Radical chlorinations comparison

Free radical chlorination of alkanes has been reviewed42. The main effort in the study of these reactions has been devoted to the role of solvent43, the effect of -substituents44 and comparison between gas- and liquid-phase processes45. 

Bottoni A. Theoretical study of the hydrogen and chlorine abstraction from chloromethanes by silyl and trichlorosilyl radicals a comparison between the Hartree-Fock method, perturbation theory, and density functional theory. J Phys Chem A 1998 102(49) 10142-10150. 

Hie free radical chlorination of three alkanes (cyclohexane, 2,3-dimethylbutane, and neopentane) was examined both in SC-CO2, and, for comparison purposes, in conventional organic solv ts. All of these experiments were carried out at 40 °C, and at alkane concentrations < 0.03 M 20). At such low alkane concentrations, little monochloride arises from reaction of Cl with alkane comprising die cage walls, ku (Scheme 1). 

Stereoselectivity of radical addition is not limited to sulfonyl radicals. The (rani-addition has also been observed for tin, bromine, chlorine, and silyl radicals. Varying degrees of selectivity has been observed for addition of carbon-centered radicals, depending on the substituents size and effect on the inversion barrier. Because the importance of negative hyperconjugation decreases for radicals in comparison to the anions, the barriers for inversion decrease in parallel. As a result, the selectivity can erode under conditions when trapping of the radical is slower than the equilibration, e.g. in the thiol-yne click reaction, which often provide a mixture of E and Z-vinyl sulfides. ... 

A comparison of the transition states during the first propagation step of radical chlorination and radical bromination. 

Atrazine is successively transformed to 2,4,6-trihydroxy-l,3,5-triazine (Pelizzetti et al. 1990) by dealkylation of the alkylamine side chains and hydrolytic displacement of the ring chlorine and amino groups (Figure 1.3). A comparison has been made between direct photolysis and nitrate-mediated hydroxyl radical reactions (Torrents et al. 1997) the rates of the latter were much greater under the conditions of this experiment, and the major difference in the products was the absence of ring hydroxylation with loss of chloride. 

The chlorine atom adds in the gas phase to propadiene (la) with a rate constant that is close to the gas-kinetic limit. According to the data from laser flash photolysis experiments, this step furnishes exclusively the 2-chloroallyl radical (2a) [16, 36], A computational analysis of this reaction indicates that the chlorine atom encounters no detectable energy barrier as it adds either to Ca or to Cp in diene la to furnish chlorinated radical 2a or 3a. A comparison between experimental and computed heats of formation points to a significant thermochemical preference for 2-chloroal-lyl radical formation in this reaction (Scheme 11.2). Due to the exothermicity of both addition steps, intermediates 2a and 3a are formed with considerable excess energy, thus allowing isomerizations of the primary adducts to follow. 

Many of the quantitative rate data on radical reactions which are to be found in the scientific literature have been obtained by comparison of reaction rates rather than by direct measurement of absolute rate constants (Ingold, 1973). For example, it is a straightforward matter to compare the rate of chlorine abstraction from CC14 by phenyl radicals with the rate of hydrogen abstraction from cyclohexane by the same species, simply by comparing the PhCl/PhH product ratio from a suitable competition experiment (Bridger and Russell, 1963). In contrast, direct measurements of the absolute rates of these reactions have yet to be carried out (although indirect estimates are available). 

The authors of primary Reference 80 present their own and selected literature values for the R—NO bond enthalpies for the hydrocarbyl cases of Me, Et, t-Bu, allyl and benzyl, as well as mixed fluorinated, chlorinated methyl radicals. We now wish to compare nitroso species with the corresponding amino and nitro compounds. Choosing what we consider the most reliable and relevant nitroso compound data, and accompanying them with the corresponding radical data, we derive enthalpies of formation of gaseous nitrosomethane, 2-methyl-2-nitrosopropane and o -nitrosotoluene81 to be 65 2, —29 4 and 174 7 kJmol-1. (By comparison, the earlier values recommended8 for nitrosomethane and 2-methyl-2-nitrosopropane were 70 and —42 kJmol-1 respectively.)... 

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. 

Chlorination can be effected by a variety of methods most of which involve free-radical mechanisms (8). In the present work, a chlorinating agent was sought that would preferentially attack the methyl group of polymethylstyrene so that a comparison with polychloromethylstyrene could be made. Both sulfuryl chloride (S02C12) and t-butyl hypochlorite (t-BuOCl) were suitable reagents for this purpose. 

Masten S J, Hoignd J (1992) Comparison of Ozone and Hydroxyl Radical-Induced Oxidation of Chlorinated Hydrocarbons in Water, Ozone Science Engineering 14 197-214. 

Iodine, less reactive than bromine, is best added to alkenes by use of IC1 and IBr (see Section 1.8.3.3). Iodine itself adds rapidly but reversibly to alkenes forming diiodides by mechanisms that can be either ionic or radical. The position of the equilibrium depends upon the structure of the alkene, the solvent and the temperature. Simple vicinal diiodides survive distillation in the dark, but are unstable toward iodine or radicals. In the presence of functions containing free OH groups, such as alumina, HI generated from I2 adds to alkenes irreversibly with the result that the HI adduct, rather than the I2 adduct, is the exclusive product.86 A comparison of the reaction of 1,5-cyclooctadiene with chlorine, bromine and iodine in CH2CI2 reveals that chlorine gas at-50 C gives a 93 7 mixture of trans- and cis-5,6-dichlorocyclooc-... 

Masten, S.J. and Hoigne, J., Comparison of ozone and hydroxyl radical-induced oxidation of chlorinated hydrocarbons in water, Ozone Sci. Eng., 14, 197-213, 1992. 

Comparison of transition states for bromination and chlorination. In the endothermic bromination, the transition state resembles the products (the free radical and HBr). In the exothermic chlorination, the free radical has just begun to form in the transition state, so the transition state resembles the reactants. 

In its relative reactivity toward toluene, ethylbenzene and cumene the more highly substituted 1-methyl-2,2-diphenylcyclopropyl radicaP , derived from the decomposition of the precursor diacyl peroxide, resembles the chlorine radical more than it does the phenyl radical (Table 3). Similarly, comparison of the relative reactivities of primary, secondary and tertiary aliphatic hydrogens toward chlorine atoms (1.0 3.6 4.2) and phenyl radicals (1.0 9.3 44) with the relative reactivities of the C-H bond in the methanol/ethanol/2-propanol series toward the 1-methyl-2,2-diphenylcyclopropyl radical (1.0 2.4 15) further confirms the low selectivity of the cyclopropyl radical. Again, this radical resembles the chlorine atom in its reactivity more than it does the phenyl radical. 

Phenol-ketone novolacs 1487, 1488 Phenol-nitrile complexes 377 Phenol radical cations 1101 fragmentation of 289-291 Phenols—see also Biphenols, Bis-phenols, Hydroxybenzenes, Polyphenols acidities of, gas-phase 310-312 acylation of 629-632, 933, 934 Lewis acid catalyzed 631 montmoriUonite-catalyzed 632 pyridine-catalyzed 631 adsorption of 944 alkylation of 606-629, 941 Brdnsted acid catalyzed 612 Lewis acid catalyzed 607-611 solid acid catalyzed 612-621 stereoselective 621-626 under supercritical conditions 621 as antioxidants 139-143, 840-901 ort/io-substituted 845 thermochemistry of 139, 140, 179 autoxidation of 1118, 1119 bromination of 649-651 jr-cation interaction of 322 chlorination of 649 comparison with isoelectronic methyl, amino and fluoro aromatic derivatives 226... 

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