C-3. However, chlorine atoms abstract the hydrogen atoms on the methyl group 50 times faster than the hydrogen atoms on C-2.973... 

Similarly, fused and spiro cyclopropane systems 31 and 33 can also be synthesized by the reaction of appropriate cycloalkenyl cobaloximes 30 and 32 with free radical precursors such as toluenesulfonyl iodide (Scheme 11). The thermal and photochemical reactions of hexenyl cobaloximes 34 with a large excess of CCI4 gives mainly the pentachloroheptane 35 (path A). On the other hand, the photochemical reactions in the presence of low concentration of CCI4 gives mainly the cyclopentyl methyl chloride 36a through homolysis of the C-Co bond followed by cyclization of the hexenyl radical and chlorine atom abstraction (path B). However,... 

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