It is apparent that a photochemical reaction has the advantage of using a green reagent, that is, light, in many cases, even natural light.1 Thus, promoting a reaction photochemically rather than thermally is "greener." As an example, rearranging an aryl ester to a hydroxyarylketone by shining light in a vessel sounds much more acceptable from the ecological point of view than carrying out the same reaction by adding an equimolecular amount of an unpleasant and waste-producing reagent, such as aluminium trichloride. However, many aspects should be considered before concluding that this is a useful perspective for industrial application. Let us first of all explore the potentiality of the method.
Photochemical reactions involve electronically excited states.2 These are high energy, if extremely short-lived, species. Thus, only reactions involving very small activation energies occur before physical decay to the ground state. The situation is completely different from that of thermal chemistry. In the latter case, the effort is toward "activating" the substrate, for example, using a strong nucleo-phile to make a reluctant (weak) electrophile react at a reasonable rate. Excited states are very energetic and have a different electronic distribution with respect to ground states. This often results in a new reaction coordinate becoming accessible; usually, however, this statement holds only for a single path, which
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involves surmounting a low enough activation energy, as the other energies are incompatible with its lifetime. The combination of high energy and the short lifetime makes reactions from the excited state both extremely fast and quite selective.
The photochemical reactions of a given molecule differ from those in the ground state and usually are much less affected by the medium, since they are much faster. During the last decades, a large body of knowledge has emerged, and now photochemical reactions are well organized and classified, as is done in thermal chemistry, according to the functionality involved in the organic molecule.2 Thus, these reactions have become part of the palette chemists can use when planning a synthesis, as in the examples presented in this chapter.
Many new paths are open. Will these be used in industrially applied processes? The answer is probably negative in the near future. The problem mainly lies in having the reagent absorbing light in the desired way. In the laboratory, photochemical reactions are usually carried out in a very simple apparatus—a cylindrical vessel into which the lamp is inserted—and using diluted solutions in order that light absorption takes place homogeneously across the vessel. To make the process acceptable for industry, much more concentrated solutions should be used, which requires the use of a more elaborate reactor, for example, a falling film reactor, where a thin film of the solution is irradiated, thus minimizing side processes. In addition, the reactor must be engineered for maximal light absorption; otherwise, carrying out the reaction will be too expensive in terms of the use of electricity. At any rate, an appropriate reactor needs to be built and this is obviously not economically reasonable as long as thermal alternatives are available, even when these involve some environmental disadvantage. If a sufficiently large series of processes for which no such alternative exists are to emerge, then photoinitiated processes will be applied, as already occurs for a (very limited) number of examples. In the meantime, and in view of the fact that more stringent regulations will foster the development of all fields of green chemistry, photochemistry should at least be considered something more than a mere laboratory curiosity.
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