Chapter 4, Problem 4.32P

Interpretation Introduction

Interpretation:

Conformational analysis of butane, looking down the C2-C3 bond, is to be performed paying attention to the relative energies of the various conformations without considering the actual energy values.

Concept introduction:

Conformational analysis is the plot of a molecule’s energy as a function of the bond’s dihedral angle. The energy values for various angles of rotation are relative to that of the lowest-energy conformation. All the rotational conformations of a compound are illustrated by Newman projection, which is a two dimensional representation of a molecule viewed down the bond of interest. In a Newman projection, the bond of interest is not visible; instead, it must be imagined as connecting the front and back carbons. The nearer atom is depicted as a point while the more distant atom is depicted as a circle. Bonds to the front converge at the point, whereas bonds to the back atom connect to the circle. Each angle of rotation defines a particular dihedral angle, $\text{θ}$, corresponding to the angle between $\text{C-X}$ and $\text{C-Y}$ bond of a generic molecule ${\text{XCH}}_2{\text{- CH}}_{\text{2}}\text{Y}$. By convention, $\text{θ}$ can assume values between ${\text{-180}}^{\text{o}}{\text{and +180}}^{\text{o}}$, where the conformations at ${\text{-180}}^{\text{o}}{\text{and +180}}^{\text{o}}$ are exactly the same because they are ${\text{360}}^{\text{o}}$ apart. In Eclipsed conformation the two substituents, X and Y, on adjacent carbon atoms are in closest proximity, indicates that the torsional angle is ${\text{0}}^{\text{o}}$. The energy associated with this conformation is highest because it possesses torsional strain. In Staggered conformation the two substituents X and Y, are at maximum distance from each other. This is the most stable conformation and thus has minimum energy value.