Question

Draw the mechanism for the elimination of the product from Question #2 (written below) using KOH and heat. The first elimination follows a classical E2 mechanism and the product is an alkene with the phenyl groups in a cis configuration. Since the bromine and hydrogen are not in an anti-coplanar arrangement, the second elimination cannot proceed through an E2 mechanism. Instead it is said to proceed through an E1cB mechanism.

Question 2 for reference: Draw the mechanism for the bromination of trans-Stilbene using Br₂. (Include stereochemistry).

Answer 1

Solution :-

Following image shows the mechanism for the bromination of the trans-stilbene

And mechanism for the elimination reaction of the product of the trans-stilbene bromination product.

In the bromination of the trans- stilbene reaction first step is the attack of the pi bond electrons on the Br-Br and forms the intermediate cation which is then stabilized by the cyclobromonion ion. Then Br^- attacks on the opposite side to open the cyclobromonium ion and gives dibromo substituted product.

In the elimination reaction of the product

The first step is the attack of the KOH to remove the H which follows the E2 mechanism and removes the 1 Br atom and generates the alkene pi bond.

Then in the second step the H is taken off from the alkene carbon and negative ion is formed which then facilitate the removal of the second Br atom and forms the triple bond.