Alkylaromatic bromination

As was noted by Jones (ref. 12) the success of a metal bromide as a catalyst for alkylaromatic autoxidations depends on the ability of the metal to transfer rapidly and efficiently oxidizing power from various autoxidation intermediates onto bromide ion in a manner which generates Br-. The fact that no free bromine is observable in this system is consistent with rapid reaction of intermediate bromine atoms with the substrate. Inhibition of the reaction by cupric salts can be explained by the rapid removal of Br2 or ArCH2- via one-electron oxidation by Cu (Fig. 10). 

The bromination of meta-nitrotoluene is an example for a high-temperature, high-pressure (high p,T) side-chain bromination of alkylaromatics [25]. 

The rate of oxidation of cyclohexane by Co(III) acetate in acetic acid is enhanced in the presence of bromide ions.265 By analogy with alkylaromatic oxidations (see Section II.B.3.b), these reactions probably involve chain transfer by bromine atoms [cf. Eqs. (20l)-(204)]. 

A further useful application of SC-CO2 as a reaction medium is the free-radical side-chain bromination of alkylaromatics, replacing conventional solvents such as tetra-chloromethane or chlorofluorohydrocarbons having no abstractable hydrogen atoms [920]. For example, bromination of ethylbenzene in SC-CO2 at 40 °C and 22.9 MPa yields 95 cmol/mol (1-bromoethyl)benzene, with practically the same regioselectivity as obtained in conventional tetrachloromethane as the solvent. Even the classical Wohl-Ziegler bromination of benzylic or allylic substrates using A-bromosuccinimide (NBS) can be conducted in SC-CO2 [920]. Irradiation of a solution of toluene, NBS, and AIBN (as initiator) in SC-CO2 at 40 °C and 17.0 MPa for 4 hours gave (bromomethyl)-... 

The success of the metal bromide catalysts in alkylaromatic autoxidation resides mainly in their ability to transfer oxidizing power from the various oxidation intermediates to bromide ions to produce bromine radicals like Br and Br2" finally (see eqs. (7) - (10)) [16]. 

Chlorination and bromination in the side chain of alkylaromatic compounds requires exclusion of the catalysts of ionic halogenation, i.e., above all of metal salts. Thus it is recommended to add substances that form stable complexes with metal salts, e.g., ethylenedinitrilotetraacetic acid (EDTA).417 Side-chain halogenation requires conditions under which radicals can be formed, illumination (100-200 household lamps or UV), high temperatures, and addition of radical formers. Discoloration occurring during side-chain chlorination, which inhibits light absorption, can be avoided by adding 0.1-5% of benzamide.418... 

For ethane at 293 K Ig Iti = 7.6 dm mob s . Chlorination of alkanes also occurs if solutions of metal chlorides (for example, AuClC or PtCb see [17]) are irradiated in the presence of an alkane. Bromination of alkylaromatic hydro-... 

The mechanism via bromine atoms is supported by molecular bromine formation in the interaction of with Br in the absence of a hydrocarbon (Bf2 is apparently formed by bromine atom recombination). This mechanism is also consistent with the fact that bromide ions, while catalyzing the oxidation in the case of alkylaromatic compounds, are not particularly effective in the case of simple alkanes. This corresponds to the difference of bromine atom reactivity with respect to alkylaromatic and aliphatic hydrocarbons. The bond energy in the H-Br molecule (85 kcal mole ) is practically equal to the energy of the C-H bond in the n.-position to the aromatic ring, so that the reaction... 

Alkylaromatic compounds Chlorinated hydrocarbons Brominated hydrocarbons lodinated hydrocarbons H2O... 

Fundamentally, Br s role in Co - Br and Co - Mn - Br catalysis is that it catalyzes the oxidation of alkylaromatics by Co(III) and Mn(III). Since the discovery of Co-Br catalysis, it has been postulated that Br is formed from Br" added initially as a salt (CoBr2, NaBr) or HBr [35, 36, 38]. Organic bromides could also serve as bromine sources, but in order to be active, the organic bromide must be converted into inorganic bromide [8, 38]. Thus, there are two questions related to the nature of the bromine species in Co-Mn-Br catalysis what is the major catalytically active form of bromide and what is the active bromine radical species ... 

If the active bromine species is indeed the Br , a critical question must be asked can HBrJ abstract benzylic hydrogens This question was addressed by Metelski and Espenson in a study where second-order rate constants between Br and RH were measured for a variety of alkylaromatics, ranging from monosubstituted monoaromatics to polysubstituted naphthalenes [31]. 

This project began with our study of the free radical bromination of alkylaromatics in SC-CO2 with the purpose of addressing the following issues ... 

Free radical brominations in conventional solvent. An appropriate quantity of alkylaromatic and 10 mL solvent were placed in a 30-mL Pyrex pressure tube (equipped with an 0-ringed Teflon needle valve and Teflon-coated magnetic stir bar). An appropriate HBr scavenger (1,2-epoxybutane) was added, and the solution was degassed 4x by the freeze-pump-thaw PT) method. Bromine was FPT degassed and distilled (via vacuum line) into the reaction mixture at -196 C. The pressure tube was sealed and the reaction mixture allowed to equilibrate at the desired reaction temperature in total darkness. The reaction mixture was irradiated with a 400 W medium-pressure mercury arc lamp at a distance of 2 ft through two Pyrex layers. Complete discharge of Br2 occurred in less than 5 min. Afterward, the solution was analyzed by GLC vs an appropriate internal standard. 

Bromination of Alkylaromatics in SC-CO2. Product Yields. The bromination of toluene in SC-CO2 proceeded smoothly as indicated in equation 6. The major reaction product, formed in >70% yield, was benzyl bromide accompanied by a small amount (10 - 20%) of / -bromotoluene (resulting from a competing electrophilic aromatic substitution process). 

Adiponitrile, waste resourcing, 146,148 Alkylaromatics, free-radical bromination, 103-104... 

We have already seen in this chapter that we can substitute bromine and chlorine for hydrogen atoms on the benzene ring of toluene and other alkylaromatic compounds using electrophilic aromatic substitution reactions. We can also substitute bromine and chlorine for hydrogen atoms on the benzylic carbons of alkyl side chains by radical reactions in the presence of heat, light, or a radical initiator like a peroxide, as we first saw in Chapter 10, (Section 10.9). This is made possible by the special stability of the benzylic radical intermediate (Section 15.12A). For example, benzylic chlorination of toluene takes place in the gas phase at 400-600 °C or in the presence of UV light, as shown here. Multiple substitutions occur with an excess of chlorine. 

We have already seen that we can substitute bromine and chlorine for hydrogen atoms on the ring of tolnene and other alkylaromatic compounds using electrophilic aromatic substitution reactions. Chlorine and bromine can also be made to replace hydrogen atoms that are on a benzylic carbon, such as the methyl group of toluene. 

JM Tanko, JF Blackert. Free-radical side-chain bromination of alkylaromatics in supercritical carbon dioxide. Science 263 203-205, 1994. 

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