Chapter 4, Problem 4.35P

Interpretation Introduction

Interpretation:

Conformational analysis of $\text{2 - methylbutane}$ looking down the $\text{C2}-\text{C3}$ bond is to be performed, and the relative energies of different conformations are to be compared.

Concept introduction:

Conformers are molecules that differ only by a rotation of one part of the molecule about a single bond.

Conformation analysis is a study of energy changes that occur during the rotation about the $\text{σ}$ bond of interest. Conformation analysis is done by rotating the front carbon along with its bonded groups, in its Newman projection, about the bond of interest.

A Newman projection of a molecule is the representation of its three dimensional structure looking along the bond of interest. The atom at the front end, typically a carbon atom, is represented by a point. The bonds to the three groups attached to it converge on this point. The carbon atom at the back is represented by a circle, with the three bonds to its attached groups ending on the circle.

Depending on the relative positions of the groups on the front and the back carbon, the conformations are classified into two main types.

Eclipsed conformation is one in which the dihedral angle is ${\text{0}}^{\text{o}}$. A staggered conformer is one in which the dihedral angle is ${\text{60}}^{\text{o}}$. The eclipsed conformer has higher energy than a staggered conformer because the groups on the two carbons are closer to each other than in the staggered conformer. The proximity of the groups in eclipsed conformer introduces a high amount of torsional strain.

Depending on the presence and interactions between bulky groups, the staggered conformation is further divided into anti and gauche conformers. In the anti-conformation, the dihedral angle between bulky groups is ${\text{180}}^{\text{o}}$ while in the gauche conformation, it is ${\text{60}}^{\text{o}}$.