MOUNT VERNON — Two Cornell College organic chemistry students have created a laboratory anomaly that has happened only once before in 25 years at Cornell — and never in 13 years at Harvard.
Students in chemistry professor Addison Ault’s class were doing their final experiment, using the concepts and applying the techniques they had learned to make a photochromic compound, 2-(2,4-dinitrobenzyl)pyridine, that maintains its light brown/yellow color in the dark but turns deep blue when exposed to sunlight. Ault has used the experiment in class for 25 years at Cornell and 13 years in the Harvard Summer School.
Here’s how the experiment works: Prepare a solution of 2-benzylpyridine in concentrated sulfuric acid; cool it to ice temperature; slowly and cautiously add concentrated nitric acid; warm the resulting mixture to the temperature of boiling water for 20 minutes; dilute the reaction mixture by pouring it onto ice. Then isolate the product, 2-(2,4-dinitrobenzyl)pyridine, by extracting into ether, concentrating the ethereal solution and then allowing the concentrated solution to stand, whereupon the product separates as many small crystals.
Four years ago Cornell student Heather Parham, a 1998 graduate, isolated her product as a single crystal weighing more than 800 milligrams (more than a 4-carat diamond). Ault thought that was a once-in-a-lifetime event – until it happened again in March. Twice.
Graham Pumphrey, a sophomore from Newton, Kan., and Rhonda Reisdorff, a junior from Billings, Mont., each isolated their product as a single crystal. Pumphrey’s weighed 891 mg; Reisdorff’s weighed 864 mg.
“They provided the conditions that permitted this random piece of luck to occur,” Ault said. “They followed the directions and were patient. They allowed the concentrated solution to stand without making any effort to induce crystal formation.”
The Journal of Chemical Education plans to publish news of the experiment in an upcoming issue.
“I knew I had something rare as soon as I saw the crystal,” Pumphrey said. “All the other crystallizations I had done yielded several small crystals. Addison said it was probably a one-in-a-trillion likelihood to get one crystal.”
There is no practical application for this compound, either as a batch of crystals or a single crystal. Ault said researchers once considered using the compound in eyeglasses that adjust to changing light conditions. But it takes 24 hours for one of Ault’s prized crystals to change from dark blue to a “colorless” light brown/yellow.