R. E. Bozak
and V. E. Alvarez California State College Hayward, California 94542
The Photoaddition of Maleic Anhydride to Benzene A simple organic experiment in
The current flurry of research activity in organic photochemistry has not found its way into the first year organic laboratory. The photochemical synthesis of benzopinacol, first described in 1900, persists as the sole pedagogical example of this fascinating field. The present, easy to perform, experiment may help to rectify the situation. The normal reluctance of the dienophile, maleic anhydride to surmount the resonance stabilization and add to benzene is overcome here by photosensitization with henzo~henoneand suhseauent 1.2-addition. The resultant 1:1 product (see equations below) now con-
a complicated system
taining a l,&diene, then enters into a standard DielsAlder Reaction with a second, hut ground state, maleic anhydride to form the insoluble, high melting photoadduct (I). Prepare a solution of 5.0 g (0.051 moles) maleic anhydride (CAUTION-take care not t o get any maleie it, hydride on your s'iin or in your eyes), and 2.0 g (0.001 moles) bennophenone in 100 ml of benzene by gently refluxing the mixture an a steam bath for -15 min. Gravity filter all or a portion of the solution (The insoluble material is maleic acid) into a Pyrex Petri dish' (size 90 mm X 40 mm) and carefully seal the exposed edge, using an appropriate size Pyrex watch glass and masking tape. The watch elms should have its convex side uo and the height of the solution rn the Petri dish should not exceLd -30 m m . ' W r a ~ sluminum foil around the outside of the Petri dish and half way up its side (see figure). Set the solution in direct sunlight for -7 d. ~~~
~
Apporotur for photooddition, A, Pyrex watch gloss; B. niasking tope; C, aluminum foil.
After exposure, filter the resultant crystals by suction filtration using No. 2 filter paper, wash with benzene and suck dry for -20 min. The yield under these conditions is usually 1.0 g. The observed melting point of the photoadduct in an inert atmosphere is 355-358' (Lit. 356"1)-using Dow Corning silicone oil No. 730 in a Thiele tube. Although no decomposition is mentioned in the literature, we have observed that "browning" of the compound starts around 300% 1 Use of a Petri dish is not mandatory-other suitable vessels include an inverted Florence flask, a n Erlenmyer flask, etc. 2 A ~ o u s H. , J. F., AND BBYCE-SMITH, D., Pme. Chem. Soe., 327 (1959).
Volume 47, Number 8, August 1970
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