Chapter 17.4, Problem 9ATS

Interpretation Introduction

Interpretation:

The structure of a conjugated diene yields the same products should be proposed for the 1,2 addition and 1,4-addition of $\text{HBr}$.

Concept introduction:

• Conjugated Dienes undergoes addition reactions with $\text{HBr}$ as similar to simple alkenes addition with $\text{HBr}$.
  • The allylic carbocation is formed as an intermediate by the addition of proton in the $\text{HBr}$.
  • The ${\text{Br}}^{-}$(nucleophilic) can attacks in either of the two resonance stabilized carbocation’s in the in the allylic intermediate. 
• There are four carbons for addition of proton in a conjugated diene, but addition of proton at C1 and C4 carbons only form the allylic carbocation. The nucleophile can attack two positions of allylic carbocation. If it is C2 carbon, then the it is called 1,2 addition of $\text{HBr}$ and if it is C4 carbon then it is called 1,4-addition of $\text{HBr}$.
• The butadiene produces two products for addition of $\text{HBr}$. Because it is forming 2-bromobutene (1,2 addition) and 4-bromobutene (1,4 addition).
• The simple cyclic dienes produces same products for addition of $\text{HBr}$, because the two formed cyclo-haloalkenes will have same names.

To draw: the structure of a conjugated diene yields the same products for the 1,2 addition and 1,4-addition of $\text{HBr}$.