Chemical Education Today
Report
Cornell College Students Achieve the Highly Improbable
W
by Addison Ault
H N
H H
C
N
O
H
C
O
light
N + O-
Photo by Dawn Goodlove
On Tuesday, March 14, 2000, students in my organic chemistry class got ready to do one of my long-time favorite experiments, making 2-(2,4-dinitrobenzyl)pyridine. This is a “photochromic” compound that maintains its light-brown color in the dark but turns deep blue in the sun. When the crystals are returned to storage in the dark, they revert to their original sandy color. The color change from light brown to deep blue takes only a few minutes in bright sunlight, but it takes about a day for the deep blue to change back to light brown. The interconversion appears to be completely reversible any number of times.
N + O-
dark
Figure 1. Graham Pumphrey (left) and Rhonda Reisdorff pose with the photochromic crystals they synthesized. NO2
NO2
brown-sugar brown
blue-jeans blue
2-(2,4-dinitrobenzyl)pyridine
a
b
c
Students really enjoy this laboratory experiment, whether or not they get large crystals. I have sometimes had students who were not in the course ask to do this lab! Very rarely do students fail to obtain at least a small yield. (The typical yield looks like brown sugar crystals.) We have determined the cause of most failures as a mistake in measuring one the materials used. The photochromic material, 2-(2,4-dinitrobenzyl)pyridine, can be prepared from 2-benzylpyridine by treatment with a mixture of nitric acid and sulfuric acid.
d
e
Figure 2. The photochromic crystals are shown turning from light brown (a) to dark blue (e) during exposure to the photographer’s floodlamp.
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Journal of Chemical Education • Vol. 77 No. 11 November 2000 • JChemEd.chem.wisc.edu
Photos by Jerrold J. Jacobsen and Addison Ault.
At the start of the class I mentioned that four years ago Heather Parham had obtained her product as a single crystal that weighed more than 800 milligrams (more than a 4-carat diamond). But I assured the class, that “this could never happen again in a trillion years”! Well, a trillion years must pass quickly in Iowa because in what seemed like less than 24 hours two students had equaled this achievement: Rhonda Reisdorff ’s single crystal weighed in at 864 mg, and Graham Pumphrey’s crystal tipped the balance at 891 mg.
H H N
H
conc HNO3
C
conc H2SO4
2-benzylpyridine
N
H
C NO2
NO2 2-(2,4-dinitrobenzyl)pyridine
We originally described this experiment in the Journal in 1974 (1) when we presented a procedure on a 32 mM scale that specified the use of red fuming nitric acid, something that should no longer be used. Since that time two modifications of this procedure have been published in the Journal (2, 3). The procedure reported by Zaczek and colleagues in 1982 (2) is on a 16 mM scale but still calls for fuming nitric acid. The procedure reported by Gilfillan and Pelter in 1994 (3) is on a 0.5 mM scale that specifies “freshly prepared anhydrous nitric acid”. Readers will be interested to know that we discovered a few years ago that ordinary concentrated nitric acid works just as well as if not better than the red fuming nitric acid that was in the original preparation. Our current procedure, 16 mM in scale and using concentrated nitric acid, is presented here.W Although some may object to the large scale of this experiment, students seem to enjoy the opportunity of working with larger amounts of materials. At the current price of 2-benzylpyridine, the cost of this starting material is less than $1 per student. W
Supplemental Material
The revised procedure for carrying out this laboratory experiment appears as supplemental material in this issue of JCE Online. Hazards The procedure calls for both concentrated sulfuric acid and concentrated nitric acid. These materials are corrosive and should be handled with the care normally taken with concentrated acids. Keep 2-benzylpyridine off the skin. Literature Cited 1. Ault, A.; Kouba, C. J. Chem. Educ. 1974, 51, 395. 2. Zaczek, N. M.; Levy, W. D.; Jordan, M. L.; Niemyer, J. A. J. Chem. Educ. 1982, 59, 705. 3. Gilfillan, E. D.; Pelter, M. W. J. Chem. Educ. 1994, 71, A4.
Addison Ault is a member of the Chemistry Department of Cornell College, 600 First Street West, Mt. Vernon, IA 52314;
[email protected]. JChemEd.chem.wisc.edu • Vol. 77 No. 11 November 2000 • Journal of Chemical Education
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