flyingbanana
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How to choose PTC?
There are so many! I've mainly been looking at quaternary ammonium salts. I understand the basic principle but I don't see the reasons why someone
would choose a chloride vs. bromide. vs. iodide salt, or how the different tails would interact with your reaction.
Practically, would it be okay to substitute different salts for a procedure that is written for something else?
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Nicodem
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You choose the PTC according to the reaction you run. You just consider that the solubility of the PTC must not be way better in one phase on the
behalf of the other.
As for the anion part it depends on the reaction type and mechanism. For example, if you would use a PTC for an oxidation you would preferably avoid
the iodides. On the other hand, if you would use it for a nucleophilic substitution, using an iodide would be just fine. The other main difference is
in the annoyingly high molecular weight. Even 5 mol% can be a lot considering that, TBAI for example, has the MW of 369 g/mol.
…there is a human touch of the cultist “believer” in every theorist that he must struggle against as being
unworthy of the scientist. Some of the greatest men of science have publicly repudiated a theory which earlier they hotly defended. In this lies their
scientific temper, not in the scientific defense of the theory. - Weston La Barre (Ghost Dance, 1972)
Read the The ScienceMadness Guidelines!
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Rubin
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You are correct in assuming that the PTC anion does often matter.
It depends on the rxn. As a trial, you could pick a PTC which someone has already used for a similar rxn.
What ion are you transferring?
What is the organic group you will be reacting it with?
What solvent would you prefer?
Read Chp 1 of Phase Transfer Catalysis - by Startks, Liotta, and Halpern, 1994, Publ.: Chapman & Hall. ISBN: 0412040719, Chp. 6 - variables in
reaction design is also worth reading, as is the whole book.
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flyingbanana
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^ thanks, i'll check the library.
tetrabutylammoniumiodide and tetraethylammoniumiodide are available to me. I'm interested in williamson ether synthesis (phenols) with various alkyl
halides, for example methyl iodide or allyl chloride, in preferably acetone since that is readily available. The alkyl halides should be soluble in
acetone, so I guess it's the phenoxide ion that needs phase transfer.
tetrabutylammonium seems to be the "standard" for some reason so I guess I'll give that a try. I read somewhere that these kinds of reactions, the
unsymmetrical PTC is best, where one tail is relatively bulky compared to th eothers...
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Rubin
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I'm a bit suspicious of acetone as a solvent here.
I don't think the iodide salt is ideal here; but it will work. In general TBAB (tetrabutylammonium bromide) acts as a very good general purpose PTC
with strong alkali. A nice one to look out for if you are buying one and you can't think of what to buy!
Williamson uses a strong base and you should use the tetrabutylammonium for reactions with strong base rather than the tetraethylammonium (due to the
decomposition of tetraammonium salt by the alkali which is not so much of a problem with alkyl chains longer than proplyl.
No matter. Read this lot:
Japanese patent Abstracts:
JP-6293750
046949-B, 1984
JP-62061975
These journal articles:
O-methylation of Phenols with (MeO)2CO: SynLett. 1998 p1893
PTC Williamson: Tetr. 44, pp6677-80 (1988)
PTC extractability of OH ions and PTC stability: TL 27, pp2361-4 (1977)
PTC Phenol Ethers: Tetr. 30, pp1379-82 (1974)
Excerpts from a book: "Phase Transfer Calalysis", Starks, Liotta, Halpern. 1994. ISBN: 0-412-04071-9
pp 266-283 (Variables in Reaction Design)
pp 410-420 (Williamson synthesis with PTC)
[Edited on 24-7-2006 by Rubin]
Attachment: PTC-Williamson.zip (703kB)
This file has been downloaded 629 times
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flyingbanana
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Thanks for the zip, it was very convenient
I will try water/DCM system with TBAI until I find a more suitable PTC. I'm just worried about how the reaction time will differ with this quat, I
don't have any TLC or anything to be able to check progress...
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flyingbanana
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ughh...I'm not a fan of PTC at all. workup has been an incredible pain...extractions yielded really bizarre layers/emulsions, at some point I had 4
phases! water and DCM would make 3 layers and sometimes the DCM would be on the top, sometimes on the bottom...really really weird and headache
inducing
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flyingbanana
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okay I'm stumped...evaporation of DCM extracts yielded a reddish brown oil, target phenyl ether is supposed to be a solid. Tried crashing out with
cold water, no luck. The oil does not dissolve in basic water, so it can't be the phenol. I tried bisulfite purification to see if it would
crystallize, I end up with the same oil. And now something in the filtrate from filtering the bisulfite adduct is crystallizing. Any way out of this
mess? I have no idea what anything is at this point. I have:
1) clear golden aqueous from the original ether synthesis. contains dissolved alkali salts and supposed unreacted phenol
2) reddish brown oil sitting on the bottom of an opaque dirty yellow 10% NaOH solution
3) cloudy light yellow aqueous filtrate from filtering bisulfite adduct with white crystals forming
I'm really tempted to just throw everything out the window and go to sleep for 100 days.
[Edited on 19-8-2006 by flyingbanana]
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triphenylphosphineoxide
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What solvent did you use to wash the bisulfite adduct? The product may have carried over.
If cold water didn't force product out of oil, try dissolving the oil in a minimum of Toluene or similar. The oil as a whole should have greater
solubility than the ether. Put aside uncapped and allow to slwly concentrate crystals may slowly grow.
I have forced my user name out of oils this way The TPPO crystals took about a week to grow.
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triphenylphosphineoxide
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Ignore the first question above, I'm a sleep deprived idiot.
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not_important
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I know that iodides and PTC are not a good combination in many cases, in part depending on the actual PTC cation and in part on the sovents.
In some cases using PTC or PEG, only a minimal water wash is done at the most, just enough to remove the hydroxide. The reaction mix is vacuum freeze
dried, then extracted with organic solvent; this can avoid the emulsion problems. Shouldn't be a problem, everyone has a freeze-drying rig, right?
Crystals forming from the aqueous bisulfite sound like addition product crystallising out; it may not be the simple adduct you're thinking of.
With DCM and hydroxide, some people have had problems when long reaction time were used. Some of the DCM is hydrolised to H2CO - formaldehyde - which
reacts with active aryl rings and carbonyls. Doesn't take much gorp to make an oil out of the product.
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leu
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| Quote: | | Shouldn't be a problem, everyone has a freeze-drying rig, right? |
Probably not since commercial units are so expensive, but it's possible to construct one very cheaply if one knows how 
Attachment: Taylor.zip (158kB)
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Chemistry is our Covalent Bond
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flyingbanana
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Update:
Combined all the solutions and isolated the oil (again).
Didn't feel like waiting so long for triphenylphosphineoxide's advice (annoying compound by the way, it's a real pain to remove) so I opted for bulb
to bulb vacuum distillation. I first tried with a warm condensor but crystals clogged it up so I set up the distillation so that the vacuum adapter
and receiving flask were connected directly to the head. Heating under water bath. Crystals started forming in the receiving flask, but I couldn't get
all of it because of the water bath...have to finish with oil bath. But the crystals look kind of strange, I think the aldehyde might have gotten
oxidized to benzoic acid because the crystals look like shards. Either way, I'll recrystallize them and then stare at the crystals and dream about
having an IR, NMR, or even melting point instruments
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