Photohalogenation

Photohalogenation. Photochemical chlorination of aUphatic hydrocarbons has been the textbook example of industrial photochemistry for decades (45). As of tiie mid-1990s it is still coimneicially impoitant and industiial-scale lialogenation has been reviewed in detail (1). In most examples of... 

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,... 

The most innovative photohalogenation technology developed in the latter twentieth century is that for purposes of photochlorination of poly(vinyl chloride) (PVC). More highly chlorinated products of improved thermal stabiUty, fire resistance, and rigidity are obtained. In production, the stepwise chlorination may be effected in Hquid chlorine which serves both as solvent for the polymer and reagent (46). A soHd-state process has also been devised in which a bed of microparticulate PVC is fluidized with CI2 gas and simultaneously irradiated (47). In both cases the reaction proceeds, counterintuitively, to introduce Cl exclusively at unchlorinated carbon atoms on the polymer backbone. 

The photoadditions of halogens, hydrogen halides, and alkylhalides to olefins have been extensively documented.<107) Photohalogenation reactions occur by absorption of light by the halogen, leading to excitation of a non-bonded p electron to an antibonding a excited level, followed by decomposition of the molecule into free radicals ... 

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

Photonitrosation is related to photohalogenation the absorbing species is nitrosyl chloride (NOCI), the reactive intermediate is a... 

In the analogous series of halogenation reactions with acetylenylcyclopropane (54), the main products were unrearranged ( )- and (Z)-(l,2-dihalovinyl)cyclopropanes. The presence of minor amounts of l-halo-2-(halomethylene)cyclobutanes indicates that both ionic and free-radical pathways are operative in photohalogenations with molecular halogens. 

The working temperature of tungsten-filament incandescent lamps lies between 2200 and 3000 K. Therefore, they emit light mostly in the visible and infrared parts of the spectrum. Such a source of radiation may be useful in photoreactions of coloured chromophores, for example in the photodissociation of bromine or chlorine molecules to initiate photohalogenation reactions (Section 6.6.1). 

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... 

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... 

Unfortunately, in most photohalogenation reactions other than those involving chlorine, the competing reactions of dissociation to heat, light,... 

As pointed out in the section on photohalogenation, the use of a polar solvent can materially alter the course of a liquid-phase photoehlorination This has been indicated in experimental work by Gonze and Britton et al. and is claimed by Neubauer et al. to give up to 30 per cent gamma isomer content in the mixture of isomers. 

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