Free radicals alkylaromatics

The scope of oxidation chemistry is enormous and embraces a wide range of reactions and processes. This article provides a brief introduction to the homogeneous free-radical oxidations of paraffinic and alkylaromatic hydrocarbons. Heterogeneous catalysis, biochemical and hiomimetic oxidations, oxidations of unsaturates, anodic oxidations, etc, even if used to illustrate specific points, are arbitrarily outside the purview of this article. There are, even so, many unifying features among these areas. 

Alkanes can be simultaneously chlorinated and chlorosulfonated. This commercially useful reaction has been appHed to polyethylene (201—203). Aromatics can be chlorinated on the ring, and in the presence of a free-radical initiator alkylaromatic compounds can be chlorinated selectively in the side chain. King chlorination can be selective. A patent shows chlorination of 2,5-di- to 2,4,5-trichlorophenoxyacetic acid free of the toxic... 

A question which inevitably arises on surveying the enormous sucess of the Amoco catalyst is why the combination Co/Mn/Br in acetic acid In order to answer this question we must first examine the mechanism of free radical chain autoxidations of alkylaromatics (ref. 4). 

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)-... 

Therefore the catalysis of the oxidation of the alkylbenzenes to the corresponding aldehydes is kept alive by the formation of an excess of Co ", formed by the oxidation of the aldehydes with oxygen. In general, oxidation intermediates like aromatic aldehydes and peroxides, which are normally more reactive than the corresponding toluenes, can regenerate highly oxidized metal species. Besides the free-radical mechanism stoichiometric and ionic reaction pathways also play an important role in the oxidation of alkylaromatic compounds. This is shown with Co " as oxidant on the left-hand side of Scheme 2. 

Reaction of N-chloramines with aromatics [(153)-(155)] has been illustrated most recently with alkylaromatics by Clerici et al. (1974). Amination of aromatics is an electrophilic rather than free-radical... 

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. 

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

Tanko JM, Blackert JF. Free-radical side-chain hromination of alkylaromatics in supercritical carbon dioxide (SC-CO2). Science 1994 263(5144) 203—5. 

A complete mechanism for the autoxidation of alkylaromatic hydrocarbons by cobalt(n) in acetic acid has not been established,25 6 although a complex rate law has been determined for tetralin. 22 The reaction most likely proceeds by a fiiee radical chain mechanism in which the purpose of the cobalt ions is to provide a hi h steady state concentration of free radicals by catalysis of the decomposition of THP. The free radical nature of the autoxidation of tettalin with the colloidal CoPy catalysts is supported by experiments which showed inhibition of the reaction by 2,6-di-rerr-butylphenol and 2,6-di-rm-butyl-4-methylphenol, and by a shortening of the induction period and increase of the reaction rate when azobis(isobut nitrile) was added to the reaction mixture as a free radical initiator. 

Thermodynamically cleavage of a C-H bond in an alkylaromatic radical cation is strongly favored over C-C fragmentation due to the much higher solvation free energy of the proton as compared to a carbocation. However, the former process is characterized by significantly higher intrinsic barriers (0.5-0.6 eV for C-H... 

---