Question

how do i assign relative energy values to conformations

Answer 1

To assign relative energy values, you have to first draw the newmann projection structure of the molecule and then have to identify how many eclipse or gauche or both the interactions are present. Each type of interaction has a perticular value of rotational energy barrier.Sum up all the applicable energy values to get the relative energy values.

Point to be remembered :

A torsional strain arises due to two bonds which eclipsed each other (eclipsed conformation).

A steric strain arises due to two atoms which are very close to each other.

Lets consider the following example.

Relative energy of I is as follows.

CH3-CH3 gauche = 3.8 kJ/mol

Relative energy of II is as follows :

CH3-CH3 eclipsed + H-CH3 eclipsed + H-H eclipsed = (11 +6 + 4 )kJ/mol = 21 kJ/mol

So, the relative energy of I is 3.8 kJ/mol and that of II is 21kJ/mol