shimikiah - 30-6-2012 at 07:14
Hey Guys--I've been having some trouble with a reaction, and with a deadline drawing near I decided to seek some help to try and help speed up the
process. Any help is greatly appreciated!
First, I'd like to note that this is my first time messing with LAH, so if it seems I am missing something obvious I may be. I am working in
anhydrous conditions (all glassware dried by flame, flask being flushed with N2). My solvent is anhydrous ether which I have not further purified (I
have been told the LAH in excess should make up for this).
After adding the malonate and allowing it to react, I am using an acidic workup: water, dilute (2N) HCl. Extract twice with ether, add K2CO3 to
aqueous layer until alkaline, extract with ether. Wash ether layers with K2CO3, dry over Sodium Sulfate. Rotavap ether off.
What I keep getting is a yellow oil--my starting material is a colorless liquid and my end compound is a white solid. I have tried to recrystallize
the oil in several different solvents to no avail. Could there be a problem with my workup? Or does my problem just lie in the separation of the
product from the oil?
unionised - 30-6-2012 at 07:33
Not really my field, but won't the LAH just deprotonate the staring material and then the work up will give you the same thing back?
barley81 - 30-6-2012 at 07:49
That's why you can use a large excess of LAH (though it might be somewhat wasteful). I think that the procedure accounted for that.
shimikiah - 30-6-2012 at 07:49
LAH should attack the carbonyl carbon and then form a complex with the oxygen. That gets kicked off and ethanol leaves. Another attack by the LAH
results in a second complex being formed with the oxygen, which is removed by the addition of acid after.
chemrox - 30-6-2012 at 12:34
Have you tried distilling the oil? You probably need a pretty good vacuum. 0.1-0.3 torr should do it. You should collect a clear oil that will
crystallize in time.
shimikiah - 1-7-2012 at 06:36
I was afraid that a vacuum distillation might be what was needed. I currently don't have the equipment to run a very low pressure one. Our common
vacuum lines go down to about 180 torr.
On that note, do you need special glassware to run a vacuum distillation at 0.1 to 0.3 torr?
chemrox - 2-7-2012 at 17:41
No, that's not a particularly 'hard' vacuum. As with any vacuum procedure make sure your glass is without internal scratches and don't us the flasks
with flat areas on the the bottom. Check everywhere for tightness and good seals.