Chapter 24, Problem 24.12YT

Interpretation Introduction

Interpretation:

For the Diels–Alder reaction shown here, ${\text{ΔH}}^{\text{o}}{}_{\text{rxn}}$ =$\text{-121 kJ/mol }\left(\text{-29kcal/mol}\right)$, whether this reaction would be easily reversible at $\text{150 °C }$ is to be determined.

Concept introduction:

The entropy term in a Diels–Alder reaction is negative because the reaction decreases the number of independent molecules (i.e., the reaction leads to less “disorder”). This means that increasing the temperature makes ${\text{ΔG}}^{\text{o}}{}_{\text{rxn}}$ less negative (because ${\text{ΔG}}^{\text{o}}{}_{\text{rxn}}$= ${\text{ΔH}}^{\text{o}}{}_{\text{rxn}}$-${\text{TΔS}}^{\text{°}}{}_{\text{rxn}}$). Under normal conditions, the prototypical Diels–Alder has a substantially negative change in standard free energy, giving it a very large equilibrium constant. As a result, the reaction is irreversible.