Photohydration of Pyridine Modification of an Undergraduate Kinetics Experiment Gerald H. Morine Bemidji State University, Bemidji, MN 56601 In an article in this Journal, Andre et al.' described an undergraduate experiment on the photochemical kinetics resulting from the photolysis of pyridine in aqueous solution. The pyridine photlyzes to give a bicyclic intermediate, Dewar pyridine, which undergoes hydrolysis to give a conjugated amine, observable by its absorption at 360 nm. My students have done this experiment for five years and some modifications in the original procedures1 have been found useful to get amounts of product accurately measurable with short photolysis times and with a consistent yield. pH Control Use of a pH 10 buffer gives the desired product more consistently than the p H 8 buffer used by Andre, et al.' When the photolysis is done a t pH 8, another absorbance sometimes appears a t 305 nm, with reduction in the desired absorbance a t 360 nm. We have used the standard NaHC03 pH 10 buffer2 with excellent results. Increasing the Yield When there are many students in the class, minimizing the photolysis time can be important. The low-cost germicidal lamp (low-pressure mercury arc) used produces the 254-nm photolyzing radiation in relatively small amounts compared to the photolysis lamps usually used in photochemistry research laboratories. The lamp and sample are entirely enclosed, but the students wear UV-absorbing safety goggles in case of accidental exposure. Addition of "inert" salts to the solutions before p h o t o l y ~ s increases the yield of pyridine photolysis product substantially. Making the solution 3.0 M in LiCl or CaC12 increases the rate of product formation by factors of 3.1 and 2.0, respectively. This is not due to asimple ionic strength effect,
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since addition of KC1 in this concentration has almost no effect on the yield. Possible explanations for this increase include the effect of changes in the activity coefficients on the rate of the Dewar pyridine photoproduct's hydrolysis to the final observed product or deceleration of the dark reaction that reforms nvridine from nart of the Dewar nvridine. The aqueous sample can be in a shallow iden dish, such as a Petri dish top, and photolyzed from above, instead of being in a quartz tube photolyzed from the side.' This increases the exposed surface area of the solution exposed to the most intense light is increased and the "internal filter" effect decreased. This photolysis arrangement works well since this reaction is not sensitive to air.' Use of a magnetic stirrer bar in the dish circulates pyridine molecules to the surface, where the light is most intense. This apparatus removes the need for relatively expensive and fragile 254nm-transmittine ouartz reaction tubes. Photolvsis lieht reflection losses are also redured by this methnd," both'hue to the lack of curved surfaces and the different interface the light must cross. Acknowledgment Undergraduate students Patricia Munnis carefully tested the open dish method and Linda Nor,ak Chandler did numerous salt solution pbotolyses with great skill and dedication. 'Andre, J. C.: Niclause. M.; Joussot-Dubien, J.; Deglise. X. J. Chem. Educ. 1977,54,387-388. Weast, R. C., Ed. CRC Handbook of ChemistryandPhysics, 65th ed.: CRC: Boca Raton. FL 1984; D D-149. Skoog. D. A. Principles of Instrumental Analysis. 3rd ed.; Saunders: Chicago, 1985; pp 101-102.
