Electrochemical fluorination mechanism

Electrochemical fluorination in anhydrous hydrogen fluoride (Simons process) involves electrolysis of organic compounds (ahphatic hydrocarbons, haloalkanes, acid halides, esters, ethers, amines) at nickel electrodes. It leads mostly to perfluori-nated compounds, but is accompanied to a high extent by cleavage and rearrangement reactions. The mechanism of the formation of carbocations according to Eq. (1) and Scheme 1 is assumed... 

Thus, it is beyond doubt that the fluorination reaction proceeds via an association between the organic substrate (or its protonated derivative) and the electrochemically generated nickel fluoride(s) on the anode surface. However, since the precise nature of this association, and the exact species involved, are not known, the next stage in the mechanism of fluorine substitution is not apparent and it has generally been the practice to make inferences from studies of product analyses. 

One postulated reaction mechanism for electrochemical fluorination involves an intermediate nickel fluoride, with nickel in the oxidation stage +III/ + IV, as the active fluorination agent. The induction period in which the nickel fluoride layer is formed at the nickel surface can thus be explained. A radical fluorination mechanism has also been postulated, with oxidation of the fluoride anion to the radical, or as discussed below in the ECEC mechanism.15 The mechanism of this process is still a matter for debate. Reference should be made to a report that does not support the postulates of this section.21 For partial electrochemical fluorination, the ECEC mechanism is postulated as follows. In the first step the starting material is oxidized at the anode (E = electrochemical step). 

Electrochemical fluorination 168,169> is a commercial process for perfluorina-tion of aliphatic compounds. The reaction is performed in liquid hydrogen fluoride -potassium fluoride at a nickel anode. The mechanism is not known free fluorine cannot be detected during electrolysis, so it seems probable that fluorination is a direct electrochemical reaction. Theoretically, hydrogen fluoride-potassium fluoride should be a very oxidation-resistant SSE, and it might well be that the mechanism is analogous to that proposed for anodic acetamidation of aliphatic compounds in acetonitrile-tetrabutylammonium hexafluorophosphate 44 K... 

The yield of the electrochemical fluorination can reportedly be improved by the addition of butadiene sulfones. ° This additive is highly soluble in anhydrous HF and increases the conductivity of the electrolyte solution. Butadiene sulfone itself is fluorinated to perfluorobutanesulfonyl fluoride and, therefore, needs to be added continuously to the reaction. The use of this additive increases the yield of perfluorooctanesulfonyl fluoride by -10% It may act by trapping radicals formed at the anode, but the mechanism by which it increases the yield of the electrochemical fluorination of long-chain sulfonic acid derivatives is unknown. 

Radical-cations may also play a role in HF-catalysed XeFj fluorination of aromatic substrates (see p. 352) and in Simons-type electrochemical fluorination, e.g. see Scheme 6. However, both Italian and Russian groups working on the mechanism of electrochemical fluorination believe that anodic complexes of the type first proposed by Burdon and Tatlow are involved the latter group, following its analysis of-anode deposits and... 

Sartori P, Ignat ev N (1980) The actual stale of our knowledge about mechanism of electrochemical fluorination in anhydrous hydrogen fluoride (Simtuis process). J Fluorine Chem 15 231... 

Selective Electrochemical Fluorination, Scheme 6 Reaction mechanism for electrochemical fluorination of sulfldes... 

The mechanism of electrochemical fluorination is still incompletely understood [6,3138-41]. Fluorination is believed to occur by fluorine adsorbed on the nickel fluoride layer formed on the anode surface while hydrogen is liberated on the cathode ... 

The three principal electrochemical methods are described by which fluorine can be directly introduced into organic compounds, namely electrolysis in molten salts or fluoride ion solutions, electrolysis in molten potassium fluoride/hydrogen fluoride melts at porous anodes, and electrolysis in anhydrous hydrogen fluoride at nickel anodes. Using examples from the past decade, it is aimed to demonstrate that electrofluorination in its various forms has proved to be an increasingly versatile tool in the repertoire of the synthetic chemist. Each method is described in terms of its essential characteristics, reaction parameters, synthetic utility, advantages and disadvantages, patent protection, and potential for commercial exploitation. The different mechanisms proposed to explain each process are critically reviewed. 

Fluorine is produced by electrolysis of molten salts on carbon anodes including KF-21TF at about 100°C, potassium bifluoride at about 250°C, and fluoride salts at about 1000°C. The decomposition potential of molten potassium bifluoride is 1.75 V at 250°C, a value close to that estimated thermodynamically [80]. The kinetics of the anodic process is characterized by a Tafel slope of 0.56 V per decade, j), = 1 x 10 A/cm [81], and by a complex reaction mechanism involving the formation of fluorine atoms on carbon. During the electrolysis, C-F surface compounds on the carbon anode are formed via side reactions. Intercalation compounds such as (CF) contribute to the anodic effect in the electrochemical cell, which can be made less harmful by addition of LiF. 

Electrochemical (anodic) fluorinations can be carried out, but may be difficult to control and over-fluorination and/or fluorination of substituents often results. The mechanism involves conversion of the substrates into radical cations, which are then trapped by fluoride, rather than electrophilic fluorination. Again, the method is more suited to robust systems such as pyrimidinones and purines. 

Among the various applications suggested for hyperbranched polymers are surface modification, additives, tougheners for epoxy-based composites, coatings, and medicines. It has been demonstrated that hydrophobic, fluorinated, hyperbranched poly(acrylic acid) films can passivate and block electrochemical reactions on metal surfaces thus preventing surface corrosion (Bruening, 1997). The lack of mechanical strength makes hyperbranched polymers more suitable as ad-... 

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