Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: shrikantvarma on June 25, 2010, 09:21:43 AM
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I am using sodium hydride for my trans-esterification reaction could anybody tell me how to quench sodium hydride in best way,by using water will it cause fire?
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by using water will it cause fire?
You will generate hydrogen gas whatever you use, which is flammable, but unless you do it next to a naked flame or a powdered metal you should not get a fire.
If you quench with water you generate NaOH which may hydrolyse your ester.
Can you give more info on the reaction?
I assume the transesterification is:
RCOOR' + R''OH + NaH ---> RCOOR'' + R'ONa + H2
If this is correct and R''OH in excess then there is no NaH left. If NaH is in excess, quench with R''OH would be my advice. If it is an intramolecular transesterification (lactonisation), or R''OH is too precious to be used in excess, then I'd suggest either a bulky alcohol (like t-BuOH) or a weak acid (like AcOH) at 0oC. I would avoid adding water if the mixture is strongly basic (or strongly acidic) as you risk hydrolysis of the ester.
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I agree with most of Dan's comments except fires. From having used it quite a bit, a simple observation we often made was that if the sodium hydride sparked in water, the reagent was fresh and good. I would do that in a sink with water with just a small amount of NaH. The best way I have seen to avoid fires with most reagents is to use an inert atmosphere.
I have seen several fires and the most common error was to not use an inert atmosphere (but no, not me, oh no, everyone else).
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I would say the easiest way to quench sodium hydride would simply be to cool the reaction to zero and quench with wet MeOH or tBuOH, Like dan said you may risk transesterifcation but its not too likely because you probably dont have much NaH to begin with since you created your original alkoxide with it. if you are still concerned you can drop a tiny piece of dry ice into your solution which will create a blanket of CO2 in your flask, keeping it under "inert" atmosphere and quench with something like wet EtOAc followed by an alcohol and finally with water. this will no doubt quench anything in there and is quite rigorous in terms of avoiding fire hazard. hope this helps
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I agree with most of Dan's comments except fires. From having used it quite a bit, a simple observation we often made was that if the sodium hydride sparked in water, the reagent was fresh and good.
Oh ok, that's the pure stuff I assume? We always use a 60% dispersion on mineral oil and an atmosphere of argon. Never seen that spark myself
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The 95% stuff will certainly be feisty if wet. i have seen people in the lab weigh it out on plastic weigh boats and subsequently burn straight though them ::)
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@Dan, clearly you were more fastidious than I. I presume the sparking came from an ignition when NaH is quenched with water. We also used the 60% in mineral oil. Consequently, small amounts of NaH would adhere to a spatula, weighing paper, etc, and we rather more carelessly than you would just wash it off in a sink with water. Doing so, we quickly began to note when some batches became less reactive (after having been opened and closed several times) than a fresh commercial sample.
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@Dan, clearly you were more fastidious than I. ... small amounts of NaH would adhere to a spatula, weighing paper, etc, and we rather more carelessly than you would just wash it off in a sink with water.
No, I do that too - your NaH is probably just of higher quality. I've only used it fresh and new once, otherwise just batches that have been intermittently exposed to the air. I think I'll be a bit more cautious next time I use a fresh batch...
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Depends upon which scale your reaction is:
Inert atmosphere is very important, not only quenching and the reaction itself generates equimolar amount of Hydrogen gas (Highly flammable). The reactor should have proper vent to take the hydrogen out (as certain percentage in the inert gas). Cooling the reaction mixture to 0 C and then quenching with t-butanol followed by water. Quick layer separation avoid ester hydrolysis.
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If you don't think it would be a problem you could just quench with acetone and get the self aldol product.