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General Comments about Handling and Destroying Alkali Metal and Organometallic Residues - BEFORE YOU START
- Before you start using alkali metals, their inorganic derivatives (e.g. MH) and their organometallics (e.g. MeLi, BuLi and tBuLi) you should think about how to clean up and dispose of them. This is a very common cause of fires and explosions through ignition of hydrogen and solvents.
- Talk to PSc about this if you are inexperienced or if you are unsure.
- Let PSc know if you need to destroy a reagent that is not mentioned below.
- Work in a fume cupboard from which ignition sources, flammable solvents and other fuels have been removed. Make sure the working surface is not wet and avoid accidentally splashing water. Think about working in a tray.
- All glassware should be dried carefully.
- Have a dry-powder extinguisher available, not CO2.
- When Destroying:
- Another person should be on hand when alkali metal residues are destroyed and the person doing the work should consider wearing a face shield. Tell your colleagues what you are doing.
- Use a diluting volume of an inert, high-boiling solvent such as toluene. A solvent like hexane will more readily ignite.
- Think about working over a tray large enough to contain the solvent - what would happen if the vessel breaks?
- Use an alcohol which will react with the reagent in a reasonable time but which will not be too vigorous (e.g. iso-propanol usually reacts too slowly with sodium, sometimes taking days). After that, add a more reactive alcohol to ensure complete reaction. Then add a little water cautiously before disposal. At each step, assume that there will be further reaction.
- Have an inert gas supply available when destroying. Gently blowing dinitrogen into a vessel (e.g. a small beaker, a conical flask or RBF) is a good way of preventing the formation of explosive mixtures of hydrogen and air, and it sometimes has the useful side-effect of cooling the mixture through evaporation. This is particularly useful when destroying the residues of solvent-drying stills where the metal or similar reagent is in a round-bottom flask. It is not such a good method in a larger beaker (>250 ml) or a crystallising dish because it is much harder to keep the air out with the stream of dinitrogen.
- nBuLi, MeLi, tBuLi. These commercially available materials (solution in hexane or ethers) are spontaneously flammable and should always be handled under N2 or Ar, preferably using Schlenk techniques rather than syringe. tBuLi is particularly reactive.
- Generally we transfer the reagent as supplied commercially from the "SureSeal" or similar bottle to an ampoule (or a graduated ampoule) with PTFE stopcock for storage. This is done by slightly over-pressuring the SureSeal with a needle attached to an argon supply and cannula-transferring to the dry, degassed ampoule. The system should not be pressurisable, and for example the argon supply should be open to a bubbler.
- Whether you are working from a storage ampoule or directly from the supplier's "SureSeal" or similar bottle, the choice of handling method is important.
- For larger volumes it is best to use a cannula method and a graduated ampoule. Don't forget that the cannula will contain residues.
- If you are going to use a syringe (e.g. for small quantities) then it must be a glass syringe with PTFE plunger. These are expensive but much safer than the alternatives as they are much less likely to stick and the plunger will not pop out under modest pressure. The needle should be long enough so that gas bubbles can be expunged.
- Never use an all-glass syringe (the plunger can come out).
- Never use a plastic syringe (the plunger can freeze).
- When destroying, add the reagent hexane/ether solution to at least an equal vol. of toluene. This mixture can be handled in air, with caution but should be blanketed with inert gas as stated above to prevent the formation of a flammable gas mixture. Begin destroying by adding 10% iso-propanol in toluene carefully with stirring. Follow this with a little neat iso-propanol, then once this is IMS carefully when the reaction has ceased. When there is no further reaction, cautiously add a few drops of water.
- Note that tBuLi is particularly reactive.
Sodium Metal Residues
- Use inert gas to disperse hydrogen as described above.
- Always keep the metal covered with toluene in a beaker. Add iso-propanol with stirring. There is usually not usually much reaction with this alcohol. If so, add IMS cautiously. When completely dispersed and reacted - large amounts can take hours - add water cautiously.
- Wired solvent bottles. Bottles containing Na wire should be clearly marked as such. Sodium which has been used to dry THF often becomes thickly coated with a deposit which may dissolve only slowly in EtOH or IMS.
- Blow inert gas into the winchester and then flood the container to a depth of 8-10 cm with IMS. Use at least 200 ml. Set aside for 1 hour swirling occasionally. The reaction may be fairly vigorous. Keep the inert gas flow going to disperse the hydrogen evolved.
- Pour the alkoxide into further alcohol, and cautiously add water dropwise to ensure completion of the reaction before disposal.
- Rinse the original container thoroughly with more IMS. Wash the container with water.
- This is much more reactive then sodium. Always keep covered with toluene in a beaker.
- Use inert gas to disperse hydrogen as described above.
- Add 25% solution of iso-propanol in toluene with stirring.
- When completely dispersed and reacted, add more iso-propanol. If there is no further reaction, add IMS cautiously, followed by a few drops of water if there is no further reaction.
Potassium, Sodium, Lithium Hydrides
- These will explode on contact with water and, if exposed to air, may catch fire without warning. Large quantities of hydrides are always dangerous.
- The dry powder should only be handled in a glovebox. Take care when removing tissues that are contaminated with hydrides as they may smoulder or even ignite on exposure to air.
- Water should never be used on metal hydride fires. Powder extinguishers may be used or the fire may be smothered by sand or soda ash. Provided that there is no flammable material in the vicinity very small fires may be allowed to burn since smothered material may re-ignite.
- These materials are usually supplied as suspensions in oil. The removal of the oil should only be conducted with the assistance of experienced personnel and after consultation with PSc.
- Small amounts of residues can be destroyed conveniently using IMS (alcohol) followed by water after any reaction subsides. Larger amounts should be treated as the alkali metals above.
- NaH/DMSO, NaH/DMF and NaH/DMAc reaction mixtures have poor thermal stabilities, posing signiﬁcant risks. These mixtures undergo exothermic decompositions at low temperatures (as low as 37 °C) and produce non-condensable gases such as H2, which has resulted in multiple reports of explosions. Safer alternative conditions should be employed to avoid incidents (e.g. use of alternative bases, NaH only used with stable solvents such as anhydrous THF, etc.).
Lithium aluminium hydride
- LiAlH4 will explode on contact with water and, if exposed to air, may catch fire without warning. Large quantities of LiAlH4 are always dangerous.
- All experiments with LiAlH4 as reagent should be done on as small a scale as possible.
- The compound may be weighed rapidly in a stoppered bottle in air and transferred to a reaction flask. The pelletised form of this reagent is very convenient and can be crushed a weighed out in the glovebox.
- Care should be taken with samples that have been stored. These may appear to be completely decomposed but there may be LiAlH4 under the surface crust.
- An excess of LiAlH4 is normally used. Small residues at the end of reactions may be destroyed with dry ethyl acetate, under nitrogen. When the reaction has completely subsided the residue should then be very cautiously poured into a large volume of cold water.
- Water should never be used on LiAlH4 fires. Extinguishers are not recommended but the fire may be smothered by sand or soda ash. Provided that there is no flammable material in the vicinity very small fires may be allowed to burn since smothered material may re-ignite.
These Procedures are for use exclusively by members of the Scott group, Warwick University. We put them here so that they can be readily accessed by members of the group. Others who carry out these procedures or even to take advice from them do so at their own risk. The Scott group and associates do not warrant or guarantee the safety of individuals using these procedures and hereby disclaim any liability for any injuries or damages claimed to have resulted from or related in any way to the Procedures herein.