![]() But before we discuss why that’s the case, let’s start by looking at the mechanism of the radical halogenation. Notice, that although it may look like you’re just “adding” a halogen to your molecule when we use the skeletal structures, in reality, we’re replacing one of the hydrogens that was in that position.Īlso, while we can potentially get all the products I have here for this example, in reality we are going to have one major product, a couple of minor ones, and some traces of for the rest. Also, we’re producing the hydrogen halide as a side product in this reaction.Īnd here’s a slightly more complicated example, where we have multiple potential places for the halogen to substitute the H. In each case, we replace one of the hydrogens in our alkane with the corresponding halogen. Here are a couple of simple examples of this reaction. ![]() We’ll go over the intricacies of the mechanism, how to find the major products in this reaction, and discuss the most important points of each mechanistic step. ![]() Diluting the starting products with an inert gas or absorbing the reaction heat with copper granulate can help in this case.In this tutorial, we are going to talk about the radical halogenation of alkanes. Nevertheless, methane fluorination may be carried out in a controlled reaction, so as to prohibit an explosion. 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. In addition, chain propagation is extremely exothermic. Therefore, they occur often enough in the reaction mixture, even if placed only at room temperature. In the case of methane fluorination, activation energies of the reactions of chain propagation are small (see also "Early and late transition states"). ![]() Otherwise, the reactive radicals formed by the initiation reaction will recombine rather than chain propagation will happen. The activation energy of a chain reaction must not be too high. As a result, the reaction itself provides enough energy for additional initiation reactions. In addition, fluorination is very exothermic, the reaction enthalpy is -431 kJ/mol. Therefore, one start reaction may initiate thousands of fluorination reactions. 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. The initial reaction (chain initiation) - that is, the homolytic cleavage of a halogen molecule - must, however, occur only a few times. Therefore, the kinetics of methane halogenation can be illustrated effortlessly.įluorination (155 kJ/mol) seems to have relatively high activation energy. The dissociation energies of all halogens are known. 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. Therefore, the thermodynamics of methane halogenation are, first of all, determined by the reaction enthalpy (ΔH°). 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. Iodine, on the other hand, does not react with methane. The reaction between methane and chlorine is easily controllable, while bromine is even less reactive than chlorine. If no precautions are taken, a mixture of fluorine and methane explodes. The reactions of fluorine, chlorine, bromine, and iodine with methane are quite differently vigorous.
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