Aromatic compounds photochemistry

Other than energy considerations, on which there is little comparative data, the most important green role for photochemistry is in improving atom economy. Although only a preliminary research result, an excellent example of this is the avoidance of the need for stoichiometric amounts of Lewis acid catalysts in the synthesis of some acylated aromatic compounds. Benzoquinone can be reacted with an aldehyde under a sunlamp to yield an acylhydroquinone in up to 88% yield. The alternative procedure would involve reaction of an acyl chloride with hydroquinone and a... 

Aromatic substitution, a quantitative treatment of directive effects in, 1, 35 Aromatic substitution reactions, hydrogen isotope effects in, 2, 163 Aromatic systems, planar and non-planar, 1, 203 Aryl halides and related compounds, photochemistry of, 20, 191 Arynes, mechanisms of formation and reactions at high temperatures, 6, I A-Se2 reactions, developments in the study of, 6,63... 

A comprehensive treatment of nitro-compound photochemistry is not attempted here and would be beyond the scope of this volume. The discussion is thus restricted to aromatic and heteroaromatic nitro compounds. The reactions preferentially reviewed are those for which at least some experimental information has been given on efforts to elucidate the multiplicity of the reacting excited state, or those in which intermediate excited triplet states seem highly probable for various reasons. 

Chapter 2, by Miranda and Galinco, provides a critical survey of the photo-Fries reaction undergone by numerous aromatic esters, amides, and so forth. This chapter is a valuable companion to Chapter 5 by Fleming and Pincock, in Volume 3. Miranda and Galinco s chapter is the sixth chapter devoted to the photochemistry of aromatic compounds in this series. 

James N. Pitts, Jr., is a Research Chemist at the University of California, Irvine, and Professor Emeritus from the University of California, Riverside. He was Professor of Chemistry (1954-1988) and cofounder (1961) and Director of the Statewide Air Pollution Research Center (1970-1988) at the University of California, Riverside. His research has focused on the spectroscopy, kinetics, mechanisms, and photochemistry of species involved in a variety of homogeneous and heterogeneous atmospheric reactions, including those associated with the formation and fate of mutagenic and carcinogenic polycyclic aromatic compounds. He is the author or coauthor of more than 300 research publications and three books Atmospheric Chemistry Fundamentals and Experimental Techniques, Graduate School in the Sciences—Entrance, Survival and Careers, and Photochemistry. He has been coeditor of two series, Advances in Environmental Science and Technology and Advances in Photochemistry. He served on a number of panels in California, the United States, and internationally. These included several National Academy of Science panels and service as Chair of the State of California s Scientific Review Panel for Toxic Air Contaminants and as a member of the Scientific Advisory Committee on Acid Deposition. 

Aromatic compounds have a special place in ground-state chemistry because of their enhanced thermodynamic stability, which is associated with the presence of a closed she of (4n + 2) pi-electrons. The thermal chemistry of benzene and related compounds is dominated by substitution reactions, especially electrophilic substitutions, in which the aromatic system is preserved in the overall process. In the photochemistry of aromatic compounds such thermodynamic factors are of secondary importance the electronically excited state is sufficiently energetic, and sufficiently different in electron distribution and electron donor-acceptor properties, ior pathways to be accessible that lead to products which are not characteristic of ground-state processes. Often these products are thermodynamically unstable (though kinetically stable) with respect to the substrates from which they are formed, or they represent an orientational preference different from the one that predominates thermally. 

Radical substitution plays a part in the thermal chemistry of aromatic compounds, but not in the photochemistry, except in so far as many radicals that attack aromatic compounds are generated by photochemical methods from other addends. The reason for this is that reactive radicals exist only in low concentrations, and electronically excited states similarly are formed only in low concentrations the rate of bimolecular reaction between two such reactive species is generally much lower than the rates of alternative processes such as attack of the radical on ground-state aromatic compounds. 

A. Gilhert, in W. M. Horspocl (ed.). Synthetic Organic Photochemistry, Plenum 11984). Photoaddition, photocycloaddition and photocyclization processes of aromatic compounds are all covered in this account of synthetic aspects of aromatic photochemistry. 

Most of the fairly extensive photochemistry of aromatic compounds has not been studied in sufficient detail to permit disentanglement of singlet and triplet mechanisms. Theoretical calculations indicate that the pattern of substituent effects on side-chain reactions of excited disubstituted benzenes should be quite different from that observed in the ground states of the molecules. One problem associated with these predictions is the question of whether or not they are appropriate for triplets as well as for corresponding singlet excited states. Consider the following system ... 

The excited states arising in the photolysis of aromatic compounds provide a good starting point. It should be noted in this connection that the expectation that benzenoid excited states differ from the parent ground states both in electronic structure and reactivity has been commented on previously in the literature (2.3) however, a general link between quantum mechanical description and experimental photochemistry lias been needed. 

Mizuno K, Maeda H, Sugimoto A, Chiyonobu K. Photocycloaddition and photoaddition reactions of aromatic compounds. In Ramamurthy V, Schanze KS, eds. Molecular and Supramolecular Photochemistry. Vol 8. New York Marcel Dekker, Inc., 2001 127-316. 

---