Fluorine atoms, abstraction

The reactions of fluorine atoms with hydrocarbons are similar to those of active nitrogen in that they provide an essentially universal response. Fluorine atoms abstract H atoms from hydrocarbons at near-collisional reaction rates. Reactions with fluorine are highly exothermic, forming strong H—F (=570 kJ mol-1) and C—F (=485 kJ mol-1) bonds while breaking much weaker C—H (=414 kJ mor1) and C—C (=368 kJ mol-1) bonds. The hydrogen abstraction reaction... 

The low energy collisions of Si+ with perfluorohexane have been studied in the search for analogies with the ion-surface reactions at fluorinated alkane monolayer surfaces. A fluorine atom abstraction product was observed, SiF+, in analogy to the corresponding ion/surface reaction, though its abundance was relatively low with respect to fluorocarbon fragment ions39. 

The experimental investigations by Fagerstrom et al have shown that at a total pressure of 1000 mbar of SF6, the reaction is still in the fall-off regime. The fluorine atoms abstraction reactions, produced by radiolysis of the gas mixture containing SF6 in great excess, can initiate the H-abstraction reactions... 

The reactions of F( P) atoms have been studied kinetically in a discharge-flow apparatus at 297 K. The major reaction channel for the F-hICl reaction forms IF + Cl, although a minor process yields CIF + I. Fluorine atoms react with CO and Xe slowly to yield COF2 and XeFj, respectively, but no reaction was detected with either He or Kr. Fluorine atoms abstracted hydrogen from HOF, generating OF radicals. The mechanisms of the reactions of F and OF radicals with NOF have been investigated by Schumacher et al. Quantum yields were shown to be... 

DFT calculations for the activation of fluoromethane and fluorobenzene at Ce and Ho have been performed revealing a harpoon-type mechanism for the metal-mediated fluoride abstraction.The reactions of the lanthanoid cations La Ce, Pr, and Yb with fluoromethane have also been investigated by DFT calculations. After coordination of Ln, electron transfer from Ln to fluoromethane takes place, followed by formation of LnF and a methyl radical. Mass spectrometric investigations which show gas-phase collisions of Nb with hexafluoro- and tetrafluorobenzene have been reported. The reaetions involve the transfer of four fluorine atoms to Nb. A systematic study on the reactivity of transition metal eations toward hexafluorobenzene in the gas phase has been performed by inductively coupled plasma (ICP) tandem mass spectrometry.Multiple fluorine atom abstraction dominates the chemistry observed for early transition metals, while association of hexafluorobenzene to the metal cation has been found for late transition metals. 

The gaseous products of the "Co y-radiolysis (in Pyrex ampoules) of liquid trifluoroacetic acid at 23 C and of the polycrystalline acid at - 196°C have been identified (CgF, CHFg, COg, CO, Ha, and CgF, CHF3, COg, respectively) and their C-values determined. Failure to detect the formation of carbon tetrafluoride, tetrafluoroethylene, or silicon tetrafluoride was taken as circumstantial evidence for the non-occurrence of reactions involving fluorine-atom abstraction by trifluoromethyl radical or the generation of... 

FIGURE 4.9 Main-chain scission process initiated by fluorine atom abstraction by hydrogen radical, proposed by Corns. (Redrawn from Corns, F. D. 2008. ECS Transactions 16 235-255.)... 

The rate of combination of trifluoromethyl radicals to form hexafluoro-ethane has been measured by the flash photolysis of trifluoromethyl iodide coupled with rapid-scan i.r. spectroscopy in the absence of an inert diluent (At, N2, or COt) carbon tetrafluoride and tetrafluoroethylene were also formed, presumably via fluorine atom abstraction from trifluoroiodomethane by hot trifluoromethyl radicals (c/. ref. 27). Photolysis of trifluoroiodomethane has been used in studies on (i) the direction of radical attack on 1,3,3,3-tetrafluoropropene [- CFj CHI CHFCF, (75%) + (CFa)jCH CHFl (25%)] (ii) the rates of hydrogen abstraction from ammonia, ammonia-ethylene oxide, silane, trimethylsilane, tetramethylsilane, and cycloalkanes and (iiQ the competitive addition of the CFj- radical to ethylene and vinylidene fluoride. Radicals formed by photolysis of the fluoroalkyl iodides CFJ, C FJ, n-C,F,I, (CF,)jCFI, (CFaljCHI, (CFalaCDI, (CFa)jCClI, and (CFa)iCPhI (the last was synthesized by treatment of CFs CPh CTj with CsF and iodine in DMF) have been... 

The fluorination reaction is best described as a radical-chain process involving fluorine atoms (19) and hydrogen abstraction as the initiation step. If the molecule contains unsaturation, addition of fluorine also takes place (17). Gomplete fluorination of complex molecules can be conducted using this method (see Fluorine compounds, organic-direct fluorination). 

Consider abstraction of a hydrogen atom from propan( by fluorine atom. This can generate either of two propy radicals, depending on which hydrogen is attacked. 

It is believed that polymer surface fluorination proceeds via a free radical mechanism, where fluorine abstracts hydrogen atoms from the hydrocarbon, and fluorine atoms are substituted.11 Of course, the precise conditions depend on the nature of the polymer in question and the surface properties required. 

To probe the effects of HCFC structure on toxicity the metabolism of three penta-haloethanes, HCFC-123, HCFC-124, and HCFC-125 were studied. The three compounds differ one from the other by the number of fluorine atoms present in the /3-carbon (Fig. 4.64). It was found that the enthalpies of activation, AHact, for hydrogen atom abstraction paralleled the rate of trifluoroacetic acid excretion suggesting that the more difficult it was... 

For the formation of indoles 2 and 3 we assumed that amines 1 first reacted with fBuLi at -110°C to give N-(2-lithioallyl)amines 4, through halogen-metal exchange 23 this was confirmed by deuteration and isolation of the deuteriated amine 5 a (Scheme 2). Intermediate 4, which is stable for several hours at —110 °C, probably undergoes proton-abstraction ortho to the fluorine atom by the additional equivalents of fBuLi when the temperature is raised to — 40 °C giving the intermediate 6. The subsequent elimination of LiF produces a benzyne intermediate 7, which is efficiently... 

There are no abstractable hydrogen atoms on CF30, so that reaction with 02 is negligible (k < 4 X 10-17 cm3 molecule-1 s-1 at 373 K Turnipseed et al., 1994 Niki et al., 1994). In addition, the C-F bond is too strong for decomposition to give a fluorine atom and CFzO to take place. However, CF3Q reacts with NO,... 

One type of isosteric substitution that should have tremendous potential in the design of safer commercial chemicals is replacement of hydrogen with fluorine. Since many bioactivation mechanisms of chemicals involve cytochrome P450-mediated hydrogen atom abstraction to yield toxic metabolites, it would seem plausible that replacement of such a hydrogen atom with fluorine would provide a safer isosteric analog, without affecting commercial efficacy. 

This has been found to be the case with valproic acid (2-propylpentanoic acid) (12), which is hepatotoxic, and 2-fluorovalproic add (22), which is much less hepatotoxic, discussed in Sedion 4.3.1. The hepatoxidty of 12 involves cytochrome P450 abstraction of its C-2 hydrogen atom. The C-2 fluorine atom of 22 cannot be removed by cytochrome P450 metabolism. It would be interesting to observe if the same isosteric replacement would reduce the hepatoxidty of other carboxylic acids, such as the widely used 2-ethylhexanoic add. 

The methyl and trifluoromethyl radicals formed in the photolysis are assumed to abstract hydrogen, but not fluorine, atoms from the parent ketone. 

No thermal reactions were observed at temperatures up to 350 °C. and there was no evidence for the abstraction of fluorine atoms. 

All aspects of the structure, reactivity and chemistry of fluorine-containing, carbon-based free radicals in solution are presented. The influence of fluorine substituents on the structure, the stability and the electronegativity of free radicals is discussed. The methods of generation of fluorinated radicals are summarized. A critical analysis of the reactivities of perfluoro-n-alkyl, branched chain perfluoroalkyl and partially-fluorinated free radicals towards alkene addition, H-atom abstraction, and towards intramolecular rearrangement reactions is presented. Lastly, a summary of the synthetically-useful chemistry of fluorinated radicals is presented. 

The trend of reactivities which is observed for hydrogen atom abstraction by partially fluorinated radicals is qualitatively similar to that for their addition to styrene. However, the absolute rates and the range of reactivities for each type of process can be seen to differ significantly. Thus, absolute rate constants for... 

Due to the multiple desorption products, the etching of the surface with halogen appears to be quite complex. A multi-step reaction mechanism has been suggested to account for the SiCl2 desorption species. In the case of fluorine atom adsorption, F atom abstraction and dissociative chemisorption mechanisms have been suggested. In order to account for the complex surface reactions, more studies are needed. 

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