Chapter 16, Problem 81CP

(a)

Interpretation Introduction

Interpretation:

The structure of the compound (2) which contains a carbon-carbon $\text{π}$ bond in Z configuration has to be identified.

Concept introduction:

There are mainly three types of pericyclic reactions,

1. 1) Electrocyclic reactions
2. 2) Cycloaddition reactions
3. 3) Sigmatropic reactions

[3,3]-sigmatropic rearrangement involving the conversion of allyl vinyl ether to an unsaturated carbonyl compound. This reaction is known as Claisen rearrangement reaction.

• E-Z designators are used as like cis-trans terminology for non-similar groups attached alkenes.
• In E-Z designations, the groups attached to vinylic positions are checked by their priority on the basis of higher molecular weight. If the higher priority groups are on the same sides, then the configuration is designated as Z. If the higher priority groups are on the opposite sides, then the configuration is designated as E. 

(b)

Interpretation Introduction

Interpretation:

The plausible mechanism for the transformation of 1 to 2 has to be determined.

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 $\text{HBr}$.
• The ${\text{Br}}^{-}$(nucleophilic) can attacks in either of the two resonance stabilized carbocation’s in the allylic intermediate. 
• Symmetrical diene like butadiene will produce only two products. Because the intermediates allylic carbocation formed are same.
• For the Butadiene derivatives, there are four carbons for addition of proton, but at C1 and C4 carbons only forms the allylic carbocation.

(c)

Interpretation Introduction

Interpretation:

The explanation for the stereochemical outcome has to be interpreted.

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. 
• Symmetrical diene like butadiene will produce only two products. Because the intermediates allylic carbocation formed are same.
• For the Butadiene derivatives, there are four carbons for addition of proton, but at C1 and C4 carbons only forms the allylic carbocation.