Photochemically Induced Substitution of Vinyl and Aryl Halides

Photochemically Induced Substitution of Vinyl and Aryl Halides 

Irradiation of both aryl and vinyl halides (bromides are commonly used) induces homolysis of the carbon-halogen bond and produces aryl radicals from aryl halides and (apparently) both vinyl radicals and vinyl cations from vinyl halides. 

photolysis of 1,2,2-triphenylbromoethene at 350 nmin ethanol (CH3CH2OH) yields products from both substitution (from vinyl cation) and reduction (from vinyl radical). These products are formed under conditions where, in the absence of light, no substitution occurs. 

Although details are obscure, it is presumed that weakening of the carbon-bromine bond (n n absorption) precedes heterolysis to a stabilized vinyl cation and bromide anion (Br ). The cation then reacts with ethanol (CH3CH2OH) and a 

The general observation that replacement of a hydrogen or carbon attached to the carbon-carbon double bond by halogen results in retardation of electrophilic addition reactions is justified by arguing that there is decreased electron density in the double bond. That is, halogen inductively withdraws electrons better than hydrogen or carbon (electronegativity, Chapter 1). 

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VIII. Photochemically Induced Substitution of Vinyl and Aryl Halides... 

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