Reactivity Order of Halogens
When comparing the enthalpies of the propagation stages of different halogens, significant differences in the exchanged heats are observed. In the case of fluorine, both steps are exothermic (including hydrogen abstraction) with an overall energy balance of -103 Kcal/mol. The activation energies of the transition states in this reaction are very low, making it the most reactive halogen.
At the other end of reactivity is iodine, whose reaction is endothermic and does not take place. Reactivity order: F2 > Cl2 > Br2 > I2
Heats Exchanged in the Propagation Stages
The following table shows the heats exchanged in the propagation stages for the halogenation of methane.
The reaction is strongly exothermic in the case of fluorine (most reactive halogen) while in the case of iodine it is endothermic by 13 Kcal/mol.
