Question

Explain in terms of thermodynamic parameters, how it is possible for a Diels-Alder reaction to proceed in the forward direction (cycloaddition) at room temperature, but in the reverse direction (decomposition of the adduct) at higher temperatures

Answer 1

In Diels Alder reaction from two molecules of reactant (Diene and Dienophile) the Diels Alder adduct is obtained. As two molecules combine to give one molecule the entropy change of this reaction is negative. Now, Gibbs Free Energy change of a reaction is calculated according to the formula

            

As, the enthalpy change for the forward reaction is negative the entropy factor is not predominating at room temperature and the reaction goes in the forward direction.

At an elevated temperature the Entropy factor can not be neglected and therefore the reaction goes in the backward direction because in this process the entropy change becomes positive as two molecules are generated from one D.A. Adduct.