Question

Account for the difference in the products obtained under photochemical and thermal conditions:

Short answer

a.

b.

Step-by-step solution

Product in photochemical condition 

In the given reaction, the reacting system has three double bonds that are a total of six pie electrons (even number of electrons); thus, under the given photochemical condition, suprafacial rearrangement will take place 1,3 hydrogen shift occurs, which involves four electrons.

A [1,3] sigmatropic migration of deuterium will result in the formation of the product shown as follows:

Step 2: Product in thermal condition

In the given reaction, the reacting system has three double bonds that are six pie electrons (even number of electrons); thus, under the given thermal condition, suprafacial rearrangement will take place 1,5 hydrogen shift occurs, which involves six electrons.

A [1,5] sigmatropic migration of deuterium will result in the formation of the product shown as follows: