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.