FireLion3 - 22-3-2014 at 21:33
What kinds of bases would be best suited to deprotonate an aldehyde but not a ketone? For my particular reaction this is needed to prevent reaction of
ketone products with initial substrates.
I read sodium carbonate is not strong enough to deprotonate a ketone, yet I have read of carbonates being used in various aldol condensations. It
would seem sodium carbonate may be the perfect base for this purpose. Hydroxides are out of the question, it would seem. Organic bases are of
potential interest due to a wide variety of PH levels. Any ideas?
This list is of interest:
http://www.aqion.de/site/191
[Edited on 23-3-2014 by FireLion3]
Galinstan - 23-3-2014 at 07:10
deprotonate a ketone? a carbonyl group doesn't have a hydrogen on it. do you mean a hydrogen on an ajasent carbon?
unless you are talking about an enol (the tautomer of a ketone)
smaerd - 23-3-2014 at 13:40
Sodium carbonate might be, but under what conditions? Another often over-looked readily available semi-strong weak base is trisodium phosphate.
As I am sure you know this is a function of pKa or more generally acidity. So it depends largely on solvent, temperature, and fortunately or
unfortunately structure. Some aldehyde a-hydrogens may be nearly equivalent to ketones with respect to their ability to donate protons. Sheesh some
ketone a-hydrogens are about as acidic as acetic acid (see meldrums acid).
In general for acyclic carbonyl a-hydrogens solvents such as DMSO(hydrogen bond acceptor) their 'acidity' is decreased. Less polar aprotic solvents
without the ability to accept hydrogen bonds (chloroform, etc...), the 'acidity' is increased. Sorry if that's phrased terribly or I'm confusing that
with something else, I have a headache.
I assume you are planning to do some kind of nucleophilic substitution or addition reaction to an enolate? Could you provide more details about the
potential substrates and products involved? There are many ways to go about this sort of thing to get a "high" yield for a desired product, but it
really is reaction specific, and will likely still require some experimentation.
[Edited on 23-3-2014 by smaerd]
DJF90 - 24-3-2014 at 04:01
How about using an enol equivivalent that you can preform from the aldehyde, such as silyl enol ether or enamine?