Chapter 17.9, Problem 21CC

(a)

Interpretation Introduction

Interpretation:

For each of the given thermal electrocyclic reactions, the major product formed should be determined.

Concept introduction:

• Electrocyclic reaction is a concerted cyclization reaction of conjugated polyenes (π-system), in which one π-bond is converted into an σ-bond and remaining π-bonds shift their positions. Thereby the newly formed σ-bond connects the two ends of π-system to form a ring.
• The mechanism of the ring formation in the Electrocyclic reaction is drawn by using arrows representing the forming σ-bond and the shifting of π-bonds in the π-system of a polyene.
• The reaction conditions (thermal or photochemical) controls the product in such a way that the rotation for end lobes (ring closure) of the HOMO of conjugated (π-system) polyene.
• According to Woodward-Hoffmann rule for thermal and photochemical Electrocyclic reactions,

π-systems	Thermal	Photochemical
4- π-electrons	Conrotatory	Disrotatory
6- π-electrons	Disrotatory	Conrotatory

• The mechanism for reverse of Electrocyclic reaction is drawn by using arrows representing the conversion of σ-bond to π-bond and the shifting of π-bonds in the π-system of a cyclic alkene.

To determine: the major product formed for each of the given thermal electrocyclic reactions.

(b)

Interpretation Introduction

Interpretation:

For each of the given thermal electrocyclic reactions, the major product formed should be determined.

Concept introduction:

• Electrocyclic reaction is a concerted cyclization reaction of conjugated polyenes (π-system), in which one π-bond is converted into an σ-bond and remaining π-bonds shift their positions. Thereby the newly formed σ-bond connects the two ends of π-system to form a ring.
• The mechanism of the ring formation in the Electrocyclic reaction is drawn by using arrows representing the forming σ-bond and the shifting of π-bonds in the π-system of a polyene.
• The reaction conditions (thermal or photochemical) controls the product in such a way that the rotation for end lobes (ring closure) of the HOMO of conjugated (π-system) polyene.
• According to Woodward-Hoffmann rule for thermal and photochemical Electrocyclic reactions,

π-systems	Thermal	Photochemical
4- π-electrons	Conrotatory	Disrotatory
6- π-electrons	Disrotatory	Conrotatory

• The mechanism for reverse of Electrocyclic reaction is drawn by using arrows representing the conversion of σ-bond to π-bond and the shifting of π-bonds in the π-system of a cyclic alkene.

To determine: the major product formed for each of the given thermal electrocyclic reactions.

(c)

Interpretation Introduction

Interpretation:

For each of the given thermal electrocyclic reactions, the major product formed should be determined.

Concept introduction:

• Electrocyclic reaction is a concerted cyclization reaction of conjugated polyenes (π-system), in which one π-bond is converted into an σ-bond and remaining π-bonds shift their positions. Thereby the newly formed σ-bond connects the two ends of π-system to form a ring.
• The mechanism of the ring formation in the Electrocyclic reaction is drawn by using arrows representing the forming σ-bond and the shifting of π-bonds in the π-system of a polyene.
• The reaction conditions (thermal or photochemical) controls the product in such a way that the rotation for end lobes (ring closure) of the HOMO of conjugated (π-system) polyene.
• According to Woodward-Hoffmann rule for thermal and photochemical Electrocyclic reactions,

π-systems	Thermal	Photochemical
4- π-electrons	Conrotatory	Disrotatory
6- π-electrons	Disrotatory	Conrotatory

• The mechanism for reverse of Electrocyclic reaction is drawn by using arrows representing the conversion of σ-bond to π-bond and the shifting of π-bonds in the π-system of a cyclic alkene.

To determine: the major product formed for each of the given thermal electrocyclic reactions.