Chapter 17, Problem 37PP

Interpretation Introduction

Interpretation:

The major product expected for the given reaction between 1,3-butadiene and $\text{HBr}$ at ${\text{40}}^{\text{o}}\text{C}$ should be drawn with the mechanism of its formation.

Concept introduction:

• 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 $\text{HBr}$.
• Symmetrical dienes like butadiene will produce only two products. Because the intermediates allylic carbocation formed are same.
• The 1,2-adduct is kinetic product, because of the proximity of C2 carbon to attack by ${\text{Br}}^{-}$.
• The 1,4-adduct is thermodynamic product, because the more substituted double bond is more stable.

To draw: the major product and the mechanism of its formations for the given reaction between 1,3-butadiene and $\text{HBr}$ at ${\text{40}}^{\text{o}}\text{C}$.