Q.65 - Organic Chemistry

Question

Explain the selectivity observed in the following reactions.

Step-by-Step Solution

Answer

The selectivity in these reactions can be explained based on anti-periplanar orientation.

In both these reactions, elimination takes place where anti-periplanar orientation is maintained. Since the position of the leaving group is different for both these reactions,  the elimination products are different too.

Anti-periplanar orientations

To undergo elimination reactions, the protons and the leaving group must be in a trans-position with each other to give an elimination product.

Observation of the reactants

In the first reaction, bromide lies in the dash position. So, to maintain anti-periplanar orientation, abstraction of proton takes place from the junction hydrogen atom. 

Mechanism 

In contrast, in the case of the second reaction, bromide lies in the dash position. So, abstraction of proton takes place from that carbon which lies in the dash position.

Representation of another mechanistic approach

Based on the explanations, the selectivity in these reactions can be explained based on anti-periplanar orientation.

Representation of the possible elimination products

In both these reactions, elimination takes place where anti-periplanar orientation is maintained. Since the position of the leaving group is different for both cases, the elimination products are different too.