Mechanism free radical direct fluorination

Scheme 2.1 Gas-phase perfluorination of a variety of hydrocarbons by the LaMar process. At the top the proposed mechanism of free radical direct fluorination of alkanes is shown [8].

Mechanism of fluorination in this process is In situ reaction with fluorine generation, that is, organic substrate of hydrocarbon reacts with free radical of fluorine and so, this process is classified in direct fluorination. 

Direct fluorination of aromatic rings is so exothermic that a tarry mixture of products is obtained. Reaction of benzene with the xenon fluorides, XeF2 or XeF4, does give fluorobenzene, but the mechanism is probably free radical rather than polar.137... 

The presence of a three-spin effect can also be revealed by triple irradiation experiments, where both the electron and the proton spin transitions are strongly irradiated. This has the effect of removing the indirect pathway for the transfer of spin polarization from the electron to the fluorine via the proton, leaving only the direct electron—fluorine mechanism. Any additional proton—fluorine Overhauser effect due to the saturation of the proton signal is negligible in solution containing free radicals. Figure 23 shows the results of such a triple-irradiation... 

In direct fluorination of powdered high-density polyethylene with the gas, diluted with helium or nitrogen, the accompanying exotherm causes partial fusion. In addition, there is some destruction of the crystalline regions [137]. On the other hand, fluorination of single crystals of polyethylene can result in fluorine atoms being placed on the carbon skeleton without disruption of the crystal structure. The extent of cross-linking, however, is hard to assess [138]. The reaction has all the characteristics of free-radical mechanism [139] ... 

The mechanism for the fluorination of benzene and its derivatives with xenon difluoride in the presence of hydrogen fluoride has been discussed independently in more detail, but similar conclusions about the participation of radical-cations in the early stages of the reaction have been reached. However, the effects of added chlorine and of hydrogen chloride on fluori-nations by this method indicate that free radical-cations are involved only in the formation of biaiyls, the halogenobenzenes themselves being formed directly from a complex of the substrate and fluorinating agent. 

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