Trichloromethyl radical, reaction

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]... 

We see that the effect of multidipole interaction plays an important role in all reactions of abstraction and addition of polar reactants. This interaction can increase or decrease the activation energy of the reaction. However, the multidipole interaction does not influence the reactions of nonpolar trichloromethyl radicals with mono- and polyatomic esters due to the nonpolar character of the attacking radical [89]. 

A similar reaction of 1,5-cyclooctadiene with trichloromethyl radicals, produced from carbon tetrachloride and dibenzoyl peroxide, leads to 2-chloro-6-trichloromethylbicyclo-[3.3.0]octane (94), with chloroform and dibenzoyl peroxide the analogue 95 is obtained and iV-t-butylfonnamide affords compound 96 (equation 57)62,63. 

FIGURE 5.5 P450-mediated reduction of carbon tetrachloride and subsequent reactions of the trichloromethyl radical. 

Laser flash photolysis at wavelengths within the charge-transfer absorption bands of 2,2,6,6-tetramethylpiperidine-./V-oxyl (TEMPO) and carbon tetrachloride yields theoxoam-monium chloride of TEMPO 291 (Xmax = 460 nm) and the trichloromethyl radical in an essentially instantaneous 18 ps) process152. The primary photochemical reaction is an electron transfer from TEMPO to carbon tetrachloride followed by immediate decomposition of the carbon tetrachloride anion radical to chloride and trichloromethyl radical (equation 140). The laser flash photolysis of TEMPO and of other nitroxides in a variety of halogenated solvents have confirmed the generality of these photoreactions152. 

Homolytic cleavage of covalent bonds is an alternative means of generating free radicals. This may be assisted by the addition of an electron as in the case of carbon tetrachloride activation. The electron may be donated by cytochrome P-450, allowing the loss of chloride ion and the production of a trichloromethyl radical (Fig. 4.7). This can initiate other radical reactions by reacting with oxygen or unsaturated lipids. 

Similar mechanisms were proposed for the aromatization of 2,4,6-tri-phenyl-4//-pyran (151c) to 387b with aryldiazonium tetrafluoroborates,356 with 2,6-di-fe/Y-butylphenoxide radical, and tetracyanoquinone dimethide357 on the basis of kinetic and electrochemical experiments. Another free radical chain pathway for the reaction of 151c with trichloromethyl radical and tetrachloromethane was also postulated353 (Eq. 22). 

The metabolism of carbon tetrachloride (a chemical solvent that was formerly in common use) attracts attention as well. Its bioactivation appears to involve consecutive one-electron reduction and the formation of chloride ion and the trichloromethyl radical. The latter radical then reacts with oxygen, giving rise to an oxygenated radical and, eventually, to highly toxic phosgene (Mico Pohl 1983). Scheme 3-70 (below) describes these reactions ... 

The hypothesized transformation pathways of CT and CF to methane are shown in Figure 2. The transformation of CT and CF to methane in the Pd/alumina system, despite the low reactivity of MeCl, indicates that the reactions do not involve sequential dehalogenation of CF to methane (i.e. MeCl is not an intermediate). The formation of C2 and C3 compounds during the transformation of both CT and CF indicates the existence of a radical pathway. However, the production of ethane (12-14%) and CF (18-23%) from CT was much lower than that of methane (51-60%). This implies that the main transformation pathway is a direct reaction of CT to methane, with a secondary pathway involving a trichloromethyl radical which then reacts to form CF and C2 and C3 species. Similarly, the relatively low production of ethane (<1%) from CF indicates that the major pathway for the reaction of CF to ethane occurs through direct transformation to methane, rather than through a dichloromethyl radical species. (Lowry and Reinhard 1999)... 

When t-butyl or trichloromethyl radicals were bombarded with ethylene no detectable reaction occurred. The reaction of normal alkyl radicals, apart from ethyl, with ethylene cannot be studied directly because the e.s.r. spectra, of the initial and final radicals are virtually identical, but this difficulty could be overcome by using deuteriated ethylene. However, when the reaction of n-heptyl radicals with deuteriated ethylene was studied, no reaction occurred. As the deuteriated ethylenes are known to undergo addition reactions more readily than the protium analogue (Feld et al., 1962), the results indicate that alkyl radicals would not add to ethylene under the conditions of these experiments. The absence of reaction allows a lower limit of about 5 kcal rnole" (see earlier discussion VIIIA) to be placed on the addition of n-alkyl (apart from ethyl), t-butyl and trichloromethyl radicals to ethylene. 

Very few data exist for these reactions but for t-butyl and n-butyl radicals the activation energies determined in the gas phase are 7-1 kcal mole and 7-3 kcal mole respectively (Kerr and Trotman-Dickenson, 1960a, b), and are consistent with a lower limit of 5 kcal mole . However, the activation energy for the addition of trichloromethyl radicals... 

Reductive decarboxylation has been achieved by heating the acid with LTA in chlorofoim as solvent and hydrogen donor. Only a moderate number of examples are known. The more facile oxidation of secondary and tertiary radicals by LTA effectively limits the method to primary carboxylic acids. It should be noted that stoichiometric quantities of trichloromethyl radicals are generated in the course of this reaction. 

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