Fluorine, elemental reactions with

Replacement of Hydrogen. Three methods of substitution of a hydrogen atom by fluorine are (/) reaction of a G—H bond with elemental fluorine (direct fluorination, (2) reaction of a G—H bond with a high valence state metal fluoride like Agp2 or GoF, and (J) electrochemical fluorination in which the reaction occurs at the anode of a cell containing a source of fluoride, usually HF. 

Thus, for a successful fluorination process involving elemental fluorine, the number of coUisions must be drasticaUy reduced in the initial stages the rate of fluorination must be slow enough to aUow relaxation processes to occur and a heat sink must be provided to remove the reaction heat. Most direct fluorination reactions with organic compounds are performed at or near room temperature unless reaction rates are so fast that excessive fragmentation, charring, or decomposition occurs and a much lower temperature is desirable. 

The reaction of trifluoromethyl radicals, generated in a radio-frequency discharge process, with elemental mercury f/7i], mercury halides [174], dime-thylmercuiy [24], or HgO [175] has been used for the preparation of CFjHgX and (CF3)2Hg. Direct fluorination of dimethylmercury with elemental fluorine gives (CF3)2Hg [176],... 

Fluorine reacts spontaneously with almost all elements, hydrogen, water vapor, and many organic compounds. Steel will melt and ignite in fluorine with a violent reaction. 

Fluorine is the most reactive of all elements, in part because of the weakness of the F—F bond (B.E. F—F = 153 kj/mol), but mostly because it is such a powerful oxidizing agent (E ed = +2.889 V). Fluorine combines with every element in the periodic table except He and Ne. With a few metals, it forms a surface film of metal fluoride, which adheres tightly enough to prevent further reaction. This is the case with nickel, where the product is NiF2. Fluorine gas is ordinarily stored in containers made of a nickel alloy, such as stainless steel (Fe, Cr, Ni) or Monel (Ni, Cu). Fluorine also reacts with many compounds including water, which is oxidized to a mixture of 02> 03> H202, and OF2. 

This element can never be controlled. Fluorine reacts violently with solid methane at -190°C. With liquid hydrocarbons at -210°C, the reaction is dangerous. All hydrocarbons react dangerously, from the first homologues to anthracene as well as lubricants. In the gaseous state, there is ignition with small quantities, and detonation with large quantities and when the mixture is made quickly. There is immediate detonation with alkenes and alkynes. With benzene, when fluorine is incorporated bubble by bubble and at a low temperature, this causes ignition on the surface. If the flow rate is substantial, there is immediate detonation. 

When heated in air at 800°C AS4S4 vapors begin to dissociate to AS2S2 which then ignites to form arsenic oxides. Ignition in chlorine produces arsenic chloride. Reaction with fluorine forms arsenic trifluoride. It is stable in water and also in the air at ambient temperatures. It does not react with hot concentrated HCl but is decomposed by nitric acid. It forms thioarsenite ion, AsS3 and elemental arsenic when warmed with caustic soda solution. Similar reaction occurs with sodium sulfide. 

Fluorine also reacts with other halogens, forming interhalogen compounds. While with bromine and iodine it reacts vigorously at ordinary temperatures, with chlorine the reaction occurs at 200°C. Such interhalogen products with these halogens include iodine heptafluoride, bromine trifluoride, bromine pentafluoride, and chlorine trifluoride. Metalloid elements, such as arsenic, silicon, selenium, and boron also inflame in a stream of fluorine, forming fluorides. 

Thallium burns in fluorine with incandescence. Reactions with other halogens form halides. Thallium combines with several elements forming binary compounds. 

Elemental fluorine can be used to replace relatively electron rich C-H bonds by C-F. In particular, tertiary hydrogen atoms which are remote from electron withdrawing substituents can be selectively replaced by fluorine and, where there is more than one tertiary hydrogen atom in the substrate, that with the higher electron density is replaced (Fig. 44). In contrast, where there is an electron withdrawing group close to tertiary hydrogen, very little reaction with fluorine takes place and, consequently, when one fluorine atom has been introduced into a molecule further reaction is often inhibited. 

However, the proponents of the Radical Mechanism would argue that, whatever the actual agent, the fact remains that the process provides a means of generating fluorine at a rate, and in a form which is conducive to controlled reaction with organic compounds, probably constrained on a favourable, possibly catalytic [189] heat-sink surface, in such a way as to overcome the two principal problems inherent in all elemental fluorinations [190], namely... 

In passing, it should be noted that the reaction between elemental fluorine and hydrocarbons (Step lb in Table 1) is possibly one of the fastest reactions from a kinetic point of view known in any field of reaction chemistry. Studies of the reaction with fluorine to extract a hydrogen from a hydrocarbon or partially fluorinated hydrocarbon in the gas phase found that the activation energy is between 0 and 1 cal mol 1 (not kilocalories). This reaction has been studied independently, with two studies leading to a zero activation energy and one study to a 1 cal mol 1 value. 

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