Radical addition fluorine atom

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). 

Etchant species (for example, fluorine atoms) diffuse to the surface of the material and adsorb onto a surface site. It has been suggested (20) that free radicals have fairly large sticking coefficients compared with relatively inert molecules such as CF4, so adsorption occurs easily. In addition, it is generally assumed (20) that a free radical will chemisorb and react with a solid surface. Further, surface diffusion of the adsorbed species or of the product molecule can occur. 

Oxidation of 44 is completely inhibited by the addition of a small amount of 2,4,6-tri-f-butylphenol. Consequently, the oxidation of the Si—Si bond with oxygen displays the following features 1) a Si—Si bond which is either angle-strained or substituted with more than two fluorine atoms is easily oxidized 2) a radical mechanism is operative 3) the insertion of oxygen into the Si—Si bond proceeds stereospecifically . Incorporation of molecular oxygen was also observed in the direct photolysis of the cyclopolysilanes 80 and 81 (equations 88 and 89) . ... 

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. 

In order to determine whether the enhanced reactivities of perfluoroalkyl radicals could be attributed to some linear combination of the individual contributions of fluorine atoms on the a-carbon (the radical center), the /1-carbon and the y-carbon atom, the absolute rates of addition of a number of parfza/Zy-fluorinated alkyl radicals to a-methylstyrene, styrene, and pentafluorostyrene were determined by LFP [70]. The data in Table 9 clearly indicated that this is not the case, that y- and /1-fluorinated-n-alkyl radicals exhibit little enhancement, while a-difluoroalkyl radicals, although more reactive, remain very much less-reactive than analogous perfluoro species. 

The relative rate of addition of atoms and radicals to methyl and fluorine substituted ethylenes... 

Theoretical studies have indicated that m-bcnzync is monocyclic with a C(l)-C(3) distance of 2.0 A whereas in tetrafluoro-w-benzyne the increased eclipsing strain between fluorine atoms stabilizes the bicyclo[3.1.0]hexatriene form with a C(l)-C(3) distance of 1.75 A.56 Computational studies coupled with gas-phase experimental studies show that appropriate substituents can be used to tune the reactivity of 1,3-arynes. Thus the presence of NH+ at C(5) makes (13) mildly carbocationic whereas the addition of OH at C(4) in (14) gives a highly reactive (bi)radical.57... 

Owing to the high electronegativity of fluorine atoms, perfluoroalkyl radicals show an electrophilic character moreover, these radicals are usually much more reactive than the nucleophilic alkyl radicals in the addition to alkenes, aromatic rings, and quinones for enthalpic reasons (Scheme 14.5b). 

Electron transfer from copper or copper salts to alkyl halides has been used to initiate atom transfer radical additions. One modification of this process involves catalytic amounts of copper powder and fluorinated alkyl iodides the radicals so generated may react in either inter- or intramolecular fashion with alkenes (equation 13)19. Alternatively, a-chloroesters with remote alkene functions undergo cyclization in the presence of cat-... 

In the case of a free-radical addition of hydrogen bromide, a type of addition limited to hydrogen bromide only, the attacking species is not proton, but a bromine atom that will join the carbon in such a way as to generate a more stable free-radical species. It will add to the carbon carrying two fluorines. In the intermediate free radical, the single electron is better accommodated at the carbon holding chlorine, because chlorine can disperse the electrons in its d-orbitals. 

Stereodectronic Effects. The conformation of a ring effects the reactivity of cyclic ethers and we have already described the evidence for such processes in the literature 10). Here, however, we demonstrate that stereoelectronic effects may influence the reactivity of amines in free radical processes. Using a series of cyclic amines [15], radical addition to a fluorinated alkene may occur but then a choice is possible (a) the intermediate radical could react with amine to give the 1 1 adduct [16] (ki) or (b), a 1,5-shift of hydrogen can occur (k2) which ultimately leads to the 2 1 adduct [171. We anticipate that k2 would vary little with the amine but, of course, ki will vary with the ease of hydrogen—atom abstraction from the amine. Correspondingly, we find that the ratio of the 2 1 adduct [17] increases substantially in the series... 

The mechanism of the fluorination has not yet been fuUy elucidated. However, radical intermediates are presumed. Normally, the fluorine atoms are found in close proximity, and the addition preferably advances not along the axis, but along the circumference of the tube. This results in domains with a high degree of fluorination existing side by side with regions that are at best scarcely attacked by fluorine. A possible explanation for this effect is based on the assumption of a... 

The major atmospheric sources of FCO are expected to be the photolysis of CFjO and FCICO however, with the input of alternative halocarbons such as HCF-134a, an additional source of FCO is carbonyl monofluoride (HFCO). Another source is the reaction of fluorine atoms with atmospheric CO. As aforementioned, the photolysis of CFjO produces FCO with quantum yields that range from 0.47 to 1. Reaction of HFCO with HO radicals, F, and Cl atoms are also sources of FCO radicals. 

The irradiation of alcohols, acids, amides, fluorocarbons, and other compounds has provided other radicals for study. Work on single crystals oriented in magnetic fields yielded the spectroscopic data for the perfluorosuccinate, XX, and the perfluoropropionamide, XXVI (51,56). The study of unoriented materials has yielded additional information concerning the coupling behavior of a-and /3-fluorine atoms in perfluoromethylcyclohexyl radical, XXVII, hydroxyalkyl derivatives, XXVIII, the perfluoroglutarate, XXIX (57,58) as well as the 2,2,2-trifluoroethyl radical (59) and other related substances (54). 

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