Radical Substitution Reactions
Radical Substitution - Mechanism
The radical chlorination of methane is a considerably exothermic reaction; its reaction enthalpy ΔH0 is approximately -104 kJ/mol. Nevertheless, this does not necessarily signify that the reaction visibly runs at room temperature, as the reaction rate is controlled by the activation energy EA and not by the reaction enthalpy. At room temperature, however, the reaction can be initiated by irradiation with ultraviolet light. This behaviour may be explained by the reaction mechanism.
The radical chlorination of methane is a chain reaction that consists of three individual steps: the chain initiation, the chain propagation, and the chain termination (see animation). In this reaction, a C-H bond of methane and a Cl-Cl bond of chlorine are broken, while a C-Cl bond and an H-Cl bond are formed. The results are the formation of the products chloromethane and hydrogen chloride.
Energetical characteristics in the radical chlorination of methane
Thus, because of the energy balance of the reaction, the bonds formed by this reaction contain 104.6 kJ/mol less energy than those that were cleaved during the process. Therefore, the reaction is very exothermic.
In the initiation step of the chain reaction, the weakest bond in the starting products, the Cl-Cl bond (ΔHDiss = 242.4 kJ/mol), is homolytically cleaved either by ultraviolet light or by heat (T > 300 °C). As a result, chlorine radicals are formed.
In the first chain propagation step, the chlorine radical then abstracts a hydrogen atom from a methane molecule, yielding hydrogen chloride and a methyl radical. A C-H bond of methane (ΔHDiss = 438.9 kJ/mol) is cleaved and an H-Cl bond (ΔHDiss = 430.5 kJ/mol) is formed in this reaction step. Thus, since the energy balance of the reaction is (-430.5 kJ/mol)-(-38.9 kJ/mol) = + 8.4 kJ/mol, the reaction step is endothermic. However, it is just slightly endothermic. As a result, enough product is available in equilibrium in order to keep the chain reaction running. In addition, the methyl radical, which is one product of the first propagation step, is continuously consumed by the second propagation step of the chain reaction. Thus, it is eliminated from the equilibrium of the first propagation step. As a result, the first propagation step continuously proceeds towards equilibrium.
The energy diagram of the chain reaction shows that the activation energy of the first endothermic propagation step is EA = 16.7 kJ/mol. In contrast, the second chain propagation step, namely the formation of chloromethane, has a lower activation energy (less than 4.2 kJ/mol) and is very exothermic. The reaction enthalpy of this step can be approximately calculated using the dissociation energies of the bonds involved: ΔH02nd step = ΔHDiss,Cl-Cl - ΔHDiss,methyl-Cl = 242.4 kJ/mol - 355.3 kJ/mol = -112.9 kJ/mol. Therefore, the second propagation step is the motive force of the chain reaction. The energy balance of the chain propagation at all thus amounts to -112.9 kJ/mol + 8.4 kJ/mol = -104.5 kJ/mol.
The critical step of the radical chlorination of methane is the abstraction of a hydrogen atom from methane by a chlorine radical (EA = 16.7 kJ/mol). This is illustrated by the interaction of the molecular orbitals. In the transition state of this reaction step, the hydrogen atom is partially bound to the carbon of methane, as well as to the chlorine atom. See also the animation of orbital interactions for the whole process of the chlorination of methane below.
The chlorination of methane follows a radical chain mechanism. If the reaction is initiated once, it may proceed for a while by the continuous repetition of the chain propagation cycle. A termination of the chain reaction occurs, for example, when two radicals recombine by forming a covalent bond and are thus eliminated from the cycle.
The biggest disadvantage of the radical chlorination of methane is the low selectivity of the reaction. As mentioned above, the reaction is not cut off at the monochloromethane stage. Instead, it yields a product mixture of mono-, di-, tri, and tetrachlorinated products - that is, monochloromethane, dichloromethane, chloroform, and carbon tetrachloride. This is a result of the fact that the remaining C-H bonds are weakened with increasing degree of chlorination. Therefore, additional radical chlorinations are preferred. The problem of multiple chlorination can be avoided by the high-dilution principle - that is, by the application of a large excess of methane.