Stirred Reaction Flask Explosion

Key Instruction Points:

  • Don't leave reaction unattended.

  • Use proper PPE.

  • Control sources of contamination.

  • Set chemical hood sash to lowest height possible.

Background

The Fire Department received an alarm from the Chemistry Building, and responded with fire and EMS personnel.  County Sheriff officers also responded.  At about 10:35am, EH&S personnel arrived at the incident site.

At about 10:10 a.m., an explosion occurred within the Chemistry Laboratory.  A Ph.D. research student, performing an experiment inside a fumehood, was injured by flying glass shards, which were generated from an explosion that occurred in a reaction flask (see photo below).  Although the fumehood sash was partially down (about half way), the researcher received injuries mostly to the right side of his face (see photo below) and to his left hand and arm.  No injuries were associated with the eyes since the researcher was wearing safety glasses with side shields.

The researcher was de-conned in the laboratory emergency shower and received first aid from laboratory personnel, who are also safety representatives for the laboratory.  After the first aid treatment, the researcher was escorted from the building to meet the arriving Fire Department EMS personnel.  The EMS personnel then transported the researcher to a near by building for further deconning in a hot water shower.  Afterwards, the researcher received additional first aid before being transported to the Hospital ER for treatment and observation.  Late in the afternoon, the researcher was release from the ER.  On the following Monday, the researcher returned to his laboratory at the Chemistry Building.

Description of Experimental Procedure

The experiment being performed was a modification of the Simmons-Smith cyclopropanation procedure for the synthesis of species for reacting with olefins.  Very simply, in a stirred reaction vessel under a dry argon atmosphere containing an ultra-low water and oxygen solvent (250 mL dichloromethane; CAS# 75-09-2) cooled to -10oC by dry ice in acetone, two reactants (diethyl zinc; CAS# 557-20-0 and methylene iodide; CAS# 75-11-6) are sequentially introduced via a fill funnel under dry argon pressure.  Photos of the experimental apparatus and equipment are shown in the Incident Investigation section below.   In between fillings, the funnel is rinsed with the solvent fed from a one-use, sterile plastic syringe.  First, diethyl zinc (13 mL) is added from its container with a double-ended needle to the solvent at about 2.5 mL/min.  Next, methylene iodide (22 mL) is added with a glass syringe to the solvent mixture at about 1 mL/min.  After the final addition, the solvent mixture is allowed to continue to stir for 20 min producing chemical species for reacting with olefins.  As the reaction goes to completion, the solvent mixture turns milky with the formation of a fine precipitate, which is normal.

Description of Incident

The experiment was performed as stated in the SOP, which was recorded in the researcher's lab notebook, up to and including the addition of the methylene iodide to the dichloromethane and diethyl zinc mixture under an inert argon atmosphere.  After the methylene iodide (22 mL) was added at a rate of about 1 mL per min over a time period of about 25 min, the researcher noted that the experiment was proceeding normally.  At this point, he left the experiment in the fumehood for the reaction to continue for about 20 min. However, he decided to return after about ten minutes to check on the experiment. Upon returning, he noted the stir bar was not rotating due to the formation of an unusual amount of precipitate in the bottom of the reaction flask. The reactant mixture was clear, but with no liquid phase separation.  The appearance of the reactant mixture was unusual; normally the mixture would appear milky white due to the suspension of a fine precipitate.  Although the stir bar was not rotating, the researcher did not perceive the risk of an explosion.  So he immediately proceeded to restart the stir bar.  During this process, when he had the reaction flask in his left hand, the contents of the flask detonated.  From the flying glass shards, the researcher sustained serious injuries to the left hand, right neck, and right side of the face from flying glass shards.

NOTE: Terms such as explosion and detonation will be used through out this report with the realization that a very rapid release of energy may have occurred without an actual detonation.   Regardless, the energy release and the subsequent pressures were so rapid and great that the neck of the flask could not vent the pressure buildup.  After the incident, there was no evidence of fire/smoke or other combustion products.

Incident Investigation

After an interview with the injured researcher, a reenactment of the experiment was performed substituting water for the chemicals: dichloromethane, diethyl zinc, and methylene iodide.  The experimental setup and equipment are shown in the photos below (clockwise: experimental apparatus, diethyl zinc container, and rinse syringe).

The experimental apparatus, under a positive-pressure argon atmosphere, is continuously fed from an argon cylinder through a drying column.  Setting atop the round bottom flask, which is the reaction vessel, is a septum-fitted funnel for feeding the reactants.  The reactants are fed via double-ended needles (diethyl zinc), plastic syringes (dichloromethane), or glass syringes (methylene iodide) into the funnel, and then fed into the flask through a stopcock.  The only difference in the mock setup and the experimental setup was the use of an open bath rather than a half-sphere Dewar.  This difference was not judged to be a factor in the incident.

Primary physical hazards associated with the chemicals components were the flammability of dichloromethane (LFL 13%, UFO 23%), methylene iodide and diethyl zinc incompatibilities (see http://xxxx.edu/~msds/), and the pyrophoricity, water reactivity, and explosive heat-sensitivity of diethyl zinc.  Of particular concern are the incompatibilities of the two reactants with other chemicals such as alkenes, oxidizers, copper-zinc alloys, potassium-sodium alloys, and potassium.  For example, alkenes in the presence of the reactant mixture could result in an explosive reaction and in the presence of potassium form a shock-sensitive mixture.  During the reenactment of the experiment, several possible causes for the incident were identified and are addressed in the following table.

Hazard Assessment Table

 Explored Cause

 Likely Effect

Introduction of water via the glassware and reusable syringes

Since there is a SOP for washing glassware and reusable syringes, it is not likely.  If it did happen, there might be no sign to small amounts of visible emissions in the flask; no signs were noted.

Injection of water with the dichloromethane or methylene iodide

The dichloromethane is dried in the purification process, which is under a dry nitrogen atmosphere; not likely contaminated.  The methylene iodide is transferred from a glass bottle, which is used by several researchers.  Probably some water could be introduced resulting in no sign to visible emissions in the flask; no signs were noted.

 Injection of oxygen with the dichloromethane or methylene iodide

Oxygen is removed from the dichloromethane in a purification process, which is under a dry nitrogen atmosphere; not likely contaminated.  The methylene iodide is transferred from a glass bottle, used by several researchers.  Probably some oxygen could be introduced resulting in no sign to visible emissions in the flask; no signs were noted.

Loss of argon atmosphere

The argon cylinder was still under pressure, and argon was still flowing after the incident.

Increase temperature of the reactant mixture

 The only heat source is the heat of reaction.  The magnetic stirrer did not have a heating element.  The final reactant was added over a 25-min time period to prevent large temperature increases.  In addition, the reactant mixture was cooled in a Dewar to -10oC by a dry ice/acetone solution.  Nevertheless, if a large temperature increase had occurred, detonation could result.

Introduction of a fourth chemical component via the glassware, syringes, or contamination in other components

Because of the very strict cleaning, rinsing, and drying procedure used on the glassware and reusable syringes, it unlikely that amounts of contamination could be introduced that could result in an explosion.  Evaluation of the solvent and diethyl zinc sources indicates it is unlikely that these are sources of contamination.  However, the methylene iodide (stabilized with copper or silver mesh) is purchased, stored, and used out of a glass bottle by many different researchers.  It is judged to be a possible source of contamination.

Conclusion

A definitive conclusion could not be made as to the specific cause of the detonation of the reactant mixture.  However, based on the above hazard assessment, two likely causes of the explosion detonation were a very rapid increase in temperature of the reactant mixture and the introduction of a fourth chemical, as a contaminant, into the reactant mixture.  The introduction of a third reactant could possible explain the formation of unusual amounts of precipitate, which settled to the bottom of the reaction vessel stopping the magnetic stirrer from rotating.  The stopping of the magnetic stirrer was judged to be the abnormal occurrence that preceded the explosion and perhaps was the causality for the explosion.

The use of safety glasses probably saved the researcher from serious eye injures.  Nevertheless, additional protection to the face and body would have reduced the number and severity of the injuries received.

Recommendations

  • Do not leave this experiment unattended; ensure that the reactant solution is continuously stirred.

  • Use additional personal protective equipment such as full faceshield and blast shield when conducting experiments with highly reactive components.

  • Assess the operation of the dichloromethane purification unit to ensure high purity.

  • Review the glassware cleaning procedure to ensure no contaminates are present.

  • Discontinue the use of sterile syringes for introducing chemicals into the experimental apparatus.  The term "sterile" is no indication of "chemical contamination levels.

  • Always set the fumehood sash at the lowest usable height.

  • Assess the use of the methylene iodide to reduce the likelihood of contamination and implement the following controls:

    • Date the container as to when received.

    • Date the container as to when opened.

    • Implement procedures to ensure minimum container open  time.

    • Set criteria for methylene iodide use, considering such parameters as color and age.