Chapter 15, Problem 15.9P

Interpretation Introduction

Interpretation:

Among alkenes, alkynes and aromatic hydrocarbons, why only alkenes exhibit cis-trans isomerism has to be explained.

Concept Introduction:

The geometric isomers are said to be the isomers which shows different orientation of groups around a double bond.  The geometric isomers are also known as cis-trans isomers.

When two similar or higher priority groups are attached to the carbon on same side, it is said to be cis-isomer.

When two similar or higher priority groups are attached to the carbon on opposite sides, it is said to be trans-isomer.

To exhibit the geometric isomerism, a molecule should have double bonded carbon atoms ($\text{C=C}$) and each double bonded carbon should be bonded to two different groups.