20.24:
Radical Anti-Markovnikov Addition to Alkenes: Thermodynamics
Recall that anti-Markovnikov addition is only observed with HBr. The radical addition with HCl and HI is thermodynamically unfavorable.
But why?
Let's examine the thermodynamics for each propagation step involved in the radical mechanism.
For a reaction to be spontaneous, the change in Gibbs free energy, ∆G, must be negative.
For the first propagation step, the sign of ∆G is dictated by the competition between the enthalpy and the entropy terms.
In the case of HBr and HCl, the enthalpy term dominates at low temperatures, and ∆G becomes negative, implying a thermodynamically favorable reaction.
However, in the case of HI, both the enthalpy and entropy terms are positive, implying that ∆G is positive, regardless of temperature, indicating a thermodynamically unfavorable reaction.
For the second propagation step, the entropy term is close to zero because the number of reactants and product molecules are equal. So, the enthalpy term dictates the sign of ∆G.
As evident, ∆G is positive for HCl, rendering the reaction thermodynamically unfavorable.
Overall, the two propagation steps are thermodynamically favorable only in the case of HBr.
20.24:
Radical Anti-Markovnikov Addition to Alkenes: Thermodynamics
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.