Ernesl F. Silversmith
Morgan State College Baltimore, Maryland 2 1 2 1 2
Demonstration of Photochemistry and -
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the Dimerization and Trapping ot Free Radicals
Most photochemical reactions are not ideally suited for use as lecture demonstrations. For example, the cis-trans isomerization of azobenzene' and photoreduction ,of henzophenone2 require 2 and "at least 24" hr, respectively. The reaction between hydrogen and chlorineSrequires fairly elaborate equipment that is difficult to transport, and the reaction involves the danger of an explosion. A reaction discovered by Hayashi and Maeda,' the chemistry of which was elucidated by White and Sonnenherg: is the basis of a safe demonstration utilizing very simple equipment. The reaction involves a rapid, readily noticeable color change. Another advantage is the fact that the dimerization and trapping of free radicals can be demonstrated with the same system. Lophine (2,4,5-triphenylimidazole,available from Aldrich Chemical Co., Inc. or Fisher Scientific Co.) is oxidized with aqueous potassium ferricyauide in ethanolio potassium hydroxide: yielding the 4,4'-dimer of the 2,4,5-triphenylimidazyl radical5 (referred to as the "piezochromic dimer" by White and Sonnenberg5). This compound is stirred with benzene (0.3 g per 100 ml works well, hut the ratio is not critical) until all the solid dissolves. The resulting deep violet color is due to the 2,4,5-triphenylimidazyl radi~al.'.~ The solution is transferred to a screw-capped jar (a thin polyethylene sheet between jar and cap slows loss of benzene by evaporation). The jar is kept in the dark (in a box or drawer, or by wrapping with foil) for 45 min or longer. After this time the solution is pale yellow, as the radical has dimerized to give the 1,2'-dimels,8 (referred to as the "photochromic dimer" by White and Sonnenherg5). This solution is used for the demonstration. When this solution is irradiated (bright sunlight, a sunlamp or an overhead projector can be used), the
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violet color the 2,4,5-triphenylimidazyl radical forms in a few seconds. Thus, the photochemical reaction is readily apparent to the audience. The violet solution can then be put into the dark for 15-30 miu; the color will revert to pale yellow,B demonstrating dimerization of the radicals.' Also, the violet solution can be treated with a small amount of a radical trap (e.g., hydroquinone or diphenylamine), causing the violet color to disappear instantaneously. The discussion accompanying the demonstration can he adjusted to the level of sophistication of the audience. The author has used it successfully with junior high school groups by stressing the fact that it is an example of interaction of matter and energy and the conversion of one form of energy into another.
~HELMKAMP, G . K., AND JOHNSON, H. W., JR., "Selected Experiments in Organic Chemistry," (2nd Ed.), W. H. Freeman and Co., San Francisco, 1968, p. 32. 2 M o ~ n r J. ~ ,R., AND NECKEBS, D. C., "Laboratory Experiments in Organic Chemistry," Reinhold Book Corp., New York, 1968, p. 138. a FuRQASON,C . M., AND MOORE, J. W., J. CHEM. EDUC.,20.41 (1943). (a) HAYABHI, T.,AND MAEDA, K., Bull. Chem. Soc. Japan, 33, 565 (1960). (b) 35, 2057 (1962). J., J. Am. Chem. Soc., 88, WHITE,D. M., AND SONNENBERG, 3825 (1966). The solution may hhrwe a. faint pink tinge because of the presence of a small amount of radidiod in thermal equilibrium with dimer. 7 Another demonstration involving a photo-induced color J. H., change has been described by GROTE, J. W., AND BARNETT, J. CHEM.EDUC.,10, 43 (1933). However, the reverse reaction requires 6-8 hr or more and the chemistry of the reaction is quite complex and apparently not completely known; see BAUDISCH, O., Science, 108, 443 (1948).
Volume 47, Number 4, April 1970
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