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Author: Subject: Claisen Reaction
awlb2
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[*] posted on 12-12-2016 at 09:59
Claisen Reaction


Hello everybody,
I have a quick question about organic synthesis: For a Claisen Condensation reaction (e.g. self-condensation of ethyl ethanoate) a methoxide or ethoxide sodium salt is used as a strong base- but what base could be used or made to perform this without these poorly available compounds ?

I have a few strong bases/alkalis such as Na2CO3, Ca(OH)2 and NaOH (solution only) - would these suffice ?

Would Ca(OH)2 react with ethanol to form the ethoxide?

Many thanks in advance if you can help!
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[*] posted on 12-12-2016 at 10:26


The trouble with using hydroxides is that they can hydrolyze the ester (giving ethanol and acetate ion) instead of the enolate that you want. Aqueous sodium hydroxide will do only that. Dry calcium hydroxide *might* be more successful.

If I were to try this reaction, I'd try making magnesium methoxide (magnesium is much more readily available than the alkali metals), and see if that worked.





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[*] posted on 12-12-2016 at 10:30


Thank you very much DraconicAcid- yes, using magnesium methoxide sounds like a good idea but the reaction between Mg and MeOH is pretty slow if I am correct. Do you think Ca(OH)2 can form methoixde with methanol like NaOH can?
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[*] posted on 12-12-2016 at 10:34


Quote: Originally posted by awlb2  
Thank you very much DraconicAcid- yes, using magnesium methoxide sounds like a good idea but the reaction between Mg and MeOH is pretty slow if I am correct. Do you think Ca(OH)2 can form methoixde with methanol like NaOH can?


If you can reflux the magnesium with methanol, with a small amount of iodine to activate the magnesium, it works nicely. That's how we dried all of our methanol back in the day.....

I don't know how well any hydroxide will react with an alcohol to give an alkoxide.




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[*] posted on 12-12-2016 at 10:36


OK, that's a good idea (adding iodine to initiate reaction). I just remembered adding iodine to initiate reaction between aluminium and 2-propanol to make Al-isopropoxide when I made that.

Thanks for your help!

[Edited on 12-12-2016 by awlb2]
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[*] posted on 12-12-2016 at 12:22


You also need to take into consideration your target molecule when choosing a base, otherwise you may end up with mixed products. If your target molecule is a beta-keto ester, then you're going to want to use the conjugate base of the alcohol formed upon the condensation of the two reactant molecules. For example, if your target molecule were ethyl acetoacetate, then the base you'd want to use would be an ethoxide salt, as the alcohol that forms in this case is ethanol. If you were to use a methoxide salt as the base instead, then you would likely also end up with methyl acetoacetate as well. Also, in addition to issues with transesterification, you also don't want to use a base that is too strong for your reaction (like using LDA to synthesize ethyl acetoacetate), as it may end up deprotonating so many of the starting reactant molecules that there aren't any left to act as electrophiles (thus no reaction).

Now on the the other hand, if your target molecule is something like a beta-diketone, then which base you choose doesn't really matter, as long as it's at least strong enough to deprotonate the initial enolizable compound. The base also wouldn't really matter if your condensation product was a beta-keto ester, but your end goal was ultimately hydrolysis to a beta-keto acid or decarboxylation to a ketone.

[Edited on 12-12-2016 by Darkstar]
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[*] posted on 12-12-2016 at 16:20


Quote: Originally posted by Darkstar  
You also don't want to use a base that is too strong for your reaction (like using LDA to synthesize ethyl acetoacetate), as it may end up deprotonating so many of the starting reactant molecules that there aren't any left to act as electrophiles (thus no reaction).


There is no such thing as using too strong of a base. When the goal isn't self-condensation, an intramolecular Dieckmann condensation, or a crossed Claisen with a non-enolizable component, superbases are specified to avoid low yields and mixed product

In a dry ice bath, LDA can quantitatively deprotonate an ester to it's kinetic enolate. The target can be whatever you choose, since the enolate is already formed. This technique is vastly more useful when you're not trying to make cheap bulk chemicals.
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[*] posted on 12-12-2016 at 17:10


I wasn't saying that really strong bases won't work, I was saying that you may need to be more careful with stoichiometry when using them in cases where both initial reactants are enolizable, especially if they are already mixed prior to enolization (as opposed to forming the enolate first, then adding the electrophilic carbonyl compound). The ester and deprotonated enolate are in equilibrium with one another. Weaker bases like ethoxide favor the ester side of the equilibrium, while stronger bases like LDA favor the deprotonated enolate side. Using a stoichiometric excess of sodium ethoxide in a classic ethyl acetoacetate synthesis just speeds up the reaction, whereas using a stoichiometric excess of LDA not only slows the reaction down, but can even stop it all together.

Again, I'm talking about using extremely strong bases like LDA in condensations that specifically involve two enolizable compounds that are initially mixed with one another, not Claisen condensations in general.

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