In conclusion, we reiterate that photoinitiated reactions have a dramatic advantage with respect to thermal reactions, since light is the green reagent par excellence. If we consider the possibility that such reactions will be applied in the foreseeable future, and in particular whether they will contribute to the development of green chemistry, we can conclude that there are both indisputable advantages and difficult to solve problems.
Pro: Photochemistry offers an inexhaustible source of new chemistry as follows:
• Distinctive chemical reactions of the excited state;
• A method for the mild generation of radicals (or other highly reactive intermediates) from unconventional (and green) precursors;
• A universal method for carrying out redox processes between organic molecules, which has no parallel in thermal chemistry;
• In every case, the photochemical step is not very dependent on conditions, affording a great deal of freedom of choice in arranging the experimental parameters (medium, additives), and thus allowing the resultant steps to be guided according to the plan.
Con: Significant limitations for an effective application are as follow:
• Photoinitiated reactions often only occur in a clean way at low concentrations, while it is difficult to have them this clean with a reduced amount of solvent;
• Light absorption must be maximized for industrial application, in order to avoid an unacceptable cost, and the knowledge of how to confront this point is not widespread;
• These reactions require an ad hoc apparatus, which make industry reluctant to invest in this direction unless large families of advantageous photochemical syntheses are developed.
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