Estimate Energy of a conformation

[y3chem]

Can someone explain how to do this if we are only given the IUPAC name of the compound?

In the textbook it uses torsional strain and multiply it by the eclipsed C-H bonds
(# of eclipsed bonds)* (4 kJ/mol)
But I don't really understand the process in which they do it :(

[irisc23]

Could you please give an example of this? Sorry it's a bit hard to explain just theoretically. Thanks.

[y3chem]

selftest 3.5A

[irisc23]

Okay, firstly, we are estimating not the absolute energy of the conformation but the relative energy of the conformations. It is very difficult to measure the absolute energy of something (since that takes into account many many factors). We measure relative energy by comparing the conformation with the lowest possible energy state of that atom.

For cyclohexane, the most stable conformation would be the chair conformation. To get the energy difference between chair and planar (or in general between the most stable conformation and the conformation in question) consider three different types of strain: 1) torsional strain, 2) bond angle strain, 3) steric strain.

In planar cyclohexane, there is definitely torsional strain as all of the bonds are eclipsed. According to the book each pair of eclipsed C-H bonds contributes +4 kJ/mol to the energy of the molecule. As there are 12 pairs of eclipsed C-H bonds, this results in +48 kJ/mol.

The book stops here with the explanation as this is already really high in comparison even to the boat conformation (already a less stable conformation). I don't know how much each 120º bond angle contributes to the energy of the conformation. If necessary that number would be given (or you could probably look it up online). Hope that helps.

[y3chem]

thank you so much! :)