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.
