Sciencemadness Discussion Board

Acetone Enolate

lacrima97 - 20-1-2006 at 17:46

I have been looking for information on the acetone enolate ion. I am having a bit of a time pinning down a good solid source of information. I have read an excerpt that says that this substance will be formed when acetone is reacted with a strong base that has a pKa higher than that of 23. I don't know how solid this information is, therefore I resort to the smart people of this board.

Thanks.

[Edited on 1/21/2006 by lacrima97]

solo - 21-1-2006 at 04:23

Quote:
I've written a lot about practical clandestine enolate chemistry, but there's always been a bit of a sticking point in much of my discussion. How does one practically make enolates at home? Enolate chemistry often requires *strong* bases, ones that mere WATER is far to acidic to have in its presence (pKa>15), so what's a bee to do?

Alkoxides.

By mixing sodium metal with an anhydrous alcohol, the sodium with strip off the hydroxy's relatively acidic proton, making an alkoxide -- a base of great strength and usefullness in making enolates at home. Just to give you an idea regarding their acidity, NaOH has a pKa of about 14, NaOMe has a pKa of 15, NaOEt's is around 26, and NaOiPr around 28-30 (all of this is measured in DMSO, and the numbers kinda vary from source to source.) Here's how.

Now its important that everything be absolutely anhydrous, and some saferty precautions should be taken. The alcohol must be 99+% pure, the glassware should be flame-dried. Good ventilation is needed, since the reaction generates hydrogen gas, and having a build-up of that can lead to Hindenburgian results. With that said, let's get cookin'!

In a 250-mL RB flask equipt with a reflux column and a magnetic stirrer, nestled in an ice bath, 25 mL of acohol (MeOH, EtOH, iPrOH) is allowed to chill with stirring. Once the alcohol is good and chilled, 1.32 grams of cleaned sodium metal is carefully added, and a tube full of dried CaCl2 to placed on the outlet of the relux column. Immediately, the reaction begins to take place, and a lot of heat and hydrogen are generated. THe solution will begin to boil as the sodium dissolves, and eventually, when all the melodrama of the disolving is over, replace the icebath with a hot water bath, and reflux an additional 15 minutes. From there, evaporate off the excess alcohol, yielding the final product (around 3.5 grams.)

Scaling up requires a little extra finess. When using EtOH or iPrOH, chilling the alcohol first in an acetone/dry ice bath, then gradually allowing the solution to come to room temperature will allow you to add the sodium metal without too many fireworks. Always keep ventilation a top priority.

The final product will be white-yellow (with ethoxide, its very yellow), and decomposes with too much exposure to air. So, once you have a nice, off-white mass of crap, keep it tightly sealed right up until you need to measure it.


.........as read here,.........solo

http://designer-drugs.com/pte/12.162.180.114/dcd/chemistry/p...

Nick F - 21-1-2006 at 07:40

Well, it depends on your reaction. Hydroxide ions will form a small equilibrium percentage of enolate ions in solution, so it can be used to form your enolate, unless the enolate is reactive towards the parent ketone. In this case you need full deprotonation of the ketone, and therefore a very strong base, to avoid messy polymerisation.

lacrima97 - 21-1-2006 at 07:55

Ahh, thank you. I know that the page you posted, solo, is the first page that comes up when "acetone enolate" is typed into google. I read the top of this page, and saw a brief description of the process, and didn't think there would be anything else to it. Thank you guys.
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So, do you think that if this final product from the sodium/alcohol reaction, was reacted with acetone, that the acetone enolate would form ok? It would be very nice if this could then be reacted with iodobenzene. I don't have any "clandestine" purposes, but I just think it would be neat to be able to make phenylacetone.

[Edited on 1/21/2006 by lacrima97]

solo - 21-1-2006 at 09:15

Just in case your mind drifts.......solo
Quote:

...............Hurtley Reaction

ArX + CH3COCH3 ----?-----> ArCH2COCH3 + ArCH2CHOHCH3

?=liquid amonia containing Na or K
X= Halide

The text goes on and says that it also occurs under UV-light when K or Na are omitted.

The reaction on unactivated halides can also be done with copper halide catalysts (the Hurtley Reaction), and with palladium complex catalysts. Compounds of the form CH3Z can be arylated by treatment with an aryl halide in liquid ammonia containing Na or K, for example (it gives this specific example):

ArX + CH3COCH3 ==NH3/K==> ArCH2COCH3 + ArCH2CH(OH)CH3

The same products are obtained, although in different proportions, when Na or K is omitted but the solution is irradiated with near-UV light. In either case other leaving groups can be used instead of halogens (e.g., NR3+, SAr) and the mechanism is the SRN1 mechanism. Iron(II) salts have also been used to initiate this reaction."

OrgSyn V, 12, 263; VI, 36, 873, 928; VII, 229.



.........March's 5th edition page 869,


Note : edited by solo, Iposted the wrong reference, it was to another thread, all corrected now

[Edited on 22-1-2006 by solo]

Nick F - 21-1-2006 at 11:39

Benzene only becomes electrophilic when it is activated with strongly electron-withdrawing substituents which stabilise the negative charge of the intermediate (F, CF3, NO2, CN, COR etc, F and CF3 working due to their massive inductive effects, the rest by conjugation of negative charge onto electronegative atoms), AND it has a good leaving group (typically halogens) which can then carry away the negative charge and restore aromaticity.

Iodobenzene will not react since that would require an SN2 mechanism, which would in turn require that the enolate was inside the benzene ring, and the attacked carbon would have to undergo stereochemical inversion, which would produce a totally silly structure. SN1 reactions aren't going to happen since phenyl carbocations are really unstable, and won't form unless you really force them to.

Edit: OK, maybe iodobenzene does dissociate fast enough.

Edit: Ah, I see they use catalysts and/or UV. That'll certainly help!

You could try chlorinating your acetone and then reacting it with benzene Friedel-Crafts style with AlCl3, but that will be messy since the alkylated benzene will be more reactive than benzene itself and will therefore react in preferance to benzene, leading to polyalkylation. It could be done, but you won't get a good yield.

[Edited on 21-1-2006 by Nick F]

[Edited on 21-1-2006 by Nick F]