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

Step 1: Understanding the Diels-Alder reaction.
The Diels-Alder reaction is a cycloaddition reaction between a diene and a dienophile. The activation barrier for a Diels-Alder reaction is influenced by factors such as the electron density of the diene and dienophile, the geometry of the reaction, and substituent effects on both the diene and dienophile.

Step 2: Analyzing the reactions.
- Reaction I: A relatively simple diene and dienophile with no strong electron-withdrawing or electron-donating groups.
- Reaction II: A more substituted system, leading to a slightly lower activation barrier.
- Reaction III: The diene and dienophile have additional steric strain, making this reaction more difficult and increasing the activation barrier.
- Reaction IV: The diene is in a strained configuration, which increases the activation barrier significantly.

Step 3: Conclusion.
The activation barrier order is II > I > III > IV, so the correct answer is (A).