16.19 : 狄尔斯-阿尔德反应与逆狄尔斯-阿尔德反应反应：热力学因素

The Diels–Alder is a reversible reaction where the reaction temperature strongly influences the equilibrium position.

Moderate temperatures favor the addition product, whereas high temperatures favor the reactants via the reverse reaction known as retro Diels–Alder.

But why?

Recall that the change in Gibbs free energy, ∆G, is a sum of the enthalpy change, ∆H, and the entropy change at a given temperature, −T∆S.

For a reaction to be spontaneous, ∆G must be negative. To predict ∆G, the values of ∆H  and ∆S are used.

In a Diels–Alder reaction, three π bonds are broken; two σ bonds and one π bond are formed. Recall that σ bonds are stronger than π bonds. So, the reaction is exothermic, and ∆H is negative.

Additionally, a decrease of moles on the product side indicates a reduction in entropy. So, ∆S is negative, and −T∆S becomes positive.

At low temperatures, the ∆H term dominates, implying that ∆G is negative, favoring the forward reaction.

In contrast, the −T∆S term dominates at high temperatures, resulting in a positive ∆G, favoring the reverse reaction.