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Beware drying nitrate-containing protic ionic liquids

“A batch of the protic ionic liquid pyrrolidinium nitrate exploded while drying it under reduced pressure at 110 °C, using a rotary evaporator with an oil bath.”

So says a paper in Green Chemistry authored by researchers from KU Leuven and Umicore, both located in Belgium (Green Chem. 2013, DOI: 10.1039/c3gc41328d).

While ionic liquids typically have low vapor pressures and high flash points, that doesn’t mean they’re hazard-free, as the documented experience indicates. KU Leuven chemistry professor Koen Binnemans and colleagues were investigating a series of pyrrolidium ionic liquids with different coordinating groups, one of which was nitrate. The researchers report:

After stirring at room temperature for 4 hours, the remaining pyrrolidine, water and nitric acid were removed on a rotary evaporator under reduced pressure (16 mbar) at 70 °C. Heating of the flask on the rotary evaporator was done by means of a hot silicon oil bath. Not all of the water could be distilled off, and the temperature was increased stepwise to 110 °C. Suddenly an explosion occurred. The glass round-bottom flask was scattered and part of the hot silicon oil in the heating bath was blasted by the shock to the walls and ceiling in the neighborhood of the rotary evaporator. At the same time, a cloud of reddish-brown nitrogen dioxide gas was visible. Luckily, nobody was injured and there was only material damage.

The explosion can be attributed to the strong oxidizing properties of concentrated nitric acid under the anhydrous conditions, resulting in a violent oxidative decomposition of the organic compounds. A search of the literature revealed that mixing of nitric acid with secondary amines like pyrrolidine
has been reported to cause violent reactions. Clark described secondary and tertiary amines as hypergolic with nitric acid.

If you need to dry a nitrate-containing protic ionic liquid, they recommend vacuum freeze-drying rather than heating.

This also seems like an opportune time to remind people of what Prudent Practices has to say about nitric acid generally:

Nitric acid is a strong acid, very corrosive, and decomposes to produce nitrogen oxides. The fumes are very irritating, and inhalation may cause pulmonary edema. Nitric acid is also a powerful oxidant and reacts violently, sometimes explosively [with] reducing agents (e.g., organic compounds) with liberation of toxic nitrogen oxides. Contact with organic matter must be avoided. Extreme caution must be taken when cleaning glassware contaminated with organic solvents or material with nitric acid. Toxic fumes of NOx are generated and explosion may occur.

Please remember to separate nitric acid from organics in both storage and waste.

1 Comment

  • Dec 7th 201300:12
    by NEAL LANGERMAN

    I really wish that chemists would look at the literature BEFORE the explosion. Bretherick discusses this and concentrated nitric acid will ultimately cause a reaction that can undergo a thermal runaway under a wide range of conditions – even in a low pressure rotovap.

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