We have two safety-related letters to the editor in this week’s issue of C&EN. The first, from Norton P. Peet, continues the discussion on sodium azide:
It was important to read the recent letters that revisited the dangers associated with the use of sodium azide (C&EN Jan. 11, page 4; April 5, page 5; and Nov. 9, 2009, page 8). None of these letters, however, mentions the hazards of exposing sodium azide to halogenated solvents.
We earlier reported the probable production of diazidomethane when methylene chloride was used as an extraction solvent in the workup of a reaction that employed excess sodium azide as a reagent (C&EN, March 14, 1994, page 4). It is convenient for chemists to use heavier than water chlorinated solvents for extraction of reaction mixtures that are diluted with water. However, if the aqueous phase contains azide, this procedure can lead to the production of the very explosive diazidomethane (N to C weight ratio of 6). Chlorinated solvents should never come into contact with sodium azide.
The second letter, from Toti E. Larson and Ruqiang Zou of Los Alamos National Laboratory, discusses problems a LANL researcher had while oxidizing dimethyl sulfoxide (DMSO) in the presence of hydrogen peroxide to form sulfate bridges in a metal-organic framework:
Referencing Ma [Angew. Chem. Int. Ed. 2008, 47, 4130], a researcher at Los Alamos National Laboratory scaled up this reaction from 1.5 mL to 12 mL DMSO and conducted the reaction in a Teflon-lined Parr vessel (model number 4745 ADB) that was placed on the bottom of a convection oven set to 150 °C and left for the weekend. The following Monday, the researcher saw that the bottom flange lip of the Parr vessel had sheared off as intended when overpressured. This shearing event was energetic enough to dent the bottom of the oven, causing the metal plate of the oven bottom to contact the heating elements, which resulted in an electrical shortage.
We were initially concerned that perchlorate salts formed during the chemical synthesis and may have detonated with the DMSO, but we have found no evidence supporting this hypothesis. Additional experiments were conducted with appropriate engineered controls to protect against a recurrence. Although nearly identical reaction conditions were used, the Parr vessel did not rupture. However, a thermocouple placed on the Parr vessel recorded an 11.4 °C temperature increase lasting approximately one hour after approximately 27 hours of heating.
Although the exact nature of this energetic release is uncertain, we have concluded that several events may have contributed. First and foremost, the exothermic nature of the oxidation of DMSO was not communicated in the publication, and the experiment was conducted close to the boiling point of DMSO (189 °C). Second, an old Parr vessel (manufactured between 1969 and 1973) with an uncertain history of use was used for experimental chemistry; the vessel was not equipped with a burst disk, which is now the preferred design. Finally, the Parr vessel was placed on the bottom plate of the oven, which was later found to have a temperature that may have been on the order of 35 °C higher than the oven set temperature of 150 °C. We believe these were critical factors that resulted in the energetic release during the experiment. …
Best practices would include checking critical published references prior to proceeding with an experiment, using appropriately designed experimental vessels, not placing reaction vessels on the bottom of ovens, and not scaling up reactions until proven safe. Researchers who plan to use DMSO for chemical experiments are directed to the review by T. T. Lam and coworkers that outlines many hazards associated with the energetic decomposition of DMSO at temperatures below its boiling point in the pure phase (J. Thermal Anal. Calor. 2006, 85, 25).