Photoreduction of Benzophenone
The study of chemical reactions, isomerizations and physical behavior that may occur under the influence of visible and/or ultraviolet light is called Photochemistry. The fundamental principles for understanding photochemical transformations are that light must be absorbed by a compound in order for a photochemical reaction to take place, and that for each photon of light absorbed by a chemical system only one molecule is activated for subsequent reaction. This "photo equivalence law" was derived by Albert Einstein during his development of the quantum (photon) theory of light. Absorption of visible and/or ultraviolet light by a molecule transfers all the energy of the photon to one molecule. This is sufficient energy to excite one bonding or non-bonding electron from its stable ground-state orbital arrangement to an Excited State orbital. The electrons then reorganize their bonding to stabilize the excited state. In some instances bonds get longer or shorter, in other instances a covalent bond may be broken. These changes depend on the energy of the light absorbed by the molecule (which is proportional to its wave number and inversely proportional to its wavelength). From the relationship E = hc / λ, we can calculate that longer¬–wavelength visible light (400 to 800 nm) is less energetic (70 to 40 kcal/mole) than Ultraviolet light (200 to 400 nm; 150 to 70 kcal/mole). Consequently, ultraviolet light is most often effective in producing photochemical change. The light required for a photochemical reaction may come from many sources. Giacomo Ciamician, regarded as the "father of organic photochemistry", used sunlight for much of his research at the University of Bologna in the early 1900's. One of the oldest and most studied photochemical reactions is the photoreduction of benzophenone (diphenyl ketone). It was discovered that solutions of benzophenone are unstable to light when certain solvents are...
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