16.19:
Diels–Alder vs Retro-Diels–Alder Reaction: Thermodynamic Factors
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.
16.19:
Diels–Alder vs Retro-Diels–Alder Reaction: Thermodynamic Factors
The Diels–Alder reaction is thermally reversible, meaning that the reaction reverts to the starting diene and dienophile under suitable temperatures. The forward reaction gives a cyclohexene derivative and is favored at low to medium temperatures. The reverse process, also called retro-Diels–Alder reaction, is a ring-opening process favored at high temperatures.
Thermodynamic factors
The influence of temperature on the spontaneity of a particular reaction can be assessed based on the change in the Gibbs free energy, ΔG. If ΔG is negative, the reaction occurs spontaneously. However, if ΔG is positive, the reaction occurs spontaneously in the opposite direction.
ΔG is a composite of two terms, ΔH and −TΔS. In a Diels–Alder reaction, two stronger σ bonds are formed at the expense of two weaker π bonds, resulting in a negative ΔH. Cycloaddition reactions proceed with a decrease in entropy. Consequently, ΔS is negative, and the term −TΔS is positive. At low temperatures, the sum of the two terms is negative, implying that the forward reaction is spontaneous. In contrast, the sum is positive at high temperatures, indicating that the reverse reaction is spontaneous.