awlb2
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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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DraconicAcid
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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.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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awlb2
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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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DraconicAcid
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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.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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awlb2
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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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Darkstar
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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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UC235
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| 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).
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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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Darkstar
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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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