Fluorine atoms, reaction + hydrocarbons

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... 

Atomic fluorine can be generated by microwave discharges in F2, CF4, or SFg. Chemiluminescence from the reaction of fluorine atoms with hydrocarbons is almost universal and results from the production of vibrationally excited HF via hydrogen abstraction. Although the HF overtone band at 880 nm can be used to detect many species that contain hydrogen, this reaction also generates chemiluminescence from C2 and CH at 470 and 431 nm, respectively, which provides selective detection of hydrocarbons. Other classes of compounds can be selectively monitored. For example, iodo compounds react to produce excited IF, which has been monitored at 580 nm, with reported detection limits of 1 pg. 

Neither fluorine nor iodine undergoes a useful photohalogenation reaction. The reaction of fluorine with carbon-hydrogen bonds is overwhelmingly exothermic and can even occur explosively. Almost no selectivity is possible (Table 11.5). In direct contrast to the reaction of fluorine atoms, reaction of iodine atoms with hydrocarbons is highly endothermic. It is so slow as to be of no synthetic utility. 

The technique of using thermalized F atoms for the study of fluorine atom reactions has proven very useful with unsaturated hydrocarbons and halocarbons, providing data on mechanisms, relative rate constants and factors controlling such reactions. The characteristic difficulties of macroscopic fluorine chemistry are often avoided at tracer levels, and analysis by radio gas chromatography can be quite straightforward. However, e3q>eriments at pressures below 0.1 atmosphere are restively difficult, and most of the usual analytical methods are inapplicable at product mole fractions <10 °. 

This reaction has often reached explosive proportions in the laboratory. Several methods were devised for controlling it between 1940 and 1965. For fluorination of hydrocarbons of low (1—6 carbon atoms) molecular weight at room temperature or below by these methods, yields as high as 80% of perfluorinated products were reported together with partially fluorinated species (9—11). However, fluorination reactions in that eta involving elemental fluorine with complex hydrocarbons at elevated temperatures led to appreciable cleavage of the carbon—carbon bonds and the yields invariably were only a few percent. 

The nonbonding electron clouds of the attached fluorine atoms tend to repel the oncoming fluorine molecules as they approach the carbon skeleton. This reduces the number of effective coUisions, making it possible to increase the total number of coUisions and stiU not accelerate the reaction rate as the reaction proceeds toward completion. This protective sheath of fluorine atoms provides the inertness of Teflon and other fluorocarbons. It also explains the fact that greater success in direct fluorination processes has been reported when the hydrocarbon to be fluorinated had already been partiaUy fluorinated by some other process or was prechlorinated, ie, the protective sheath of halogens reduced the number of reactive coUisions and aUowed reactions to occur without excessive cleavage of carbon—carbon bonds or mnaway exothermic processes. 

In contrast to saturated hydrocarbons, the unsaturated hydrocarbons react with atomic fluorine by two pathways, i.e. (atomic fluorine addition at >C=C< double bond and hydrogen substitution by fluorine atoms. The reaction of fluorine with aromatic hydrocarbons proceeds with the formation of F-derivatives and hydrogen atoms break off ... 

Secondly, it must be understood that each time a hydrogen atom is extracted from a hydrocarbon upon collision with fluorine, the free radical site created is subsequently, capped with fluorine by reaction with a fluorine atom or fluorine molecule. Each such step is 103 kcal mol 1 exothermic. As is well known, AC = — RT log K, thus when one is reacting fluorine with a hydrocarbon containing 10 to 20 hydrogen atoms each reaction adds 103 kcal mol 1. 

Fluorination of Alkanes. Fluorination of alkanes is extremely difficult to control. The reaction usually results in substantial C—C bond rupture and can readily lead to explosion.136 However, several methods for controlled direct radical fluorination of hydrocarbons have been developed. The key and obvious observation was that the only reaction sufficiently exothermic to cause fragmentation is the termination step between a carbon radical and a fluorine atom. Consequently, if the atomic fluorine population and the mobility of hydrocarbon radicals are minimized, controlled fluorination becomes feasible. 

Paramagnetic centers containing a sulfur atom in different oxidation states, (=Si-0)3Si-0-S = O, (=Si-0)3Si-0-S 02, (=Si-0)3Si-0-S02-0, and (=Si-0)3Si-0-S02-0-0, were obtained in Ref. [118]. Their radio-spectroscopic parameters were determined, and the mechanism of free radical oxidation of S02 molecules in this system was established. The mechanism of the initial steps of free radical polymerization and copolymerization of hydrogen- and fluorine-substituted unsaturated hydrocarbons was studied in Ref. [117]. The pathways were found and the kinetic parameters were determined for reactions of intramolecular H(D) atom transfer between r (CH3, CD3, CH2-CH3) and r (CH2-CH2, CD2-CD2), in the structure of (=Si-0)2Si(r)(rI) [120]. 

Fluoroolefins are key compounds playing a fundamental role in synthetic fluoroorganic chemistry. Modification of the properties of the double bond by the introduction of fluorine atoms permits the development of reaction routes that are missing in the hydrocarbon series. This creates excellent opportunities for the syntheses of definite structures,... 

Infrared chemiluminescence measurements [581] of the HF products from the reactions of fluorine atoms with a wide range of pure and halogenated olefinic and aromatic hydrocarbons (see list in Table 7)... 

Simons process — Electrochemical polyfluorination reactions of organic compounds are the only efficient way to industrial production of perfluorinated compounds. The reaction proceeds in the solution of KF in liquid HF (b.p. 19.5 °C), where the starting substances as alcohols, amines, ethers, esters, aliphatic hydrocarbons and halo-hydrocarbons, aromatic and heterocyclic compounds, sulfo- or carboxylic acids are dissolved. During anodic oxidation, splitting of the C-H bonds and saturation of the C=C bonds occur and fluorine atoms are introduced. 

In contrast to the cyclobutane systems, fluorinated cyclopropanes show a lesser thermal stability than the parent hydrocarbon. Trotman-Dickenson et al. (refs. ° )have studied thethermally induced unimolecular isomerisations of partly fluorinated cyclopropanes to mixtures of the corresponding fluoro-propenes. At 450 °C the rate of decomposition of monofluorocyclopropane is about three times that of cyclopropane products formed are 1-fluoropropene (79 %) cis-trans mixture), 2-fluoropropene (9 %) and 3-fluoropropene (11 %). The substitution of further fluorine atoms in the cyclopropane ring further increases the rate of isomerisation ° (see Table 2). Unlike the partly fluorinated cyclopropanes, perfluorocyclopropane does not isomerise to propene compounds but decomposes at 250-300 °C by a first-order reaction to perfluoroethylene . The rate coefficient is expressible as, k = 1.78 x 10 exp (-38,600//JjT) sec , and the decomposition is consistent with the mechanism... 

The highly exothermic cesium fluoride catalyzed isomerization of hexafluorobutadiene (9) via 10 to hexafluoro-2-butyne (11) is another example [5] that shows that the fluorine atom is preferably bonded to the carbon atom through the sp3-hybridized orbital rather than through the sp2 orbital [6]. And here again, Bent s rule plays an important role in the isomerization [7]. The thermal reaction of hexafluoro-1,3-butadiene provides perfluorotricyclo[3.2.0.02,6]octane (15) as a stable final product via sequential intra-and/or intermolecular cycloaddition at 200°C [8]. In contrast, the parent hydrocarbon (16)... 

The reactions of pentafluorobenzoylpyruvic acid with N-nucleophiles primarily involve the a-carbonyl group however, intramolecular cyclization replacing the ort/zo-fluorine atom in the aromatic ring is also possible, giving new types of heterocycles. This kind of cyclization is not characteristic of the nonfluorinated hydrocarbon analogs. 

The gas-phase chemistry reviewed in this chapter shows how the pattern of substitution of chlorine and fluorine atoms on the parent hydrocarbon affects its atmospheric degradation. Although numerous reaction details need to be investigated and some discrepancies resolved, a core of reliable data is available from which one can deduce which HFC or HCFC may have deleterious atmospheric consequences and which potential substitute appears environmentally acceptable. However, the work is not finished. This work has dealt only with the gas-phase chemistry. Research is still needed into the heterogeneous chemistry of HFC and HCFC degradation products and into the biological effects that they may have. It is then up to the atmospheric modelers to combine this scientific information with emission scenarios and meterology to ascertain the feasibility of a particular CFC substitute. 

Reactions of perfluorinated alkenes, such as hexafluoropropene, with fluoride ion give perfluoroalkylcarbanions which can act as nucleophiles in S Ar reactions with perfluoroheteroaromatic systems (Fig. 8.13). These reactions are another example of mirror-image chemistry and reflect well-known Friedel-Crafts reactions of hydrocarbon systems that proceed by reaction of the corresponding electrophile and carbocationic intermediates. Poly substitution processes are possible and, indeed, all five fluorine atoms may be replaced upon reaction with an excess of tetrafluoroethylene and fluoride ion. ... 

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