Fluorine reaction with alkanes

The reactivity of the halogens decreases in the order F2 > CI2 > Br2 > I2. Fluorine is an extremely aggressive oxidizing agent, and its reaction with alkanes is strongly exothennic and difficult to control. Direct fiuorination of alkanes requires special equipment and techniques, is not a reaction of general applicability, and will not be discussed further. 

Chlorination of alkanes is less exothermic than fluorination, and bromination less exothermic than chlorination. Iodine is unique among the halogens in that its reaction with alkanes is endothermic and alkyl iodides are never prepared by iodination of alkanes. 

Fluorine, being much more reactive than chlorine, is even less selective than chlorine. Because the energy of activation for the abstraction of any type of hydrogen by a fluorine atom is low, there is very little difference in the rate at which a 1°, 2°, or 3° hydrogen reacts with fluorine. Reactions of alkanes with fluorine give (almost) the distribution of products that we would expect if all of the hydrogens of the alkane were equally reactive. 

Halogenation of alkanes is common with Br2 and Clj. Fluorine, Fj, is seldom used because its reactions with alkanes are so exothermic that they are difficult to control and can actually cause C—C bond cleavage and even explosions. Iodine, Ij, is seldom used because the reaction is endothermic and the position of equilibrium favors alkane and I2 rather than iodoalkane and HI. 

Reaction of 1,1,1-trifluoroethane with fluorinated ethylenes requires pure antimony pentafluoride as a catalyst. Reaction with TFE gives a monoalkylation product in high yield addition to CH2=CF2 results in formation of a branched alkane 61 [7]... 

Alkanes do not usually react with xenon difluoride at room temperature, while thermally initiated fluorinations of organic molecules have received much less attention than liquid-phase reactions. Zajc and Zupan16 have shown that several hydrocarbons react with xenon difluoride when heated to 95-120 °C. Reproducible results can be observed only when a teflon jacket is used in the stainless reactor with appropriate preconditioning. Cyclohexane is converted to fluorocyclohexane, while the reaction with -hexane gives three monofluoro-substituted products. Fluorination of adamantane results in the formation of four products, from whose distribution it is evident that the difference in the reactivity between the secondary and the tertiary carbon atom is much larger than the difference between the reactivities of the primary and secondary carbon atoms of hexane. Liquid-phase functionalization of a tertiary carbon atom is observed in reactions in carbon disulfide, where 1-fluoroadamantane is formed17. 

Heat or light is usually needed to initiate this halogenation. Reactions of alkanes with chlorine and bromine proceed at moderate rates and are easily controlled. Reactions with fluorine are often too fast to control, however. Iodine reacts very slowly or not at all. We will discuss the halogenation of alkanes in Chapter 4. 

In this reaction, X can be fluorine, chlorine, or bromine, but not iodine. Iodine does not react well with alkanes. A common use of a halogented hydrocarbon is shown in Figure 23-3. 

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