I am trying benzylation by using NaH as a base and THF as a solvent. I want to know if it is essential to quench NaH after the completion of a reaction.
It is always advised to quench with some acidic solution like aq. NH4Cl. But you can also isolate the compound without quenching.....it works...
It is always advised to quench with some acidic solution like aq. NH4Cl. But you can also isolate the compound without quenching.....it works...
NaH is produced as a 50% dispersion in oil that makes it reasonably safe to use. The oil coats the surface of the NaH so that it does not burn in air. Leftover NaH in a reaction flask, or reactor is pyrophoric because the oil has been removed from the surface. The NaH must be quenched. Proper selection of the quenching agent is important in making the reaction mixture easy to work up.
Hello Karl,
On Sigma Aldrich bottle, it was mentioned sodium hydride, dry 95% and CAS no. is 7646-69-7
Same holds true. The material is pyrophoric in air. If you try to filter the unreacted NaH away from the crude reaction mixiture, you are set up for a fire. On a commercial scale, the oil dispersion is almost a requirement for safety.
quench with 10% ammonium chloride solution and extract with suitable solvet
Quench it with MeOH. This is the simplest and safest method. Safer than any water based methods, additionally it is usable when your reaction solvent is not miscible with water (e.g. toluene, iPr2O, etc).
It is necessary to quench always, because the NaH content is not exact (usually limits are seen on the label), and you might use it in excess.
NaH should be quenched with saturated solution of ammonium chloride at 0 0C by dropwise addition. Since it libertes hydrogen gas and reaction is very vigorous so the addition must be very slow.
Dear Helio,
Commonly, after completion of reaction water is used to quench propargyl bromide.
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