Radical brominations chlorination comparison

Selected examples of such oxidations of straight-chain aliphatic compounds are listed in Table 1. The remarkable monohalogenation and co-1 selectivities in these reactions have been attributed to polar, steric, and conformational effects. The yields of monohalogenated derivatives, based on converted (60-80"/)) substrate, are generally quantitative. Comparison of these selectivities with a number of oxidations using bromine, chlorine or rerf-butoxy radical chain carriers shows that aminium radical-mediated oxidation is far superior to others for synthetic, indeed industrial, applications [28]. The high selectivity and clean monooxidation displayed by the aminium radical chain process has been referred as an enzyme-mimetic reaction ... 

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. 

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-... 

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. 

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... 

When one compares the selectivity of Br and Cl, the more reactive species (Cl) is clearly the less selective one. For example, bromine atoms abstract tertiary H-atoms 17(X) times faster than the primary H-atom. By comparison, chlorine radical is only 7 times faster. In fact, the electronic factor m for those two radicals has almost identical values (m close to 2) and RSP is obeyed. When changes in reactivity are not due only to changes in reaction energy (A °, A/7° or AG°), the postulates of physical organic chemistry are no longer verified. For example, Br and CHj have almost the same reactivity with respect to CH4. According to RSP one would expect that both radicals should have identical selectivities for H-atom abstractions. Nevertheless, Br is much more selective, because the electrophilicity... 

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