Chapter 24, Problem 24.33P

Interpretation Introduction

Interpretation:

Which of the molecules shown will react faster as a diene in a Diels–Alder reaction with ethene is to be determined.

Concept introduction:

The Diels-Alder reaction is the cycloaddition of diene and dienophile. The diene has a pair of conjugate double and contributes its $\text{4π}$ electrons. The dienophile has single double bond and contributes its $\text{2π}$ electrons. These six electrons must flow in a cyclic fashion. Thus, the reaction proceeds through a cyclic transition state, which makes it a pericyclic reaction. Therefore, the Diels-Alder reaction is called $\text{[4+2]}$ cycloaddition. The two $\text{C=C}$ bonds in diene must be conjugated and able to attain the $s\text{-cis}$ conformation. The Diels–Alder reactions cannot take place with the diene in the $s\text{-trans}$ conformation. In $s\text{-cis}$ conformation, the two ends of the $\text{π}$ system point in nearly the same direction, and in $s\text{-trans}$ conformation, the ends of the two reacting species are farther apart. The steric factor reduces the Diels–Alder reaction rate as the reaction proceeds through cyclic transition state.