Sciencemadness Discussion Board

Extremely strong bases for enolization

Doc B - 4-11-2012 at 19:48

Is there any suggestions for a convenient extremely strong base, obtainable for the hobbiest, that is able to tautomerise acetone to the enol?

http://en.wikipedia.org/wiki/Keto-enol_tautomerization

sargent1015 - 4-11-2012 at 20:19

Is NaOH not an option?

UnintentionalChaos - 4-11-2012 at 21:32

Quote: Originally posted by sargent1015  
Is NaOH not an option?


Not if you want quantitative conversion. Probably sodium amide is a solid choice.

Doc B - 5-11-2012 at 00:44

SGT: no NaOH's conjugation pKa is not high enough.

UC: the sodium aide is defiantly an option, however it's acquisition &/or synthesis (al la Vogel) is currenty precluding it.

kristofvagyok - 5-11-2012 at 01:37

Sodium-isopropoxide or sodium-tertbutoxide will do it.

NaOH is a weak base fot this.

DJF90 - 5-11-2012 at 08:31

Sodium hydride is a good choice, as is sodamide. What do you want quantitative enolate formation for? What reaction are you doing? Have you tried using an enolate equivalent, e.g. enamine?

Hydroxide is actually a very strong base its just the fact most reactions are performed using it in aqueous solution. In methanol solution, I've used KOH to deprotonate an acetylene (pKa of phenylacetylene is ~28) prior to reaction of the anion. Phase transfer catalysis in say toluene has also been used to deprotonate very non-acidic protons, those that would typically be taken using sodamide or LDA or KHMDS. I think the problem with this application is the fact that quantitative conversion needs to occur rapidly, to avoid self condensation. I don't know how such systems stand up on this basis.

AndersHoveland - 6-11-2012 at 04:11

Quote: Originally posted by UnintentionalChaos  

Probably sodium amide is a solid choice.

I have a feeling that sodium amide would cause a different reaction, particularly in the absence of any other reactant.

"The cleavage of ketones by sodium amide was discovered in 1906 by Semmler"
see the Haller-Bauer reaction

R-CO-R' --> R-CO-NH2 and H-R'

not sure if this only applies to non-enolisable ketones

[Edited on 6-11-2012 by AndersHoveland]

zed - 7-11-2012 at 15:02

Ummm. Enolize Acetone....Might be difficult to do by ordinary methods. A relatively useful method for forming the enol of Acetone, is reacting acetone with Ketene.

The resulting product is Isopropenyl Acetate. The Acetate Ester of Acetone Enol.

The product is commercially available at less than $100.00 U.S./Liter.

Ketene itself, isn't too hard to generate, but its use is fairly dangerous. Given the chance, it will rip though your lungs like a raging Roto-Rooter.

Doc B - 7-11-2012 at 22:12

The MeOH sounds the most convenient. Ketene is another good option and easy to obtain for the hobbyist. However being that my eyes still feel like sand paper from my last experiment synthesising formic acid and alkyl alcohol, I'm not sure I'm an enthusiastic enough hobbyist to risk a good waft of ketene at the moment.

The enol is theorised to be used in a halo condensation. Most of the super bases mentioned will work but the acquisition is out of my range at present. I'll give the MeOH a go first and report back.

Cheers

DJF90 - 7-11-2012 at 23:23

A halo condensation? You mean halogenation? Depending on your substrate, you may suffer from regioselectivity issues. If the molecule is symmetrical (e.g. acetone) then this is of little concern. Monohalides are typically formed under acidic conditions, if that helps.

Doc B - 12-11-2012 at 06:33

Quote: Originally posted by DJF90  
A halo condensation? You mean halogenation? Depending on your substrate, you may suffer from regioselectivity issues. If the molecule is symmetrical (e.g. acetone) then this is of little concern. Monohalides are typically formed under acidic conditions, if that helps.


The enol displaces the halogen, I'm not exactly sure of the reactions name. I guess it's an Adol condensation?

SM2 - 12-11-2012 at 08:29

Quote: Originally posted by zed  
Ummm. Enolize Acetone....Might be difficult to do by ordinary methods. A relatively useful method for forming the enol of Acetone, is reacting acetone with Ketene.

The resulting product is Isopropenyl Acetate. The Acetate Ester of Acetone Enol.

The product is commercially available at less than $100.00 U.S./Liter.

Ketene itself, isn't too hard to generate, but its use is fairly dangerous. Given the chance, it will rip though your lungs like a raging Roto-Rooter.


if you filled a flower jar w/ acetone, and had a nice coated Pt catylist floating and fixed above, could you just heat the catylist, and then, it would be exothermic, with the evaporation sendingg a stream of acetone over the glowing Pt?

ScienceSquirrel - 12-11-2012 at 08:36

Quote: Originally posted by Fennel Ass Ih Tone  
Quote: Originally posted by zed  
Ummm. Enolize Acetone....Might be difficult to do by ordinary methods. A relatively useful method for forming the enol of Acetone, is reacting acetone with Ketene.

The resulting product is Isopropenyl Acetate. The Acetate Ester of Acetone Enol.

The product is commercially available at less than $100.00 U.S./Liter.

Ketene itself, isn't too hard to generate, but its use is fairly dangerous. Given the chance, it will rip though your lungs like a raging Roto-Rooter.


if you filled a flower jar w/ acetone, and had a nice coated Pt catylist floating and fixed above, could you just heat the catylist, and then, it would be exothermic, with the evaporation sendingg a stream of acetone over the glowing Pt?


That sounds like an excellent way to get a good solvent fire going.
Acetone is quite volatile and flammable.
This is how you should go about it.
http://www.orgsyn.org/orgsyn/orgsyn/prepContent.asp?prep=cv1...

Salmo - 12-11-2012 at 11:09

I think you need a really strong base to move the equilibrium between the two tautomers, LDA maybe? i think sodium amide would be a nucleophile other than a base.
sodium/potassium tertbutoxide would work too, something that is a strong base but too big to be a good nucleophile too..

DJF90 - 12-11-2012 at 11:31

Its alpha-halogenation of a ketone/aldehyde. Like I said, if you want mono-halogenation, a good method is to use e.g. bromine in acetic acid. Under basic conditions, halogenation tends to be exhaustive e.g. methyl ketones undergo the haloform reaction. Complete formation of the enolate and reaction with an electrophilic halogen source (e.g. N-Halosuccinimide) will also work. Again, I'll mention the regioselectivity issue you may or may not have; you'll get more help if you can be more transparent regards your substrate.

chemrox - 12-11-2012 at 17:10

Na shot in xylene - it's cheap and convenient

DJF90 - 12-11-2012 at 21:52

You sure you wouldn't get competing pinacol coupling, Chemrox?

TheChemINC - 13-11-2012 at 14:32

what about something like sodium methoxide?