I want to reduce ester with LAH. I want to know how to quench it after reaction.
Hello,
I suppose you used Et2O for the reduction of the ester to the corresponding alcohol. In this case, I normally use careful addition of water while cooling the reaction mixture with ice bath to destroy the excess of LiAlH4. The Al(OH)3 formed (gel-like suspension) can subsequently be dissolved by acidification. Lastly, just separate organic (etheral) layer ...
Kind regards.
Hello, Simon Drescher
I am going to use THF as a solvent in the reduction of ester by LAH, then how should I quench the LAH after reaction?
Hello,
for THF you can also use the method mentioned above. Again, you should be very careful with the addition of water due to the very exothermic reaction! You can also add ice water dropwise ... For the acidification I normally use 10% H2SO4 until the percipitate (AL(OH)3) has dissolved. Since THF is miscible with water, Et2O shoul be used for extraction.
Kind regards.
Cant you use Rochele's salt? It will help a lot not to get those nasty aluminium emulsions! Check it up, I recommend it if it is compatible with your product
From my experience, when using rochelles salt you do the following procedure:
Cool the reaction to 0 deg Celsius
Add some EtOAc to quench unreacted LiAlH4 (you get EtOH, should go in the aq wash) while stirring.
Add a water solution of rochelle's salt (ca 3-5 mol equiv rochelle, and calculate the amount of water from there)
Let the temperature rise to r.t. and stirr for 30 minutes or longer if necessary. No need to add small pieces of ice (at least not for me).
THF is not good for the extraction. You can always dilute the organic phase with e.g. ether (although we do not like to use ether here), or toluene or other organic solvent with poor water solubility.
Hope this helps.
In our lab we use the following method even upto 30 g of LAH
Keep the RB containing LAH at 0 deg then for X gm of LAH add X ml of distilled water dropwise, followed by Xml oh 15%NaOH solution, followed by 3X ml of Distilled water, stirr for 1hr, then filter and wash the solids with ether 2 or 3 times and then rotavap.
For each (one) gram of lithium aluminium hydride used, add dropwise (SLOWLY) one ml water at zero degree celcius followed by one ml of 15% aqeuous NaOH and, finally, 3 ml of water. Filter the resulting solid and rinse solid with plenty of ether and evaporate. additionally one might add anhydrous Magnesium sulfate.
If ur using THF .... better solvent is Et2o. Reduction with LAH is good campair to NaBH4... the procedure for quechin is cool the solution to o degree and prepair 1:1 hcl r 6m r 18% solution cool it to 0 degree and every thing should be cool ... add slowly drop by drop hcl to reaction mixture with vigarous starring and filter it and extract with et2o.
To work up a reaction containing x g lithium aluminum hydride
Dilute with ether and cool to 0°C
Slowly add x mL water
Add x mL 15 % aqueous sodium hydroxide
Add 3x mL water
Warm to RT and stir 15 min
Add some anhydrous magnesium sulfate
Stir 15 min and filter to remove salts
you can try with saturated aqueous solution of Rochelle's salt (sodium potassium tartrate). Rochelle's salt is an excellent ligand for aluminium and breaks the aluminium emulsion. The procedure is simple. Cool your finished reduction down to 0 degrees C, or lower depending on the situation (For my large scale reduction I cooled it with acetone/dry ice) and quench excess reducing agent with something non-protic. For example ethyl acetate or acetone works well. Just remember to use something you can easily evaporate off when things are done. Don't be impatient and add it dropwise with vigorous stirring. Use a addition funnel for larger scale reactions. When the quench is complete remove the cooling bath.
or
When the reaction is complete (> 10 min) cool the flask in water and ice and cautiously add saturated aqueous sodium sulfate (12 ml) dropwise to the stirred reaction mixture to decompose the excess of reagent. An aqueous solution is safe here because only a small quantity of reagent is present and it is in solution under an atmosphere of nitrogen. It is important to ensure that all of the excess reagent is destroyed before continuing (ask a demonstrator if in doubt). Then add 10% (v/v) sulfuric acid (95 ml). Separate the two layers (if present) and re-extract the aqueous layers with ether (4 x 30 ml). Dry the combined organic layers over anhydrous sodium sulfate.
or
When the reaction is complete, add water (3 ml) to decompose the excess of the reagent. It is important to ensure that all of the excess reagent is destroyed before continuing (ask a demonstrator if in doubt). Add 10% (v/v) sulfuric acid (25 ml), then separate the ether layer and extract the aqueous layer with ether (2 x 50 ml). The combined layers are dried over anhydrous sodium sulfate.
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