Question

Explain why the following compounds are not optically active:

a. the product obtained from the reaction of 1,3-butadiene with cis-1,2-dichloroethene

b. the product obtained from the reaction of 1,3-butadiene with trans-1,2-dichloroethene

Short answer

Diels -Alder reaction is a syn addition reaction. So, each of the following syn addition reactions forms a product with two new asymmetric centers with forms a pair of enantiomers. Now as enantiomers are a mixture of two opposite optically active compounds, the overall optical activity of the mixture get nullified.

Step-by-step solution

Definition of Diels-Alder reaction

Diels Alder reaction is a pericyclic reaction where a diene and dienophile reacts to form a cyclic entity. This reaction is a reversible reaction. At high temperature, the cyclic compound breaks to get back the reactants

Stereochemistry of Diels-Alder reaction

If a Diels–Alder reaction creates a product having an asymmetric center, the product will be a racemic mixture and we know, in racemic mixture, both the optically active compounds are present in 50-50 amount. So, the overall mixture becomes optically inactive.

Reaction mechanism

The reaction mechanism for both the reactions are given below,