Chapter 4.3, Problem 4.7E

(a)

Interpretation Introduction

Interpretation: The Newman projections of $\text{C–C}$ bond in cyclopropane, cyclobutane, cyclopentane,and cyclohexane should be drawn in most stable conformation.

Concept introduction: Various interconvertible forms that result from rotation around the $\text{C–C}$ bond in ethane are termed as its conformers. To depict such conformation Newman projections are written. These are obtained by conversion of hashed-wedged line structure simply when molecule is viewed in a plane along the $\text{C–C}$ as illustrated below:

  

As rotation is carried out along $\text{C–C}$ axis, energy changes occur due to bond rotation from staggered to eclipsed conformation. This change amounts to torsional strain. Energy difference between eclipsed and staggered conformations corresponds to barrier that must be overcome for rotation. Staggered conformation is regarded most stable conformation for any cycloalkane due to least torsional strain, whereas the eclipsed has large amount of torsional strain due to steric repulsions of methyl groups as illustrated below.

  

(b)

Interpretation Introduction

Interpretation: The indicated molecules should be drawn in chair conformations.

  

Concept introduction:In chair form, all the $\text{C—C—C}$ bond anglesare $109.5°$ .Since this is ideal tetrahedral angle so there is no angular strain in chair form. The staggered $\text{C—H}$ bond further allows for no amount of torsional strain. There are two kinds of hydrogens in cyclohexane namely axial and equatorial indicated as follows: