Question: Rank the following conformations in order of increasing energy.

The energy of a conformation is based on its structure. Energy is based on the stability or repulsion of the groups. The conformation that is stable has less energy as compared to the unstable conformation.

Compounds A and D are staggered conformation. The energy of staggered conformation is less than the eclipsed conformation as there would be less repulsion between the bulky groups.

Compound D is more stable than compound A as it has less repulsion between the bulkier groups. The energy of compound D is the least. Thus, the order is $D<A$ .

Compounds B and C have eclipsed conformation. There would be more repulsion between the bulkier groups (methyl-ethyl) in compound C compared to compound B (ethyl -hydrogen). Thus, the energy of compound B is less than that of C.

The increasing order of energy is mentioned below:

$D<A<B<C$