Question

on A PHOTOCHEMICAL SYNTHESIS PHOTOREDUCTION OF BENZOPHENONE TO BENZOPINACOL

Molecular oxygen is known to inhibit this photochemical reaction. Explain.

Answer 1

Photochemistry is the study of the chemical interactions which occur when molecules interact with light. Organic chemists are interested in photochemical preparations of molecules since light can act as a catalyst (or reactant), causing interesting reactions to occur. For example, we have previously discussed the use of ultraviolet light as a catalyst for the formation of radicals in the halogenation of alkanes. In this laboratory, we will use ultraviolet light from sunshine to promote a radical reaction. This particular reaction is not catalyzed by light; rather, it is driven by light. In other words, we can think of photons of light as being a needed reactant in this process.

The light is needed in this reaction to break the π bond between the carbon and oxygen in the carbonyl group of benzophenone forming a diradical. Of the bonds in the two reactants, this bond is weakest making it most susceptible to the energy provided by the ultraviolet light.

After the diradical forms it immediately becomes a scavenger, its oxygen atom quickly finding a relatively easily removed hydrogen in the 2° position in the plentiful solvent molecules. In this step the diradical becomes a benzhydrol radical, while the solvent molecule becomes a new radical.

This newly formed radical is now highly reactive, enabling it to offer a hydrogen atom to the oxygen of another benzophenone diradical, thus resulting in the formation of a stable molecule, acetone, as well as another benzhydrol radical.

Eventually one benzhydrol radical will collide with another leading to the formation of the final stable product.