Photochemical halogenations

An extraordinary variety of reactions of organic compounds are known to occur under the influence of visible and ultraviolet light. Some of these, such as the photochemical halogenation of alkanes and photosynthesis in green plants, already have been discussed (see Sections 4-4D and 20-9). It is notour purpose here to review organic photochemistry in detail — rather, we shall mention a few types of important photochemical reactions and show how these can be explained by the principles discussed in the preceding section. 

To differentiate between these alternative mechanisms, H. C. Brown, M. S. Kharasch, and T. H. Chao, working at the University of Chicago, carried out the photochemical halogenation of optically active S-(H-)-l-chloro-2-methylbutanc. A number of isomeric products were, of course, formed, corresponding to attack at various positions in the molecule. Problem What were these products ) They focused their attention on just one of these products i,2--dichloro-2-mcthyl-butane, resulting from substitution at the chiral center (C-2). 

Figure 7.17 Photochemical halogenation of monosaccharide derivatives (gluco, manno and galacto, but not talo, compounds were investigated).

Photochemical halogenations may also be convenient on the laboratory scale. As an example, benzylic bromination is conveniently carried by using HBr/H202 under irradiation with a common incandescent lamp (see Scheme 2.22)[37]. Apropos halides, it may be added that fragmentation of the carbon-halogen bond has been considered above for aromatic derivatives, but is useM also for aliphatic derivatives. A typical example is the photocyclisation of iV-phenyl chloroaceta-mides and Af-arylalkyl chloroacetamides (see Scheme 2.23)[38]. 

Halogenation. Liquid-phase monochlorination of ben2otrifluoride gives pronounced meta orientation (295) in contrast, vapor-phase halogenation favors para substitution (296). Sealed tube, photochemical, or dark chlorination (radical initiator) forms... 

Halogen radicals account for about one-third of photochemical ozone loss observed in the spring in the lower stratosphere (below 21 km) at 15—60°N latitude (76). The following three cycles (4—6) are the most important. Rate constant data are given in Reference 11. 

In the 1960s materials became available which are said to have been obtained by chlorination at lower temperatures. In one process the reaction is carried out photochemically in aqueous dispersion in the presence of a swelling agent such as chloroform. At low temperatures and in the presence of excess chlorine the halogen adds to the carbon atom that does not already have an attached chlorine. The product is therefore effectively identical with a hypothetical copolymer of vinyl chloride and symmetrical dichloroethylene. An increase in the amount of post-chlorination increases the melt viscosity and the transition temperature. Typical commercial materials have a chlorine content of about 66-67% (c.f. 56.8% for PVC) with a Tg of about 110% (c.f. approx. 80°C for PVC). 

The replacement of hydrogen with halogen takes place under thermal or photochemical conditions [J9, 20, 21] (equations 10-12). 

Thts chapter includes migration of halogen atoms, catalyst assisted reatrange ments, and thermal and photochemical rearrangements... 

In cases where hydrogen atom transfer gives primarily reduced products, Bu3Sn-SnBu3 under photochemical generates the radical that can cyclize, but a halogen atom transfer agent such as iodoethane is usually present (see 15-44). 

The biological activity of several halogenated herbicides in water is destroyed by ultraviolet irradiation (18). Irradiation seems to be a promising method for decontaminating small quantities of pesticides. The chemical similarity between the chlorinated dioxins and other chlo-rinted aromatic compounds suggested that if there were parallels in their photochemical behavior, sunlight might destroy dioxins in the environment. 

Photons are very clean reagents, leaving no residues. A photo-initiated process therefore has potential advantages even when compared to reactions initiated by the use of catalysts. Such processes may use fewer raw materials compared to non-photochemical alternatives, e.g. comparison of photo- and radical-initiated halogenation reactions. 

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