Aromatic-halogen bond formation

I > Br > Cl > F. In nucleophilic aromatic substitution, the formation of the addition intermediate is usually the rate-determining step so the ease of C—X bond breaking does not affeet the rate. When this is the ease, the order of reactivity is often F > Cl > Br > I. This order is the result of the polar effeet of the halogen. The stronger bond dipoles assoeiated with the more eleetronegative halogens favor the addition step and thus inerease the overall rate of reaetion. 

We should point out, however, that depending on the relative importance of the various reactions, kohs may not be a simple function of pH and temperature, and that product formation may strongly depend on these two variables. Furthermore, we note that many environmentally important organic compounds exhibit halogen atoms bound to a carbon-carbon double bond, be it an olefinic (e.g., chlorinated ethenes) or an aromatic (e.g., chlorinated benzenes, PCBs) system. In many cases, under environmental conditions, these carbon-halogen bonds undergo SN or E reactions at extremely slow rates, and we therefore may consider these reactions to be unimportant. 

Intermolecular photoreaction of an aryl halide with another aromatic compound may lead to the formation of biaryls. In this section several examples of such reactions will be discussed. In some cases, information concerning the reaction mechanism is available but the depth to which mechanisms have been investigated varies greatly. In many cases aryl radicals formed by homolysis of the carbon-halogen bond are the reactive species. Such radicals may also be produced via electron transfer, followed by departure of halide anion. In some cases aryl cations have been proposed as intermediates. Intermolecular bond formation may also be preceded by charge transfer within an exciplex or by formation of radical ion pairs. 

The aryl group participation process is mechanistically a Friedel-Crafts alkylation. The r value of the Friedel-Crafts alkylation by alkyl carbocations was found to be significantly lower than that of protonation or of halogenation of aromatic substrates (Yukawa et al., 1966). Olah interpreted the low r value in terms of an earlier transition state, i.e. less advanced aryl C bond formation with r = 0.6 at the transition state prior to formation of the... 

Fe(III) salts are known to oxidise electron-rich centres to foster the formation of radical species. They are particularly efficient in the oxidation of aromatic systems or a carbanion to the corresponding carbon-centred radical which undergoes C-C bond formation to yield the coupled products. For a successful synthesis, it is important to work in the absence of reactive synthetic molecules other than those which form the combination of radicals. Barton et al. used a simple water-soluble diselenide derivative that shows radical scavenger properties towards alkyl and hydroxyl radicals in Fenton-type chemistry (Fe2+-H202)4 The reaction rate between the produced alkyl radical and the diselenide overwhelms self-termination and halogen transfer reactions. The ability of diselenide to scavenge alkyl and hydroxyl radicals [ 3(0 °C) = 6.1 x 108 M-1 s-1] could be exploited as a new tool in both synthetic and mechanistic work conducted in aqueous media (Scheme 8.5).4... 

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