Chapter 21, Problem 70A

Interpretation Introduction

Interpretation:

The pair of geometric isomers need to be identified from the following structures and the relation of third structure with other two needs to be explained:

Concept introduction:

The molecule with same molecular formula and connectivity but differ in spatial arrangements of atoms in the molecule are said to be geometric isomers of each other.

Answer to Problem 70A

The pair of geometric isomers is (b) and (c). Compound (a) is a structural isomer of the other two compounds.

Explanation of Solution

Geometric isomerism, or cis-trans, isomerism is most common in alkenes. The two forms exist due to no free rotation about carbon-carbon double bond. It exists only when the double bonded carbon atoms are joined to two different groups or atoms.

In cis- isomer, the two identical groups or atoms are close to each other whereas in trans- isomer, two identical groups or atoms are farther apart.

Now, in order to determine the geometric isomers pair from given structures, the compounds containing same atoms and differ only in orientation at a carbon atom should be selected so, compound (b) and (c) contains same atoms (2 methyl and 2 ethyl groups attached to same carbon atom across the double bond) and differ in orientation that is (b) is trans- and (c) is cis-.

The relation of third structure that is (a) with other two, (b) and (c) will be:

The molecular formula of all the three compounds is ${\text{C}}_{\text{8}}{\text{H}}_{\text{18}}$, but compound (a) has different connectivity of across the double bonded carbon atoms so, it is a structural isomer of the other two compounds.