Carbon tetrabromide, photolysis

Another halogenated photolysis (30), using carbon tetrabromide to produce hydrogen bromide and subsequent reaction with spiropyran (5), produces a highly colored spiropyrilium bromide salt. 

At very short times, very little motion of reactants has occurred so that little, if any, reaction will have taken place. But the manner of creation of the mixture of A and B reactants should be considered. A very simple means of preparing a reaction mixture is by photolysis. For instance, consider a solution of anthracene and carbon tetrabromide. Photostimulation of anthracene with an extremely short duration light pulse produces excited singlet (and triplet) states. The carbon tetrabromide quenches the excited singlet state fluorescence very efficiently. Just before the photostimulation event, the quencher (i.e. B) is randomly distributed throughout the system volume and for a short time after photostimulation, it remains randomly distributed. With the exception of the location where the fluorophor A is, there is no preferred location of the quencher B. No... 

Historically, the first color-forming reaction to be discovered which involves electron transfer is probably the photoinitiated reaction of diphenylamine with carbon tetrabromide, which forms blue colors [42]. In fact, the major path for color formation is due to radical reactions, initiated by photolysis of C—Br bonds to produce Br3C and bromine atoms. An alternative mechanistic path, possible when the light is absorbed by the diphenylamine, would involve electron transfer. MacLachlan has shown that such processes do occur durin> photolysis of aminotriarylmethanes in the presence of electron acceptors such as CBr4 and CC14 [43]. Other electron deficient species (quinones, nitroaromatics) were also demonstrated to be effective. 

Other methods for ketene generation that are occasionally used are conceptually similar to the elimination of acyl chlorides but use different carboxyl activating groups. Activation of a carboxylic acid by Mukaiyama s reagent, for example, followed by treatment with triethylamine to generate a ketene in situ, has been used on occasion. A very mild method for ketene formation involves treatment of the carboxylic acid with triphenylphosphine and carbon tetrabromide in the presence of the imine. The photolysis of metal-carbene complexes, particularly chromium carbonyl carbenes, has been used but this necessarily involves more effort in the preparation of the necessary ketene precursor. ... 

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