Question

Question: (a) What product is formed when each compound undergoes a thermal electrocyclic ring opening? (b) What product is formed when each compound undergoes a photochemical electrocyclic ring opening?

Short answer

Answer

a.

b.

Step-by-step solution

Thermal electrocyclic ring opening

Reactions occurring under heat conditions are thermal electrocyclic reactions. Conjugated polyenes comprising odd pi electrons take place in a disrotatory fashion for thermal electrocyclic reactions.

Photochemical electrocyclic ring opening

Reactions requiring light are photochemical electrocyclic reactions. Conjugated polyenes consisting of odd pi electrons take place in a conrotatory fashion for photochemical electrocyclic reactions.

Product formed by the thermal electrocyclic ring opening and photochemical electrocyclic ring opening

(a)The skeletal structure of compound a can be given as:

Skeletal structure of compound a

The skeletal structure of compound b can be given as:

Skeletal structure of compound b

Compound A gets subjected to thermal electrocyclic ring opening to lead to a product comprising three pi bonds. The ring opening happens in a disrotatory manner. The compound formed by the thermal electrocyclic ring opening of compound a can be given as:

Thermal electrocyclic ring opening of compound a

Compound B gets subjected to thermal electrocyclic ring opening to give a product with three pi bonds. The ring opening occurs in a disrotatory manner. The compound formed by the thermal electrocyclic ring opening of compound b can be given as:

Thermal electrocyclic ring opening of compound b

(b)Compound a gets subjected to photochemical electrocyclic ring opening to give rise to a compound containing three pi bonds. The ring opening happens in a conrotatory manner. The compound formed by the photochemical electrocyclic ring opening of compound a can be given as:

Photochemical electrocyclic ring opening of compound a

The product obtained after photochemical electrocyclic ring opening of compound B has three pi bonds. The ring opening happens in a con-rotatory manner. The compound formed by the photochemical electrocyclic ring opening of compound b can be given as:

Photochemical electrocyclic ring opening of compound b