Aromatic side chains, free-radical

Titov claims that the free radical mechanism applies for nitration of aliphatic hydrocarbons, of aromatic side chains, of olefins, and of aromatic ring carbons, if irf the latter case the nitrating agent is ca 60—70% nitric acid that is free of nitrous acid, or even more dil acid if oxides of nitrogen are present... 

Reaction 1 has been postulated both in oxidations of alkanes in the vapor phase (29) and in the anti-Markovnikov addition of hydrogen bromide to olefins in the liquid phase (14). Reaction 2 involves the established mechanism for free-radical bromination of aromatic side chains (2). Reaction 4 as part of the propagation step, established in earlier work without bromine radicals (26), was not invoked by Ravens, because of the absence of [RCH3] in the rate equation. Equations 4 to 6, in which Reaction 6 was rate-determining, were replaced by Ravens by the reaction of peroxy radical with Co2+ ... 

The most important reactions of the alkylbenzenes are outlined below, with toluene anti ethylbenzene as specific examples essentially the same behavior is shown by compounds bearing other side chains. Except for hydrogenation and oxidation, these reactions involve either electrophilic substitution in the aromatic ring or free-radical substitution in the aliphatic side chain. 

The copper center is situated on the surface of domain 2, close to its sevenfold axis. It is surrounded by a multitude of aromatic side-chains, many of which probably participate in the generation or stabilization of the free radical [30]. The copper ion is coordinated by four internal and a single external ligands the O from Tyr 272, N< 2 from His 496, His 581 and an acetate ion almost form a square around the copper ion (Table 8). The O from Tyr 495 is the fifth, axial ligand and is located furthest from the central ion (Fig. 23), indicating a relatively weak coordination of this ligand [159]. 

Radicals derived from monocyclic substituted aromatic hydrocarbons and having the free valence at a ring atom (numbered 1) are named phenyl (for benzene as parent, since benzyl is used for the radical C5H5CH2—), cumenyl, mesityl, tolyl, and xylyl. All other radicals are named as substituted phenyl radicals. For radicals having a single free valence in the side chain, these trivial names are retained ... 

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... 

The newly formed free radical may terminate by abstraction of a hydrogen atom, or it may continue cracking to give ethylene and a free radical. Aromatic compounds with side chains are usually dealkylated. The produced free radicals further crack to yield more olefins. 

Titov and co-workers, although conceding the validity of the ionic nitration mechanism for liq phase nitrations with coned acids, believe that many nitrations occur via a free-radical mechanism involving the free radicals (at any rate molecules having an unpaired electron) N02, N03, and NO. For vapor phase nitration of hydrocarbons, nitration of side chains of aromatic compds in... 

In its original form the Hammett equation was appropriate for use with para and meta substituted compounds where the reaction site is separated from the aromatic group by a nonconjugating side chain. Although there have been several extensions and modifications that permit the use of the Hammett equation beyond these limitations, it is not appropriate for use with ortho substituted compounds, since steric effects are likely to be significant with such species. The results obtained using free radical reactions are often poor, and the correlation is more appropriate for use with ionic reactions. For a detailed discussion of the Hammett equation and its extensions, consult the texts by Hammett (37), Amis and Hinton (12), and Johnson (47). 

Figure 8-10 shows another pair of reactions for the halogenation of an aromatic compound. The reaction of the side chain is a free-radical substitution. Figure 8-11 shows the mechanism of this free-radical substitution. 

Multivalent radicals of aromatic hydrocarbons with the free valences in the side chain are named in accordance with Rule A-4. Examples ... 

Irradiation of powdered titanium dioxide suspended in solutions containing aromatic compounds and water under oxygen has recently been shown to induce hydroxylation of aromatic nuclei giving phenolic compounds and oxidation of side chains of the aromatic compounds (50-55). These reactions have been assumed to proceed through hydroxyl and other radical intermediates, but the mechanism for their generation, whether reactive free radicals result from oxidation of water, from reduction of oxygen, or from oxidation of the substrates on the surfaces of the excited titanium dioxide, has not been clear. 

An attempt was made to reveal the mechanism for the formation of free radicals upon irradiation of titanium dioxide in the presence of benzene and toluene. Careful examination of the effects of oxygen and water showed that the presence of oxygen is essential for the reaction, and that under oxygen the oxidation of water contributes to the aromatic hydroxylation and the oxidation of toluene as a substrate leads to oxidation of its side chain (56). 

Chlorination.1 C120 is particularly useful for side-chain chlorination of deactivated aromatic substrates by a free-radical process involving CIO . Although it cun be used to convert methylarenes to mono- and dichloromethyl derivatives, it is particularly useful for conversion to trichloromethylarenes. Thus it converts p-nitrotoluene into p-nitrobenzotrichloride in 99.8% yield. It is also useful for selective replacement of benzylic hydrogens. 

Two-electron redox phenomena are rare for Cu, and therefore Cu is not a suitable catalyst for epoxidations, for example. However, Cu is useful for free radical reactions such as deep oxidation of organic molecules in waste streams and ring or side chain oxidation of aromatic compounds. 

The chlorination of alkyl aromatics by sulfuryl chloride promoted by free-radical initiators, which was originally discovered by Kharasch and Brown990, can be modified by incorporation of transition metal complexes. Matsumoto and coworkers have observed that, upon addition of Pd(PPh3)4, in place of a radical initiator, the side-chain monochlorination of toluene is substantially more selective991. Davis and his colleagues992 have extended this study and report that Pt(0) and Pd(0) are effective initiators for side-chain chlorination of toluene by sulfuryl chloride and dichlorine. Mn, Re, Mo and Fe complexes, on the other hand, behave more like Friedel-Crafts catalysts. Gas-phase chlorination of olefins to allyl chlorides is catalyzed by PdCl2 or by PtCl2993. 

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