20.17 : 自由基卤化：热力学

Radical halogenation of alkanes follows a different order of reactivity for different halogens.

This difference can be understood from the thermodynamics of the reaction.

For alkane halogenation, the entropy term is negligible as the number of reactants and product molecules are equal. Therefore, ΔG becomes equivalent to ΔH.

Consider the radical fluorination of methane. The enthalpy of the reaction is estimated from the bond dissociation energies of the bonds broken and the bonds formed.

The overall ΔH for the reaction is negative and large, making the reaction thermodynamically favorable but highly explosive and impractical.

Conversely, for iodination, the positive ΔH value makes it thermodynamically unfavorable and impossible to achieve. 

Therefore, only chlorination and bromination with negative ΔH values are thermodynamically favorable and practically feasible.

A comparison of the two reactions reveals that bromination is slower than chlorination.

A closer look at the individual propagation steps shows that the first propagation step of bromination is more endothermic, which affects the overall reaction rate.

Since the net reaction is still exothermic, bromination occurs, but slowly.