Chapter 20, Problem 20.19P

Interpretation Introduction

Interpretation: Among the given two molecules, the major product that is formed by $1,2$-addition of $\text{HCl}$ to cyclopentadiene has to be found and explained.

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.