
(a)
Interpretation:
For the given reactions, the product formed should be explained whether it is meso- compound or a pair of enantiomers.
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 |
- Meso-compound is superimposable to its mirror image.
- Enantiomers are the non-superimposable mirror images of each other.
To explain: the product formed is meso compound or a pair of enantiomers for each of the given reactions.
(b)
Interpretation:
For the given reactions, the product formed should be explained whether it is meso- compound or a pair of enantiomers.
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 |
- Meso-compound is superimposable to its mirror image.
- Enantiomers are the non-superimposable mirror images of each other.
To explain: the product formed is meso compound or a pair of enantiomers for each of the given reactions.
(c)
Interpretation:
For the given reactions, the product formed should be explained whether it is meso- compound or a pair of enantiomers.
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 |
- Meso-compound is superimposable to its mirror image.
- Enantiomers are the non-superimposable mirror images of each other.
To explain: the product formed is meso compound or a pair of enantiomers for each of the given reactions.

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Chapter 16 Solutions
Organic Chemistry
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