Chapter 17, Problem 39PP

Interpretation Introduction

Interpretation:

For the given reaction in Problem 17.38, the two of the formed products become the major products after the flask is heated to ${\text{40}}^{\text{o}}\text{C}$ and the product distribution has no change further the flask is cooled to ${\text{0}}^{\text{o}}\text{C}$ are needed to be explained.

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 it is called 1,2-addition of $\text{HBr}$ and if it is C4 carbon then it is called 1,4-addition of $\text{HBr}$.
• Temperature is influenced in the predomination of a product. At elevated temperature, the formation of 1,2-adduct and 1,4-adduct are reversible to their allylic intermediates. Eventually, the more stable or the thermodynamic product will predominated over the less stable or kinetic product.
• 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 two of the formed products become the major products after the flask is heated to ${\text{40}}^{\text{o}}\text{C}$ and the product distribution has no change further the flask is cooled to ${\text{0}}^{\text{o}}\text{C}$ for the given reaction.