Chemical safety: peroxide formation in 2-propanol

Two years ago we experienced an explosion in our lab at the end of 2-propanol distillation. Luckily, it was a small-scale distillation. To elucidate what happened, we prepared several samples of 2-propanol and kept them on a shelf away from direct sunlight at room temperature.

We filled transparent glass bottles, tinted glass bottles, and polyethylene bottles 75% full with high-purity commercial 2-propanol. We prepared additional transparent glass bottles with the following:

a) 97% 2-propanol and 3% deionized water.

b) 2-propanol refluxed for 1.5 hours with magnesium chips and a small amount of iodine, followed by distillation; the first 10% of distillate was discarded.

c) 2-propanol from sample (b) after additional distillation; fresh 0.3-nm molecular sieves were added to the final sample to 20% by volume.

We replaced the air above the 2-propanol every two months by flashing the interior of the bottles with an airstream. After 24 months, we tested the content of all containers for peroxides using the standard iodide method (ASTM International 2008, DOI: 10.1520/e0298-08).

We found peroxides only in sample (b), which had a peroxide concentration of approximately 0.004 mol/L. We suggest that diisopropyl ether, a well-known peroxidizable substance, formed during refluxing of 2-propanol with magnesium. Diisopropyl ether boils at 69 °C, which is 14 °C lower than the boiling point of 2-propanol, and should be removed by distillation. Our results indicate that the first distillation performed without a Vigreux column was not sufficient [sample (b)], while an additional second distillation led to complete removal of all of the formed peroxidizable substance [sample (c)].

Prior to this incident, we were not aware that primary and secondary alcohols are peroxidizable. We hope this report is a reminder to the chemistry community that they are and that distillation procedures should take the hazard into account. For more information, see the 7th edition of “Bretherick’s Handbook of Reactive Chemical Hazards”; Chem. Health Saf. 2001, DOI: 10.1016/s1074-9098(01)00247-7; and J. Chem. Educ. 1988, DOI: 10.1021/ed065pa226.

Svetlana N. Kholuiskaya and Alexei Y. Metlin
N. N. Semenov Institute of Chemical Physics
Moscow