Question

1. The dehydrohalogenation of dibromostilbene occurs through two stages. Why does the second stage in particular require so much energy to complete?

2. How do the substituents on the diene and the dienophile affect the rate of reaction in a Diels-Alder reaction?

Answer 1

the activation energy for the generation of a vinyl cation intermediate would be higher than that for a lower energy intermediate. This is illustrated for alkenes versus alkynes by the following energy diagrams.

The energy level diagram of vinyl cation intermediate which very higher energy than the other alkyl cation intermediate. At the second step of dehydrohalogenation produce intermediate which is higher energy like carbocation intermediate so for that it required strong base to deprotonation.