Chapter 16.5, Problem 10PTS

Interpretation Introduction

Interpretation:

For the thermodynamic controlled given reaction, the predominated product is 1,2-adduct rather than 1,4-adduct should be explained.

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 diene 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 explain: the predominated product 1,2-adduct rather than 1,4-adduct for the given thermodynamic controlled reaction.