Chlorine atoms, abstraction reactions

Other classes of radicals have also been examined with TRIR methods. The rate constant for P-scission of alkoxy radicals (Scheme 2.6) was found to be enhanced in polar solvents by monitoring the rate of production of the product carbonyl compound [74]. The decarboxlyation of peroxyesters [75], the reactivity of azidyl radicals [76], and chlorine atom abstraction reactions [77] have also been investigated. 

Selective chlorination of the 3-position of thietane 1,1-dioxide may be a consequence of hydrogen atom abstraction by a chlorine atom. Such reactions of chlorine atoms are believed to be influenced by polar effects, preferential hydrogen abstraction occurring remotely from an electron withdrawing group. The free radical chain reaction may be propagated by attack of the 3-thietanyl 1,1-dioxide radical on molecular chlorine. 

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. 

It has been proposed that this reaction intermediate could decompose to produce HCN and CH3 [55], Chemiluminescence from alkanes can be greatly enhanced by addition of HC1. The proposed explanation is that energy transfer from active nitrogen dissociates HC1 to produce chlorine atoms, which have rapid hydrogen-atom abstraction reactions with alkanes,... 

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. 

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. 

What a beautiful tool is cyanuric chloride for the chemist working in chemical synthesis Three chlorine atoms offer reaction with a large proportion of the chemicals listed in the Beilstein Handbook or the Chemical Abstracts Index. Not only that the chlorine atoms are reasonable enough not to react simultaneously but, under adequate conditions, stepwise, allowing myriads of potential combinations. Furthermore cyanuric chloride has been and is a relatively cheap key material it can be produced quite easily from such basic materials as chlorine and hydrocyanic acid. 

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. 

Radicals can undergo other reactions as well as monomer addition. Atom abstraction reactions usually involve transferof a hydrogen or halogen atom. An example from micromolecular chemistry involves the chlorination of hydrocarbons at about 200°C or during irradiation with light of wavelength less than 4875 x 10 m ... 

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. 

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. 

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,... 

There has been a good deal of study of the polyhalogenated methanes in hydrogen atom abstraction reactions toward hydroxyl (HO ) and chlorine radicals. These reactions are involved in both the atmospheric destruction of such compounds as well as their involvement in ozone depletion. Information is needed about these reactions to model the environmental impact of the compounds. 

The behavior of 1 -adamantyl radical in chlorine atom abstraction has been investigated recently [18f]. Initiators of radical reactions and some metal complexes induce dark chlorination of saturated hydrocarbons with Cl2or CCU[19]. 

The rate constants for HCFC removal show a correlation with respect to the chlorine content of the molecule. For molecules of the form RCXj- Cl, where R = H, CH3 or CF3 and X = H, F or a combination of the two, the reaction rate increases with n. For example, the reaction of 0( D) with HCFC-141b is faster than the reaction with HCFC-142b, and the reaction with HCFC-21 is faster than that with HCFC-22 This is consistent with an abstraction mechanism in which chlorine atom abstraction is preferred because of the weaker C—Cl bond. 

Each chlorine atom formed in the initiation step has seven valence electrons and is very reactive. Once formed, a chlorine atom abstracts a hydrogen atom from methane as shown in step 2 in Mechanism 4.4. Hydrogen chloride, one of the isolated products from the overall reaction, is formed in this step. A methyl radical is also formed, which then reacts with a molecule of CI2 in step 3 giving chloromethane, the other product of the overall reaction, along with a chlorine atom. The chlorine atom then cycles back to step 2, and the process repeats. Steps 2 and 3 are called the propagation steps of the... 

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