Photochemistry of Aryl Halides

Authored by: Luca Pretali , Angelo Albini

CRC Handbook of Organic Photochemistry and Photobiology

Print publication date:  March  2012
Online publication date:  March  2012

Print ISBN: 9781439899335
eBook ISBN: 9781466561250
Adobe ISBN:

10.1201/b12252-17

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Abstract

Aryl halides have an important role in photochemistry since a long time. Historically, studies of the phosphorescence of aromatics and of their halogenated derivatives have been instrumental in defining the modern concept of triplet state, and both the intersystem crossing [1] and the fragmentation of phenyl halides have been long known [2]. In the early years of these studies, attention was given more often to photophysics than to photochemistry. This is because the introduction of electron-withdrawing substituents, such as halogen atoms, in the ring further increases the already high stability of aromatic compounds, both in the ground and excited states. Indeed, many aromatic compounds are quite photo-stable and the absorption of a photon leads to reemission rather than to reaction, since excitation causes only a partial decrease of the bonding character in the multiple π-electron array of such compounds and the rigid σ-bond skeleton disfavors geometric modifications. Indeed, stability to chemical agents and to irradiation is characteristic of many aryl halides, and this family includes some notoriously persistent environmental pollutants, such as DDT and polychlorobiphenyls. However, photochemistry, albeit often inefficient, is one of the few available paths for the degradation of such compounds and thus this is an important topic in environmental studies. Furthermore, new discoveries are revealing new reactions and a more various palette of transformations is forming for aryl halides, as a part of the development of the studies on aromatics that form a fast growing field in contemporary photochemistry.

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