Halogenation of Alkanes
Radical Halogenation of Methane
The reactions of fluorine, chlorine, bromine, and iodine with methane are quite differently vigorous. Fluorine is the most reactive. If no precautions are taken, a mixture of fluorine and methane explodes. The reaction between methane and chlorine is easily controllable, while bromine is even less reactive than chlorine. Iodine, on the other hand, does not react with methane.
The reaction entropy of methane halogenation is approximately zero, since two molecules of gaseous products are formed from two molecules of gaseous starting products in the reaction. Therefore, the thermodynamics of methane halogenation are, first of all, determined by the reaction enthalpy (ΔH°). The activation energy of methane halogenation is equivalent to the dissociation energy of the respective halogen, as the halogenation is a gas-phase reaction with a homolytic bond breakage. The dissociation energies of all halogens are known. Therefore, the kinetics of methane halogenation can be illustrated effortlessly.
Fluorination (155 kJ/mol) seems to have relatively high activation energy. The initial reaction (chain initiation) - that is, the homolytic cleavage of a halogen molecule - must, however, occur only a few times. The subsequent reactions (chain propagation) between a halogen radical and methane, and then between a methyl radical and a halogen molecule, yield another halogen radical. Therefore, one start reaction may initiate thousands of fluorination reactions. In addition, fluorination is very exothermic, the reaction enthalpy is -431 kJ/mol. As a result, the reaction itself provides enough energy for additional initiation reactions.
The activation energy of a chain reaction must not be too high. Otherwise, the reactive radicals formed by the initiation reaction will recombine rather than chain propagation will happen. In the case of methane fluorination, activation energies of the reactions of chain propagation are small (see also "Early and late transition states"). Therefore, they occur often enough in the reaction mixture, even if placed only at room temperature. In addition, chain propagation is extremely exothermic. The reaction heat cannot be eliminated from the reaction mixture quickly enough with the consequence that the temperature and thus the reaction rate steadily increase. As a result, an explosion occurs. Nevertheless, methane fluorination may be carried out in a controlled reaction, so as to prohibit an explosion. Diluting the starting products with an inert gas or absorbing the reaction heat with copper granulate can help in this case.
As a result of higher activation energy in chain initiation as well as a less exothermic character (ΔH° = -115 kJ/mol) of the chain propagation, the reaction rate of methane chlorination is comparatively lower than that of the fluorination. Therefore, the reaction of methane chlorination is easier to control.
In the chain initiation of methane bromination, the activation energy is lower than that in chlorination. However, the chain propagation is far less exothermic, and the first reaction of the chain propagation is even much more endothermic (+75 kJ/mol) than in the case of chlorination (+8 kJ/mol). Therefore, the chain propagation proceeds extremely slowly, even at 300 °C, and bromine is by far less reactive than chlorine against methane.
In the chain initiation of methane iodination, the activation energy is even lower than it is in fluorination. Therefore, one could assume that methane iodination runs more rapidly than fluorination. However, this is not the case! The complete chain propagation (+54 kJ/mol), and, in particular, the first reaction (+142 kJ/mol), is very endothermic. As a result, the radical iodination of methane does not take place.