Halogen compounds photohalogenation

Photohalogenation a photoinitiated reaction between a halogen donor, such as a halogen molecule and a substrate proceeds typically via a radical chain mechanism therefore, the reaction quantum yield, closely related to the average chain length, is greater than unity.155 

The absorption spectra of chlorine and bromine molecules have 2max at 330 nm (s = 651 mol 1 cm and 420 nm =1651 lmol 1 cm ), respectively.155 Since their dissociation energies are 243 and 192 kJ mol which correspond to photons of 492 and 623 nm wavelength, respectively, they readily undergo homolytic fission upon excitation, even when using visible (incandescent) low-power radiant sources. 

In contrast to photochlorination, the corresponding hydrogen abstraction or addition steps in photobromination are usually reversible, endothermic and more selective.155 The hydrogen abstraction rates in the allylic or benzylic positions can be relatively high, however. Table 6.19 shows the relative reactivities of the benzylic C—H bonds in a series of alkylbenzenes in the photobromination reaction carried out in CCI4.1304 It is apparent that not only electronic but also steric effects control the reaction kinetics. 

Case Study 6.34 Organic synthesis - photobromination of progesterone 

Although organic photochemistry has experienced remarkable growth, applications of photochemical synthetic methods in the chemical industry have been mostly limited to radical reactions, such as photohalogenation (this section), photopolymerization (Section 6.8.1) and to some extent photosulfochlorination, photooxidation (Section 6.7) and photonitrosylation, although some other reactions are also being used.155 

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A more energy-efficient variation of photohalogenation, which has been used since the 1940s to produce chlorinated solvents, is the Kharasch process (45). Ultraviolet radiation is used to photocleave ben2oyl peroxide (see Peroxides and peroxide compounds). The radical products react with sulfuryl chloride (from SO2 and CI2) to Hberate atomic chlorine and initiate a radical chain process in which hydrocarbons become halogenated. Thus, for Ar = aryl,... 

In presence of unsaturated compounds, chain reactions can be generated by halogen atoms. For photohalogenation reactions... 

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