The photobromination of cinnamic acid

This reaction is chosen as an example of still a third class of photochemical reaction-—one in which the quantum yield is considerably greater than unity. It is a typical chain reaction. 

The bromination of cinnamic acid dissolved in carbon tetrachloride or other inert solvent.offers a convenient system for study. The dibromocinnamic acid produced remains in the carbon tetrachloride solution. The thermal reaction is so slow that it can barely be measured at room temperature and it is entirely negligible in comparison with the photochemical reaction at ordinary intensities. The quantum yield is so large that considerable reaction occurs even if the intensity of light is much reduced by the monochromator or other device for confining the light to a narrow range of frequencies. Furthermore, the reaction is easily and accurately followed by titration with sodium thiosulfate. Potassium iodide is added and the iodine liberated is a measure of the remaining bromine. 

The reaction has been studied by several investigators and the influence of concentration and other factors on the quantum yield have been made the subject of an extended study.39 

Experimental Results. The general procedure was similar to that already described. With the monochromator utilizing a large hollow, glass prism filled with ethyl cinnamate the quartz capillary lamp gave about 25,000 ergs per second of quite pure radiation 

The quantum yields were calculated in the standard manner as illustrated with the following example. 

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According to another hypothesis frequently suggested, the bromine atoms combine with bromine molecules to give triatomic bromine molecules which are rather unstable and after reaction they regenerate the bromine atoms, ready to repeat the cycle. This chain was applied to the photobromination of cinnamic acid by Purka-yastha and Ghosh.41... 

The behavior of a photochemical chain reaction can sometimes be described rather simply in terms of the number of photons absorbed and the chain length. In the photobromination of cinnamic acid it is seen from Fig. 32 that the quantum yield is nearly proportional to the concentration of bromine cB in moles per liter. Then as an approximation... 

Example Photobromination of cinnamic acid to dibromocinnamic acid was carried out in blue light of wavelength 440 nm at 310 K using light intensity of 1.5 10 3 J per second. An exposure of 20 minutes produced a decrease of 0.075 millimole of bromine. The solution absorbed 80 per cent of the light passing through it. Calculate the quantum yield of the reaction. 

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