Question

Question: Draw the product formed when each triene undergoes electrocyclic reaction under [1] thermal conditions, [2] photochemical conditions

Short answer

The stereochemistry of the product of an electrocyclic reaction depends on the number of pi bonds present in the system and the reaction condition (thermal/photochemical).

Step-by-step solution

Electrocyclic reactions of polyenes

The stereochemistry of the product of an electrocyclic reaction depends on the number of pi bonds present in the system and the reaction condition (thermal/photochemical).

Woodward-Hoffman rules for electrocyclic reactions

The Woodward-Hoffman rules for electrocyclic reactions are as follows:

$\begin{array}{ccc}\mathrm{Number}\mathrm{of}\mathrm{\pi }\mathrm{bonds}& \mathrm{Thermal}\mathrm{reaction}& \mathrm{Photochemical}\mathrm{reaction}\\ \mathrm{Even}& \mathrm{Conrotatory}& \mathrm{Disrotatory}\\ \mathrm{Odd}& \mathrm{Disrotatory}& \mathrm{Conrotatory}\end{array}$

It describes the kind of rotation the orbitals undergo under different conditions.

Electrocyclic reactions of the triene under thermal conditions

The triene (a) undergoes disrotation, yielding a cis product.

Electrocyclic reaction of a under thermal conditions

The triene (b) undergoes disrotation, yielding a trans product.

Electrocyclic reaction of b under thermal conditions

Electrocyclic reactions of the triene under photochemical conditions

The triene (a) undergoes conrotation, yielding a trans product.

Electrocyclic reaction of a under photochemical conditions

The triene (b) undergoes conrotation, yielding a cis product.

Electrocyclic reaction of b under photochemical conditions