Alkylaromatics, free-radical bromination

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

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. 

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)-... 

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

---