Dinitration of BBenzylpyridine: Microscale Synthesis of a Photochromic Compound Elizabeth D. Gilfillan and Michael W. ~ e l t e r ' Purdue University Calumet Hammond, IN 46323
photochromic properties of the product and the hazardous nature of the reactants, this synthesis is ideal for the microscale laboratorv. Less of the toxic. irritant 2benzylpyridine is required, and the small quaAtity of anhydrous nitric acid can be prepared by the students immediately prior to use (5).
Preparation of 2-[(2,4-Dinitrophenyl)methyl]pyridine In a n attempt to address a common complaint from orAll reagents used in the experiment are available from ganic chemistry students that the products are either colAldrich Chemical or Fisher Scientific and were used withorless liquids or white crystals, we have adapted the synout further purification. thesis of 2-[(2,4-dinitrophenyUmethyllpyridine(1) to In a 5-mL conical vial containing a magnetic spin vane microscale techniques (eq 1).This synthesis of this comi b ~(1) b i ~ and fitted with an air condenser is placed 0.5 mL of wncenpound was first reported by ~ ~ ~ h i t ~ ~inh1925 trated sulfuric acid. The vial is cooled to below 5 'C in icewere studied in the early and the p ~ o t o c ~ r o mproperties ic salt bath with stirring and 80 pL (0.50 mmol) of 21960's (2). benzylpyridine is added dropwise. While keeping the temperature below 5 'C, 150 pL (calibrated Pastenr pipet) of freshly prepared anhydrous nitric acid (5)is slowly added. The ice bath is removed, and the re(1) action vessel is heated in a sand bath at 80 'C with stirring for 20 min. The warm reaction mixture is poured onto NO2 1 -1-2 g of crushed ice in a 50-mL Erlenmeyer flask. The 5-mL conical vial is In the absence of light, crystals of 1 are tan in color. rinsed with two l-mL portions of ice water and these are When exposed to sunlight, the crystals turn a deep blue added to the contents of the Erlenmeyer flask. The flask is color within a few minutes. Loss of color occurs over the placed in a n ice bath and, with stirring, 2 M NaOH is period of a day when the crystals are returned to the dark. added dropwise until the pH is above 12 (approximately The interconversion is apparently indefinitely reversible. 8-12 mL) and the mixture is a milky white color. ImThe source of the photochromic behavior is a tautomeric mediately, 10 mL of ether is added, and the two-phase mixtransfer of a benzyl hydrogen to an oxygen atom of the ture is stirred for 30 min to extract the product into the ortho nitro group (eq 2) (2). organic layer. The aqueous layer is removed with a Pasteur pipet, and the organic layer is dried over sodium sulfate, transferred via Pasteur filter pipet to 5-mL conical vial (in portions), and concentrated (I.0.1 mL) on a (') warm sand bath in the hood using a slow stream of nitrogen gas. Addition of a minimum amount (
