Interaction between bromine and tert-butyl methyl ether
In a development laboratory, bromine was added to unstirred tert-butyl methyl ether in a glass addition funnel at ambient temperature. Following the completion of the addition, rapid pressurisation occurred leading to the sudden expulsion of the mixture and destruction of the glass flask.
The usual literature sources on hazardous reactions and binary mixtures were consulted and no specific reference was found to the incompatibility of these two materials (although the hazardous interaction of bromine with THF and diethyl ether was noted). Since the review was inconclusive, the interaction was repeated and calorimetric and gas evolution measurements were made under conditions similar to the actual incident.
The possibility of photocatalysed reaction was also noted.
The investigation:
During the course of the bromine addition a rapid, but steady, rise in temperature was observed from 22°C to 40°C. After a delay of 15-20 seconds following the completion of the addition an extremely rapid, exothermic event occurred causing the temperature to rise to greater than 95°C in less than 20 seconds.
Notes:
- Exothermic heat of mixing (physical) takes place between bromine and TBME.
- A spontaneous, strongly exothermic interaction occurs with (approx.) 1:1 molar mixtures (1.2:1 in reality).
- The violent reaction appears concentration dependent since the full charge of bromine needed to be added before initiation took place.
- The temperature dependence of the reaction has not been determined (this would require repeating this procedure at, say, 0°C).
- The reaction occurs in darkness - photocatalysis not essential.
- The calculated heat of reaction will underestimate its potential since some heat was lost to the suddenly vaporised solvent. Likewise the adiabatic temperature rise will exceed the calculated value
- Violent pressurisation is most likely to be due to vapour generation rather than permanent gas since during runaway, no gas evolution was detected until the TBME boiling point (55°C) was exceeded.
- Reaction was not due to sudden mixing as there was efficient agitation (unlike the lab. incident which occurred in a static system)
Conclusion:
A spontaneous and strongly exothermic interaction occurs between bromine and tert-butyl methyl ether at a molar ratio of approximately 1:1 resulting in uncontrolled and violent pressurisation due to the rapid vaporisation of the solvent. The calculated heat of reaction was in excess of -350 kJ/litre and the associated adiabatic temperature rise was greater than 150°C.
Once initiated, the reaction between these two materials is uncontrollable and therefore this combination of reagents should be avoided.