Chapter 2.5, Problem 2.20E

(a)

Interpretation Introduction

Interpretation: Structures of the five isomeric hexanes should be drawn.

Concept introduction: Molecules that possess identical molecular formula but vary in the manner of their connectivity of the bonds are referred as structural isomers. These are essentially found in alkanes that can be branched by removal of $\text{H}$ group and its replacement with some alkyl group.

For instance, smallest branched alkane possible is $\text{2-methylpropane}$ . Its molecular formula is identical with butane ${\text{C}}_{\text{4}}{\text{H}}_{\text{10}}$ but it has different connectivity. The molecular models of $\text{n-butane}$ and $\text{2-methylpropane}$ are illustrated below.

  

The two compounds infact constitute a pair of constitutional isomers. These kinds of isomers are formed by cleavage and replacement of bonds, unlike the conformational isomers that result due to rotation around the bond.

(b)

Interpretation Introduction

Interpretation: Structures of all the possible next higher and lower homologs of $\text{2-methylbutane}$ should be drawn.

Concept introduction: Each member of a particular functional group series differs from the next lower by the addition of a methylene group $\left({\text{– CH}}_2\text{–}\right)$ . Series thus created are called homologs of each other and the corresponding series is termed as homologous series.