Fluorination radical chain

Right) Luminescence lifetimes of tris and ternary Nd111 complexes as a function of the fluorinated radical chain length. Filled and opened triangles stand for tris complexes with benzoylacetonate and thienylacetonate derivatives, respectively filled and opened circles for the corresponding ternary complexes formed with phen. 

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). 

The reaction with fluorine occurs spontaneously and explosively, even in the dark at low temperatures. This hydrogen—fluorine reaction is of interest in rocket propellant systems (99—102) (see Explosives and propellants, propellants). The reactions with chlorine and bromine are radical-chain reactions initiated by heat or radiation (103—105). The hydrogen-iodine reaction can be carried out thermally or catalyticaHy (106). 

Fluorine reacts explosively by a radical chain reaction as soon as the gases are mixed. A mixture of hydrogen and chlorine explodes when exposed to light. Bromine and iodine react with hydrogen much more slowly. A less hazardous laboratory source of the hydrogen halides is the action of a nonvolatile acid on a metal halide, as in... 

In addition to oxidation, many other reactions occur as free radical chain reactions polymerization, decomposition, fluorination, chlorination, etc. All chain reactions have a few important general peculiarities [1—3]. 

It is assumed that all similar fluorination reactions proceed via an intricate radical chain-reaction mechanism. The overall reactions for the substitution of hydrogen by fluorine (RH + F2 - RF + HF, AH298 -430 kJ/mol per carbon atom) are more exothermic than the reactions for adding fluorine to the double bonds... 

Since fluorine is less than 1 % dissociated at room temperature, the concentration of fluorine atoms may not be sufficient to initiate a radical chain process. An alternative initiation step lb (Table 1), originally suggested by Miller [31-33], probably occurs but conclusive evidence for this pathway has not been established. 

Hadley and Bigelow fluorinated methane with fluorine/nitrogen mixtures in the vapor phase over copper gauze. All four fluoromethanes were obtained, together with hexafluoroethane and octafluoropropane. As an explanation, the first specific free-radical chain mechanism to be published for a fluorination process was invoked. 

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 this section, each method of generating carbon-based fluorinated radicals will be introduced and discussed in terms of mechanism, and it will be seen that virtually all of the useful methods for generating perfluoro and partially fluorinated radicals in a practical manner involve well defined and controlled free radical chain reactions. Some representative examples which demonstrate the preparative use of these methodologies will be presented in the final section of this review. 

This similarity in reactivities probably derives from a fortuitous cancellation of substituent effects in 11. Fluorination increases chain stiffness and creates an unfavorable polarity mismatch between an electrophilic radical and an electron-poor double bond, but this is offset by the significant decrease of 7r-bond energy in 11. The vinyl ether 12 analog cyclizes about seven times faster than 11, which is consistent with the known lower 7r-bond energy and higher free-radical reactivity of perfluorovinyl ethers vs perfluoroalkenes [142]. 

The chlorination of methylchlorosilanes is realised by free chlorine in the liquid or gaseous phase. The reaction takes place by the radical chain mechanism its initiators (substances capable of generating free radicals) can be peroxides (benzoyl, kumyl, etc.), the dinitrile of 2,2 -azobis(isobutyric) acid, fluorine, 60Co y-rays, as well as UV rays. 

The trifluoromethyl group is the most prominent fluorinated side chain. An excellent review on all aspects of the introduction of the trifluoromethyl group into organic compounds is available (92T6555). The trifluoromethyl group can be introduced as radical, nucleophilic and electrophilic species as well as by functional group transformations. 

The functionalization of polymers is another useful feature of the hydrosilation reaction. The introduction of highly fluorinated alkyl chains by the hydrosilation of Si-H groups of the polymer with fluorinated aUcenes is a typical example. Poly(phenylsilanes) obtained by the dehydrocouphng polymerization of phenylsilane undergo AIBN initiated free radical hydrosilation with aUcenes, ketones and aldehydes. Cross-linking and branching of polymers can also be easily accomplished using hydrosilation. 

Reactions between hydrocarbons and elemental fluorine are extremely exothermic because of the high heats of formation of bonds from fluorine to carbon and hydrogen (approximately 456 and 560kJmol , respectively) [27, 111]. The value of A// for the dissociation of fluorine is very low ca. 157kJmol ), so it is frequently assumed that the preferred fluorination process proceeds by a radical chain mechanism (Figure 2.20), although this may not always be the case. 

Also, it has been suggested that dimerisation of haloalkenes observed during the interaction with fluorine at low temperatures (< —50° C) arises by a free-radical chain mechanism initiated in this way [115]. 

This plastic is a partially fluorinated straight-chain polymer with a very high molecular weight. It is produced by free-radical polymerization mechanism in a solvent or a hybrid (a solvent/aqueous mixture) media, using an organic peroxide initiator. Copolymerization of tetrafluoroethylene and ethylene (CH2=CH2, molecular weight 28, CAS number 74-85-1) proceeds by an addition mechanism. 

On the laboratory scale, the tendency of elemental fluorine to initiate radical chain reactions resulting in tar formation can be controlled by appropriate choice of solvent. The solvent system CFCI3/CHCI3, sometimes with additional 10% ethanol, serves as an effective radical scavenger. The reaction enthalpy is controlled by dilution of the substrate in this solvent, by dilution of the fluorine gas with nitrogen or helium, and by use of a low reaction temperature. Under these conditions, the selective fluorination of cyclohexane derivatives in the tertiary axial position is possible in reasonable yields [16] (Scheme 2.4), supposedly by an electrophilic mechanism. 

As early as the 1940s Emeleus and Haszeldine [17] discovered that perfluoroalkyl iodides are not only cleaved into perfluoroalkyl radicals by light but also that they add readily to a variety of olefins to yield telomers and 1 1 adducts [18]. This kind of radical chain reaction can also be initiated by high temperatures (Scheme 2.100). The addition of perfluoroalkyl iodides to olefins is a very important method for synthesis of partially fluorinated alkanes, polymers, oligomers, and their derivatives [19]. The synthesis of some perfluoroalkyl aromatic compounds can also be achieved [20]. 

Scheme 2.106 Examples of the addition of alkyl radicals to highly fluorinated olefins (Rfh = CF2CHFCF3) (beloiv), and the mechanism ofthe radical chain reaction (above) [32a].

Another, related type of reaction is the halodifluoromethylation of nucleophiles by dihalodifluoromethanes (e.g. CF2Br2) [9]. This reaction is always initiated by a single electron transfer from the nucleophile to the CF2XY species (X and Y denote halogens other than fluorine). The subsequent fate of the resulting radical ion pair depends on the ability of the nucleophile to form a stabilized radical, and also on the choice of solvent [10]. For phenoxides [4a, 5, 11] and thiophenoxides [4c, Ila] a reaction pathway via difluorocarbene is usually preferred whereas enamines and ynamines are halodifluoromethylated by a radical chain mechanism (see also Section 2.2.1) [12] (Scheme 2.169). 

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