The UC Center for Laboratory Safety

What Happened?

A graduate student wanted to perform an ozonolysis reaction to cleave an alkene using ozone (O3). The student set up a glass flask with the alkene in methylene chloride solvent and a gas inlet tube that was connected to a commercial ozone generator producing a flow of oxygen gas containing 1-5% ozone. Instead of cooling the reaction flask to -78 °C with an acetone/dry ice cooling bath, it was instead cooled to 77 K (-196 °C) with a liquid nitrogen cooling bath. Soon after the gas flow was started the student noted a bluish color in the solution, but did not recognize it as condensed oxygen caused by supercooling with liquid nitrogen. The student lowered the cooling bath to get a better look at the blue solution. When he wiped the frost off the glass flask with a finger the flask exploded.

The explosion impaled a shard of glass into the lens of the researcher’s safety glasses, but fortunately there were no serious injuries. After the explosion, the graduate student reported the incident to his research advisor.

What Was The Cause?

The immediate cause of the explosion was accidental collection of liquid oxygen into an organic solvent. Both oxygen and ozone are blue colored when liquefied. Further, ozone condenses to a liquid at -193 °C and is highly explosive. However, given the low concentration of ozone gas in the oxygen stream, the explosion was due to reaction of condensed oxygen with organic material, a known very explosive combination. The root cause of the accident was lack of clearly written procedures and training on how to perform ozonolysis reactions and use oxygen gas.

What Corrective Actions Were Taken?

• The incident was discussed at the next lab meeting
• Detailed Standard Operating Procedures (SOP) were developed for:
  • Ozonolysis reactions
  • Oxygen gas usage
  • Dry ice cooling baths
  • Liquid nitrogen cooling baths
• All of the students in the lab were trained on the new SOPs

How Can Incidents Like This Be Prevented?

• Follow all steps of your written procedures for performing ozonolysis
• Never supercool oxygen or ozone-oxygen gas streams
• Keep the fume hood’s sash fully down while passing ozone-oxygen through your sample

Resources

• Ozone Safety
• Ozonolysis

OZONE

• Boiling point of -112 °C (161 K; -170 °F)
• Reactive and toxic gas
• Solid and liquid ozone is highly explosive
• Reacts with alkenes and other organic molecules to form explosive peroxides
• Powerful oxidant
• Explosive when concentrated
• Explodes with many reducing agents; Br2, HBr, N2O4 

OXYGEN

• Boiling point of -183 °C (90 K; -297 °F)
• Liquid oxygen can cause freeze burns due to extreme temperature
• Powerful oxidant
• Explosive when mixed with combustible material at most temperatures 

LIQUID NITROGEN COOLING BATHS

• Use pure liquid nitrogen with a boiling point of -196 °C (90 K; -297 °F)
• Can cause freeze burns due to extreme temperature
• Release large quantities of gas
• Will condense oxygen gas, so must avoid that potential hazard 

DRY ICE COOLING BATHS

• Use solvent, often acetone or isopropanol,   cooled with dry ice which sublimes at -78 oC (195 K; -109 °F)
• Can cause freeze burns due to low temperature
• Release large quantities of gas
• Often made with a flammable solvents