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Radical fluorination is a type of fluorination reaction, complementary to nucleophilic and electrophilic approaches. [1] It involves the reaction of an independently generated carbon-centered radical with an atomic fluorine source and yields an organofluorine compound .
Organofluorine compounds are prepared by numerous routes, depending on the degree and regiochemistry of fluorination sought and the nature of the precursors. The direct fluorination of hydrocarbons with F 2, often diluted with N 2, is useful for highly fluorinated compounds: R 3 CH + F 2 → R 3 CF + HF
In reductive hydrodefluorination the fluorocarbon is reduced in a series of single electron transfer steps through the radical anion, the radical and the anion with ultimate loss of a fluorine anion. An example is the conversion of pentafluorobenzoic acid to 3,4,5-tetrafluorobenzoic acid in a reaction of zinc dust in aqueous ammonia.
Radical fluorination with the pure element is difficult to control and highly exothermic; care must be taken to prevent an explosion or a runaway reaction. With chlorine the reaction is moderate to fast; with bromine, slow and requires intense UV irradiation ; and with iodine, it is practically nonexistent and thermodynamically unfavored.
Electrophilic fluorinating reagents could in principle operate by electron transfer pathways or an S N 2 attack at fluorine. This distinction has not been decided. [2] By using a charge-spin separated probe, [3] it was possible to show that the electrophilic fluorination of stilbenes with Selectfluor proceeds through an SET/fluorine atom transfer mechanism.
Fluorination of lactones can provide heterocyclic fluorides, although ring opening has been observed for γ-butyrolactone. The six-membered lactide does not experience ring opening. [8] Fluorination opens epoxides to give either geminal or vicinal difluorides in most cases. Monoarylepoxides give geminal products with migration of the aryl group.
The reaction typically involves free radical pathways. The regiochemistry of the halogenation of alkanes is largely determined by the relative weakness of the C–H bonds. This trend is reflected by the faster reaction at tertiary and secondary positions. Free radical chlorination is used for the industrial production of some solvents: [2]
The traditional Balz–Schiemann reaction employs HBF 4 and involves isolation of the diazonium salt. Both aspects can be profitably modified. Other counterions have been used in place of tetrafluoroborates, such as hexafluorophosphates (PF 6 −) and hexafluoroantimonates (SbF 6 −) with improved yields for some substrates.