Chlorination bromination, radical-catalyzed

Ozone is formed by the photolytic decomposition of NO2 yielding oxygen radicals and by the reaction sequence NO2 — HNO3 — NO3 O3. In particular, the reaction of NO2 with hydroxyl radicals to form HNO3 increases ozone concentration because two radicals, NO2- and -OH, which catalyze ozone decomposition, are removed. Other radicals are also important for ozone destruction in the stratosphere, especially chlorine oxides see Chlorine, Bromine, Iodine, Astatine Inorganic Chemistif). The mechanism of ozone destruction is complicated as there many compounds involved. Chlorine nitrate and dinitrogen pentoxide can act as reservoir species for CIO-, NO2-, and NO3- radicals. 

Radical-catalyzed chlorination and bromination occur readily on alkanes and substituted alkanes, both in the gas phase and in solution. Both thermal and photochemical generation of the halogen atoms are employed for chlorinations, sulfuryl chloride in the presence of an initiator such as dibenzoyl peroxide is also used (Scheme 4.17). 

Michael Faraday reported in 1821 that chlorine addition to alkenes is Stimulated by sunlightand today this is taken to indicate the involvement of a free radical process (equation 26). Free radical chain mechanisms were proposed in 1927 by Berthoud and Beraneck for the isomerization of stilbene catalyzed by Br2 (equation 27), and by Wachholtz for bromine addition to ethyl maleate (equation 28).Later studies showed inhibition of halogen addition by reaction of the intermediate radicals with oxygen, and a free radical chain mechanism for solution and gas phase halogenations as in equation (26) was shown (equation 29). Kinetic and mechanistic... 

For electrophilic and more bulky radicals, for example CnHal2n+i, selectivity increases dramatically. Whereas the selectivity for the radical chlorination (Table 1, entry 7) and bromination (Table 1, entry 8) of adamantane with Hal2 in CC14 is only approximately 5, for chlorinations with CC14 (Table 1, entries 9 and 10) and brominations with BrCCl3 (Table 1, entry 11) the selectivity increases to 21.5-27. Halogenations under metal-catalyzed GIF conditions (Table 1, entry 12) or in the presence of dioxirane (Table 1, entry 13) are less selective, because of the participation of oxygen-centered (GIF) or methyl (DMD) radicals. 

From a mechanistic point of view, two different ionic mechanisms have to be considered (due to the presence of oxygen the radical chain mechanism plays no role in the technical process) first, the uncatalyzed reaction of ethylene and chlorine and second, the metal halide catalyzed reaction. Both routes compete in this process. The uncatalyzed halogenation was studied extensively for the bromina-tion of olefins [14, 15] (Scheme 4). It is commonly accepted that the halogenation of olefins starts with formation of a 1 1 -complex of halogen and alkene followed by formation of a bromonium ion. Subsequent nucleophilic attack of a bromine anion leads to the dibromoalkane. However, when highly hindered olefins (such as tetraneopentylethylene) are used, formation of a 2 1 r-complex, as an intermediate between 1 1 ir-complex and a bromonium ion, is detectable by UV spectroscopy. In the catalyzed reaction the metal halide polarizes the chlorine bond, thus leading to formation of a chloronium or carbonium ion. Subsequent nucleophilic attack of a chloride anion gives the dichloroalkane [12] (Scheme 5). 

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... 

There are in principle three possibilities for reaction of halogens with aromatic hydrocarbons, namely, addition, substitution in the nucleus, and substitution in a side chain. The last of these is discussed on pages 152 and 157. Substitution of benzene by chlorine or bromine is an ionic reaction,114 whereas photochemical or peroxide-catalyzed addition of these halogens involves a radical-chain mechanism.115 Substitution in the side chain also proceeds by a radical mechanism,116 addition rather than side-chain substitution being favored by higher chlorine concentrations.115... 

A significant fraction of the removal is caused by the presence of chemical radicals X, such as nitric oxide (NO), chlorine (Cl), bromine (Br), hydrogen (H) or hydroxyl (OH), which serve to catalyze reaction (5.145), termed cycle 1 ... 

---