A Photochromic Compound: A Preparation for the Introductory Organic Laboratory Crystals of the compound 2-(2.4-dinitrobenzy1)pyridine (I) have the most unusual property of turning a very deep blue color when exposed to sunlight, and of reverting to their original sandy color during storage in the dark. Color formation takes onlv a few minutes in hrieht - sunliebt. - . hut the loss of color takes about a dav. The interconversion aopears to he completely reversible any number of times. One explanation for this phenomenon is the reversihleformation of the tautameric form (II).'
The photochromic compound (I) is easily prepared by dinitration of the commercially available 2-benzylpyridine.
Procedure Place 25 ml of cone. sulfuric acid (0.45 mole) in a 250-ml boiling flask. Cool the acid t o 5" or below by means of an ice bath. Arrange the flask so that it can be stirred or swirled in a n ice bath and add gradually with good mixing 5.0 ml (5.3 g; 0.031 mole) of 2-henzylpyridine. To this coaled and wd-stirred mixture add dropwise over a period of about 3 min 3.0 ml of red fuming nitric aeid (density = 1.5 g/ml; 0.070 male). The addition of the first few drops of the nitric acid will cause a color change to deep brown, but the mixture will become lighter in color as the remainder of the aeid is added. After the nitric acid has all been added, heat the mixture for about 20 minon the steam bath. At the end of the heating period, pour the mixture onto about 400 g of ice in a 2-1 flask. Basify the solution to a pH of about 11 by adding almost all of a solution of 40 g (1 mole) of sodium hydroxide in about 500 ml of water. Toward the end of the addition of base, the product separates to give a milky yellow suspension. Add about 400 ml of ether and stir the mixture for 10-15 min to extract the product into the ether layer. Separate the ethereal solution, dry i t over anhydrous magnesium sulfate for a few minutes, filter, and distill to reduce its volume to about 50 ml. The product will sometimes crystallize during the concentration of the solution. Complete the crystallization by cooling the mixture in an ice bath, collect the large sandy prisms by suction filtration, and wash them with a small amount of cold 95% ethanol. Yield: about 4 g (50%). The product may be recrystallized from 95% ethanol with 90% recovery using about 10 ml/g; use bf decolorizing carbon helps a little. Wousa, J. A,, and Weinstein, J., J. Org. Chem., 27, 3155 (1962); Bluhm, A. L., Weinstein, J., and Sousa, J. A,, J. Org. Chem., 28,1989 (1963). Addison Ault Craig Kouba Cornell College
Mt. Vernon, Iowa 52314
Volume 5 1 , Number 6. June 1974
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