Question

For the Diels-Alder reactions I–IV, the activation barriers follow the order 

• A. II > I > III > IV
• B. I > III > IV > II
• C. III > IV > II > I
• D. IV > III > II > I

Hint:

In the Diels-Alder reaction, steric strain and electron-withdrawing/donating substituents play a major role in determining the activation energy.

Answer 1

The Correct Option is C

The question involves determining the order of activation barriers for the given Diels-Alder reactions I–IV. The Diels-Alder reaction is a [4+2] cycloaddition reaction between a conjugated diene and a dienophile. The activation barrier is influenced by the electron density of the diene and the electron-withdrawing nature of the dienophile. Let’s analyze each reaction:

1. Reaction I: The diene is maleic anhydride, a highly reactive dienophile due to its electron-withdrawing groups, reacting with cyclopentadiene.
2. Reaction II: Involves furan as the diene and maleic anhydride as the dienophile. Furan is a less reactive diene because of the oxygen atom which provides some electron-donating character, making it less reactive than the olefinic dienes like those in reaction I.
3. Reaction III: Here the diene is thiophene and the dienophile maleic anhydride. Thiophene, due to its sulfur atom, is less reactive than furan as a diene, raising the activation barrier compared to reaction II.
4. Reaction IV: Involves pyrrole as the diene, which is less reactive than both furan and thiophene due to the nitrogen, increasing the reactivity barrier further owing to its aromatic stability.

Conclusion:

The order of activation barriers is influenced by the nature of the diene and dienophile, wherein the dienes with heteroatoms like sulfur and nitrogen exhibit higher activation barriers due to less effective π-system overlap and aromatic stability. The order from highest to lowest activation barrier is:

• III: Thiophene diene
• IV: Pyrrole diene
• II: Furan diene
• I: Cyclopentadiene diene

Thus, the activation barriers follow the order: III > IV > II > I.