20.19: Radical Halogenation: Thermodynamics
The thermodynamic favorability of a reaction is determined by the change in Gibbs free energy (ΔG). ΔG has two components- enthalpy (ΔH) and entropy (ΔS). The entropy component is negligible for alkane halogenation because the number of reactants and product molecules are equal. In this case, the ΔG is governed only by the enthalpy component. The most crucial factor that determines ΔH is the strength of the bonds. ΔH can be determined by comparing the energy between bonds broken and bonds formed.
Based on the thermodynamics of the reaction, radical halogenation of alkanes has a different order of reactivity for fluorination, bromination, and iodination. The ΔH for radical iodination is positive (+55 kJ/mol), which suggests that the ΔG value is also positive for this reaction. Therefore, iodination is thermodynamically unfavorable, and the reaction does not take place. On the other hand, the overall ΔH for the radical fluorination of methane is large and negative (-431 kJ/mol), making the reaction thermodynamically favorable but highly exothermic and not having any practical use. The ΔH value for chlorination and bromination is -104 kJ/mol and -33 kJ/mol, respectively, making these reactions thermodynamically favorable and practically feasible. The reaction rate comparison between chlorination and bromination shows that bromination is slower than chlorination. The rate-determining step for this reaction is the first propagation step or the hydrogen abstraction step. The first propagation step for chlorination reaction is exothermic, and the energy of activation is small, while for bromination, this step is endothermic, and the energy of activation is large, which explains why bromination is slower than chlorination.