The anti-Markovnikov addition of hydrogen halides to an alkene is thermodynamically feasible only with HBr. The radical addition reaction with other hydrogen halides like HCl and HI is thermodynamically unfavorable.

Thermodynamic factors

The temperature influences the spontaneity of a reaction, which can be evaluated based on the change in the Gibbs free energy, ΔG. If the change in Gibbs free energy, ΔG, is negative, the reaction occurs spontaneously. As shown below, ΔG can be evaluated directly from the values of the two terms, ΔH and −TΔS.

The thermodynamic feasibility of anti-Markovnikov addition with HBr can be predicted from each propagation step involved in the radical mechanism.

In the first propagation step of the radical reaction, the ∆G sign is determined by the competition between the enthalpy and the entropy terms. At lower temperatures, radical addition with HBr and HCl dominates the enthalpy term. Consequently, ΔG is negative, indicating that the reaction is thermodynamically favorable. However, at high temperatures, the reaction is endothermic and thermodynamically unfavorable. In contrast, with HI, irrespective of temperature, both the enthalpy and entropy terms are positive. As a result, ∆G becomes positive, signifying that the reaction is thermodynamically unfavorable.

In the second propagation step, the entropy term is close to zero as the number of reactants and product molecules are equal, meaning that the ∆G sign depends on the enthalpy term. Since the enthalpy term dominates for HBr and HI, the reaction is exothermic and thermodynamically favorable. However, with HCl, ∆G is positive, meaning that the reaction is endothermic and thermodynamically unfavorable.

In summary, the two propagation steps in the reaction are thermodynamically favorable only with HBr.