Answer:

Electrocyclic Reaction:

Electrocyclic reaction refers to the breaking and formation of bonds in a cyclic molecule. It is a type of pericyclic reaction that involves the reorganization of electrons in a cyclic system. The term "pericyclic" means that the reaction occurs through a cyclic transition state.

Conrotatory and Disrotatory:

In an electrocyclic reaction, the cyclic molecule can undergo two types of rotations: conrotatory and disrotatory. Conrotatory rotation involves the simultaneous clockwise or counterclockwise rotation of all the sigma bonds in a cyclic system. Disrotatory rotation involves the alternate clockwise and counterclockwise rotation of the sigma bonds in the cyclic system.

Thermal and Photochemical:

Electrocyclic reactions can occur under both thermal and photochemical conditions. Thermal reactions occur due to the energy provided by heat, while photochemical reactions occur due to the energy provided by light.

Even Number of pi-bonds:

If a cyclic molecule has an even number of pi-bonds, it will undergo a conrotatory electrocyclic reaction. This is because the conrotatory rotation of the pi-bonds results in the formation of a new pi-bond, which maintains the even number of pi-bonds in the system.

Conrotatory, Thermal Reaction:

If the conrotatory electrocyclic reaction occurs under thermal conditions, it requires a high activation energy and occurs slowly. The reaction is usually reversible, and the product can revert back to the starting material.

Conrotatory, Photochemical Reaction:

If the conrotatory electrocyclic reaction occurs under photochemical conditions, it requires less energy and occurs quickly. The reaction is usually irreversible, and the product cannot revert back to the starting material.

Disrotatory Electrocylic Reaction:

If a cyclic molecule has an odd number of pi-bonds, it will undergo a disrotatory electrocyclic reaction. This is because the conrotatory rotation of the pi-bonds results in the formation of an odd number of pi-bonds in the system, which is unstable.

Conclusion:

In conclusion, a molecule with an even number of pi-bonds will undergo a conrotatory electrocyclic reaction under both thermal and photochemical conditions. The reaction occurs through the rotation of the pi-bonds and results in the formation of a new pi-bond. The reaction can be reversible under thermal conditions and irreversible under photochemical conditions.