Question

Which conformational isomer of ETHANE should be LEAST stable?

Answer 1

Consider the diagram.........

Explanation:

We are looking down the `C-C` vector, i.e. the `C-C` bond of `"ethane"`, `H_3C-CH_3`.......The maximum energy of interaction occurs when the hydrogens are eclipsed, i.e. the situation on TOP of the energy well, i.e. the torsional angle `/_H-C-C-H=0^@`. The minimum energy of interaction occurs when the hydrogens of each methyl group are STAGGERED, i.e. the torsional angle `/_H-C-C-H=60^@`.

This is an example of conformational analysis........and your text will have a lot of examples of this sort of approach, especially with regard to carbocyclic 6-membered rings........

Note that when you try to interpret the relative energies of such conformations there is NOTHING childish or facile in using a model to inform your reasoning, and help you draw the conformation. In my own experience, I have seen a lot of good students FORGO the use of models in exams, and their marks have suffered accordingly.