Q51P - Organic Chemistry (Mcmurry)

Question

Question: 1-Chloro-1,2-diphenylethane can undergo E2 elimination to give either cis- or trans-1,2-diphenylethylene (stilbene). Draw Newman projections of the reactive conformations leading to both possible products, and suggest a reason why the trans alkene is the major product.

Step-by-Step Solution

Answer

Answer

Step1: Newman projection

It represents the arrangement of the atom from front atom to back. The front atom is represented by a dot and the back atom is represented by a circle. It represents the 3-D structure of the molecule.

Step 2: E 2 reaction

It is a nucleophilic elimination reaction in which an old bond is broken and a new bond is formed both the process occur simultaneously and no intermediate is formed.

Here, the carbon-hydrogen or halogen bond breaks to form a new double bond. Hence, it’s a single-step reaction.

The E2 elimination occurs when anti pre planer geometry is present.

                 H and Cl show anti pre planer geometry

Step 3: Formation of product

Both the transition state A and B can undergo E2 elimination to form the alkene product 

Transition state A form cis and B form trans. Transition state A is less stable than Transition state B because of the crowding of the phenyl group present in adjacent position whereas in B both phenyl groups present anti to each other hence, a more stable and trans product is the major product.

                                      Newman Projection