Chemical & Engineering News (27 Nov 2000) Vol. 78, No. 48, p. 2.
I wish to report a laboratory accident that might be of interest to your readers. One of my colleagues was trying to prepare 6,6'-dibromo-2,2'-bipyridine according to the procedure described in a paper in the Canadian Journal of Chemistry [69, 1117 (1991)], although at a different scale (one-tenth the scale of the authors' measurements). During the work-up, the reaction flask (a 100-mL round-bottom flask) exploded violently and the researcher's arm was injured. The reaction is an Ullmann-type coupling of an organolithium reagent with copper(II), and involves bubbling of O2 after addition of the copper salt. The explosion occurred after O2 bubbling when hydrochloric acid was added for work-up. I suspect that O2 reacted with the organocopper (or remaining organolithium) reagent to form some peroxides, which caused the explosion.
While it describes fairly large-scale preparation of a very useful compound, the paper contains no cautionary comments. I do not intend to blame the authors for not describing the danger, but all chemists should be aware that this procedure could be dangerous.
Procedure: 2,6-Dibromopyridine (6.0 g, 25 mmol) was dissolved in dry ether (50 mL) in a 100-mL round-bottom flask and lithiated with 1.6 M n-butyllithium (15.7 mL, 25 mmol) at -90 °C under argon. Copper(II) chloride (1.7 g, 13 mmol) was added, and after 30 minutes of stirring at -50 °C, oxygen was bubbled through the mixture for one hour at -50 °C. When 25 mL of 6 N HCl was added to the mixture, the explosion occurred.
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