Sciencemadness Discussion Board - problem in claisen condensation reaction

problem in claisen condensation reaction

tomar kapil roorkee - 27-3-2012 at 09:58

Hi everyone,
I have tried to synthesize 1,3-diketone (claisen condensation) from methyl nicotinate and 4-acetyl pyridine using NaH as base in THF according to published procedure (Inorg. Chem. 2011, 50, 8879–8892). But unfortunately, it doesn't work. according to procedure, light yellow ppt should form but in my case, dark red color appears after an hour. i don't know what happened. Does this reaction really needs strong anhydrous conditions?? what will happen if I use lithium amide as base instead of NaH. Or should I take another solvent instead of THF. Many peoples have used ether, toluene, benzene and DMF also. I have also attached the procedure from the article for this reaction.

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GreenD - 27-3-2012 at 10:06

Yes needs to be anhydrous when working with NaH. Otherwise you're going to end up with H2 + NaOH.

tomar kapil roorkee - 28-3-2012 at 08:39

I have even used distilled THF but it seems as if the problem is something else. Can the self condensation of the ketone be the problem. Or the ester is too unreactive?? I dont find any solution to this failure of my reaction

Chordate - 29-3-2012 at 00:21

Quote: Originally posted by tomar kapil roorkee

I have even used distilled THF but it seems as if the problem is something else. Can the self condensation of the ketone be the problem. Or the ester is too unreactive?? I dont find any solution to this failure of my reaction

THF forms an azeotrope with water, 5 % water by weight boiling at 65 degrees. It is quite hard to dehydrate even using zeolites. I believe the common procedure is to reflux under inert atmosphere over sodium metal and benzophenone until the mix turns purple, then collect the distillate, discarding the first little bit that comes over.

Arrhenius - 7-4-2012 at 10:22

In general, procedures like this can be difficult to reproduce. Isolating a product by precipitation is often very difficult. Especially when the procedure says "dissolved in 50ml diluted acetic acid solution" - that's not very descriptive, and something that dissolves will not easily precipitate. Frankly, color means nothing. If I had to guess, the reaction probably turns red, but the product is obtained as a yellow solid. You need to be a chemist and not a baker. Monitor the reaction by TLC, watch the reaction when you add the NaH (H2 evolution?), and figure out what's going in. No one here can help you do the chemistry properly. Are you forming the product, but not isolating it, or is the reaction not going? Is the reaction not complete after 5h? Does the reaction need to be heated? Don't just follow the recipe.

To answer your questions:
-THF works very well for reactions with sodium hydride.
-Ketones generally do not undergo aldol-type reactions with themselves.
-This reaction is known, and the NMR is consistent, so the ester is probably sufficiently reactive.
-Lithium amide can promote the Chichibabin reaction so don't use this. You could try using sodium methoxide in methanol though.

Also, this procedure is backwards from what I would do. One would be wise to suspend the NaH in THF (if a LOT of H2 evolves your solvent is too wet), then add the 4-acetylpyridine, then add the methyl nicotinate. This avoids the product (a malonate-type ion) reacting with an excess of the ester.

[Edited on 7-4-2012 by Arrhenius]

AndersHoveland - 7-4-2012 at 15:06

Quote: Originally posted by Arrhenius

-Ketones generally do not undergo aldol-type reactions with themselves.

If there is some water present, the NaH may be reacting with the water, forming NaOH. Ketones can undergo self-condensation/degredation in the presence of a strong base.
see http://www.sciencemadness.org/talk/viewthread.php?tid=2744

jon - 7-4-2012 at 15:20

also NaH has to be used under an inert atomosphere

Arrhenius - 7-4-2012 at 16:36

AndersHoveland

Quote:

Ketones can undergo self-condensation/degredation in the presence of a strong base. see http://www.sciencemadness.org/talk/viewthread.php?tid=2744

I don't know why you feel compelled to chime in on everything when you don't really know chemistry. Sorry, nothing personal, but stick to facts. The reality is that ketone enolates don't readily react with ketones, due in part to the fact that the reaction is reversible. The Claisen condensation works, because the loss of alkoxide renders the addition irreversible.

Furthermore:
1.) NaOH + acetone does not give polymer. You can dry acetone by refluxing it over anhydrous K2CO3 even - which is quite a strong base.
2.) The mixture of chloroform, base and acetone also does not give polymer either, nor does it dichlorocarbene. It gives 1,1,1-trichloro-2-methylpropan-2-ol in a quite exothermic reaction.

[Edited on 8-4-2012 by Arrhenius]

chemrox - 8-4-2012 at 19:54

I wonder if you could do this in toluene using Na beads as the base. This combination worked well for another Claisen I've run that didn't yield nearly as much using NaH/THF.