Grignard stalls then runs away
During a routine plant preparation of a Grignard, the reaction stalled, too much halide was added to try to get the reaction started, and when it did start there was so much energy released that material was ejected from the reactor.
It was found that the reaction stalled as a second drum of halide was started, this was not the same batch as the first drum. It appears that the quality of the second drum was not as good, and this is what caused the stalled reaction
Naturally it is known that if too much halide is added then there is a distinct risk of the ensuing reaction being uncontrollable.
- The halide used must be of consistent quality if the reaction is to proceed smoothly, if there is any doubt, then it is better to pre-mix the halide charge.
- The reaction must be monitored throughout, if the reaction appears to stall - i.e. comes off reflux, then the cause must be investigated before further halide is added.
When a particular halide has shown signs of stalling then further investigation
should be applied to ensure that there is no repeat, and that future batches
proceed with less difficulty. In this respect the following options
can be considered:-
- Purchasing a better grade of halide
- Increasing the drying of materials - water is a known problem
- Changing the halide (e.g. from chloro to bromo or bromo to iodo)
- If changing all the halide is too expensive then consider adding a proportion of more reactive halide
- Adding a proportion of methyl iodide - methyl iodide is very reactive, its use could assist in keeping the reaction going, but it does lead to an amount of methyl magnesium bromide, this will give other products downstream, it will need to be decided if they can be easily removed on work up.
- As above, but use dibromoethane - the resultant Grignard, will form ethylene, which will be eliminated.