Chapter 11, Problem 11.45P

Interpretation Introduction

(a)

Interpretation:

All three carbocation intermediates possible from protonation of the given triene are to be drawn. The most stable carbocation intermediate among them is to be identified.

Concept introduction:

In an electrophilic addition of acid to a conjugated alkene, ${\text{H}}^{\text{+}}$ adds to a $\text{π}$ bond in the first step. Addition of a proton in the first step of the mechanism occurs to give the more stable carbocation intermediate. The carbocation intermediate produced by the addition of ${\text{H}}^{\text{+}}$ to a terminal C is resonance stabilized and hence more stable.

Interpretation Introduction

(b)

Interpretation:

All halogenated products formed by attack of ${\text{Br}}^{\text{-}}$ on the most stable carbocation are to be drawn.

Concept introduction:

In an electrophilic addition of acid to a conjugated alkene, ${\text{H}}^{\text{+}}$ adds to a $\text{π}$ bond in the first step. In step 2, the anion attacks the newly formed carbocation intermediate in a coordination step. Three products are produced from the same carbocation intermediate. The ${\text{H}}^{\text{+}}$ and anion, if added to the triene to produce an halo alkene separated by two C atoms, make it the product of 1, 2-addition. On the other hand, another is the product of 1, 4-addition as the ${\text{H}}^{\text{+}}$ and anion are added to the triene that is separated by four C atoms. The ${\text{H}}^{\text{+}}$ and anion, if added to the triene to produce an halo alkene separated by six C atoms, make it the product of 1, 6-addition.

Interpretation Introduction

(c)

Interpretation:

The product of the given reaction that is expected to be formed in the greatest amount at low temperature is to be identified.

Concept introduction:

At low temperatures, electrophilic addition to a conjugated triene takes place under kinetic control, so the major product is the one that is produced most rapidly.

Interpretation Introduction

(d)

Interpretation:

The product of the given reaction that is expected to be formed in the greatest amount at high temperature is to be identified.

Concept introduction:

At high temperatures, electrophilic addition to a conjugated triene takes place under thermodynamic control, so the major product is the one that is lowest in energy. The more highly alkyl substituted C=C is the more stable alkene that is lowest in energy.