Chapter 20, Problem 20.21P

Interpretation Introduction

Interpretation: On addition of one mole of ${\text{Br}}_{\text{2}}$ to cyclopentadiene, the two constitutional isomers that are formed for the given molecular formula have to be drawn.

Concept Introduction:

Addition reaction of dienes:

The diene with a four carbon system is $\text{1,3-butadiene}$. When it is allowed to react with two moles of $\text{HBr}$, it undergoes two type of addition reactions such as 1,2-addition and 1,4-addition as shown here:

$\text{1,3-butadiene}$ reacts with $\text{H-Br}$ and formed a resonance stabilized carbocation. The addition of a bromine atom to each of the two resonance forms has formed the corresponding product. So, two products are obtained. In one product the bromine atom has been attached to the 2^nd carbon atom indicating that it has been formed from 1,2-addition whereas in the other product, the bromine atom has been attached to the 1^st carbon atom indicating that it has been formed from 1,4-addition.

The constitutional isomers have same molecular formula and different connectivity of the substituents in the same structural formula.

Example:

$\text{2-Chlorohexane}\text{and}\text{3-Chlorohexane}$ are the two constitutional isomers of mono $\text{chlorohexane}$.