Question

Why is a Diels-Alder reaction favored at lower temperature? Please give a clear and specific answer.

Answer 1

• Dienes can be prepared by elimination reactions (review) of unsaturated alcohols and alkyl halides.
• The outcome of eliminations typically favors the more stable product,
• Since conjugated dienes are more stable than isolated dienes, the formation of the conjugated diene is usually favored over the isolated diene unless the structure prevents the formation of the conjugated system.

Now consider what happens as we alter the reaction temperature and so the average energy of the molecules changes.

1. At low tempearture, the reaction preferentially proceeds along the green path to P1 and stops since they lack sufficient energy to reverse to SM, i.e. it is irreversible, so the product ratio of the reaction is dictated by the rates of formation of P1 and P2, k₁: k₂.

2. At some slightly higher temperature, reaction 1 will become reversible while reaction 2 remains irreversible. So although P1 may form initially, over time it will revert to SM and react to give the more stable P2.

3. At high temperature, both reaction 1 and 2 are reversible and the product ratio of the reaction is dictated by the equilibrium constants for P1 and P2, K₁ : K₂.