Question

Isopentane (a.k.a. 2-methylbutane) is a compound containing a branched carbon chain. A Newman projection is given for six conformations about the C2-C3 bond of isopentane. On the curve of potential energy versus angle of internal roation for isopentane, label each energy maximum and minimum with one of the conformations.

I am pretty sure the eclipsed conformations where the CH3 and CH3 are directly next to each other will make the peaks with the highest potential energy. I am however unsure of how I know the order by which they will exist on this potential energy diagram.

Please explain and not just give me the answer.

Thank you!

Answer 1

Concepts and reason

Conformation are formed due to free rotation around carbon-carbon single bond. The different conformations of isopentane is shown by the Newman projection. To label the conformation on the potential energy diagram, relate the energy corresponding to each of the isomer based on the stability parameter.

Fundamentals

In Newman projection, there are basically two forms, that is, staggered and eclipsed which are formed alternatively by the rotation of around C-C single bond and represents the various conformation of a molecule.

Generally, staggered conformation due to least interaction between the substituents is more stable than the eclipsed conformation. The conformation which is more stable will have lower energy.

Draw the given Newman projection of isomers of isopentane and mark them as eclipsed and staggered as shown below:

Arrange the given isomers in increasing order of stability as shown below:

Arrange the given conformations in increasing order of energy as shown below:

Draw the given energy level diagram and place the conformation on the given spaces according to their energies as shown below:

Ans:

The complete potential energy diagram is shown below: