Melgar
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Most practical way to oxidize a primary alcohol to an acid?
Say I have a primary alcohol, n-butanol, and want butyric acid. Most of the preparations on the internet say to use either potassium permanganate,
which is both expensive and messy, some form of hexavalent chromium, which is extremely toxic and expensive, or some platinum group catalyst which
obviously wouldn't be my first choice. Could I get a cleaner reaction with, say, sodium hydroxide and 30% H2O2? Presumably, i could then evaporate
and acidify with H2SO4 or H3PO4, then distill. (Using acidic peroxide solution and distilling from that seems like it could result in unwanted
free-radical oxidation, ie, piranha solution.)
Incidentally, "base piranha" also is a thing, but uses ammonium hydroxide, and needs to be heated in order to work, and also seems to be a somewhat
complex reaction involving formation of amides. If I want it to stop at the acid, or a soluble salt of the acid, sodium hydroxide or sodium carbonate
would seem to be the way to go. And sodium carbonate can complex with hydrogen peroxide to give sodium percarbonate, AKA "Oxyclean". I'm not finding
much in the literature on this, presumably because either it doesn't work, or it's not very difficult to do, and I'm not sure which it is. But
considering that long-chain alcohols seem to be a lot cheaper and more readily available than long-chain acids, I'd like to have a reliable, clean,
go-to reaction for getting their corresponding acids. Anyone here have enough experience doing this to advise me, or at least point me in the right
direction?
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CuReUS
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see this thread -http://www.sciencemadness.org/talk/viewthread.php?tid=29620
although the paper title says esters,you can also get only the acid if you want
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Maroboduus
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I thought you could change the longer chain alcohols to acids by using bleach in acetic acid, but I'll be damned if I can find a reference.
Lots of stuff about making ketones out of secondary alcohols that way, but I suppose that's a more popular reaction.
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aga
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Why would you think that ?
An alcohol in an aqueous solution of sodium hypochlorite and acetic acid ?
The mind Boggles, but then, it does that a lot.
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blogfast25
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Quote: Originally posted by Melgar  | | Say I have a primary alcohol, n-butanol, and want butyric acid. Most of the preparations on the internet say to use either potassium permanganate,
which is both expensive and messy, some form of hexavalent chromium, which is extremely toxic and expensive, or some platinum group catalyst which
obviously wouldn't be my first choice. |
I'd recommend strong nitric acid (> 70 %). The advantage is that it leaves no residual cations and any excess NA can be distilled off.
Dichromate (hexavalent Cr) isn't really that toxic and is easily managed. But it's becoming very hard to get. And you need to separate the butyric
acid from the Cr3+, K+ and SO42-.
| Quote: Originally posted by Maroboduus | I thought you could change the longer chain alcohols to acids by using bleach in acetic acid, but I'll be damned if I can find a reference.
Lots of stuff about making ketones out of secondary alcohols that way, but I suppose that's a more popular reaction. |
Bleach means a TON of water. More separation work!
[Edited on 25-10-2016 by blogfast25]
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Melgar
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Thanks, I actually do have some oxone that I nearly forgot about (sold as chlorine-free pool shock), but the DMF requirement is a nonstarter. I don't
really mind if I have a 10% drop in yields because I used a non-ideal solvent, since I'd rather lose a small amount of butanol than a large amount of
DMF. The OP in that thread didn't seem to use it though, so perhaps all will be good? Perhaps if done in an alkaline media, the acid will form salts
rather than esters? Since butyric acid has a characteristic strong smell, I should be able to do some preliminary tests to see how well these various
oxidation systems work.
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Texium
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| Quote: Originally posted by blogfast25 | | Dichromate (hexavalent Cr) isn't really that toxic and is easily managed. But it's becoming very hard to get. And you need to separate the butyric
acid from the Cr3+, K+ and SO42-. |
Yes, and that separation is
something that's harder to do in practice than it looks on paper. I tried making butyric acid once using the dichromate method, and it seemed to be
going well until the step where you neutralize the acid and precipitate the Cr(III). The precipitate was very fine and filtering it was a nightmare,
even using a fine frit and a vacuum pump. Once the solution began to come through the filter, it foamed up massively, threatening to get sucked into
the vacuum pump. Also, the acidic conditions of the reaction promoted the formation of some butyl burtyrate, based on the pleasant fruity smell that
became more apparent after the mixture was neutralized and the smell of butyric acid was no longer covering it up.
I haven't tried it before, but the oxidation using permanganate in basic conditions is supposed to be much easier and cleaner. I have quite a lot of
potassium permanganate and a cheap source of it, so I plan on trying the method sometime in the future.
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blogfast25
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| Quote: Originally posted by zts16 | | I tried making butyric acid once using the dichromate method, and it seemed to be going well until the step where you neutralize the acid and
precipitate the Cr(III). The precipitate was very fine and filtering it was a nightmare, even using a fine frit and a vacuum pump. Once the solution
began to come through the filter, it foamed up massively, threatening to get sucked into the vacuum pump. |
It should be possible to control the precipitate by the conditions of formation: from dilute solution you get more fluffy, easier to filter
hydroxides. But of course than means more damned water too...
But even then, there's still the K2SO4 to contend with.
[Edited on 25-10-2016 by blogfast25]
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Maroboduus
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| Quote: Originally posted by aga |
Why would you think that ?
An alcohol in an aqueous solution of sodium hypochlorite and acetic acid ?
The mind Boggles, but then, it does that a lot. |
I thought that because I thought I'd seen such a reaction.
And certainly bleach in acetic acid is used for the oxidation of secondary alcohols to ketones, and aldehydes to carboxyllic acids, and bleach
(whether in acidic or basic media I don't recall) cleaves alkylated aromatics to benzoic acid.
So just why would your mind boggle at the reaction I mentioned? If you have some reason to believe that HOCL (the active species in those reaction
conditions) wouldn't oxidize primary alcohols perhaps you should actually explain it.
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blogfast25
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@Maroboduus:
aga's mind boggles a lot when it's ladled with EtOH, take no notice.
Hypochlorite is more than potent enough to oxidise alcohols to acids. How practical a method that is, is another matter. 
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Maroboduus
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Thanks, for the info.
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CuReUS
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that's not a bad idea,but I think you should try what wack suggested before changing the pH of reaction.Like he said,keep the quantity of butanol to a
minimum so that there isn't a lot of it around to react with the butyric acid formed.
perhaps if you slowly dripped in butanol into a flask of oxone,NaCl and water,you could reduce the amount of ester formed.
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Melgar
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| Quote: Originally posted by CuReUS |
that's not a bad idea,but I think you should try what wack suggested before changing the pH of reaction.Like he said,keep the quantity of butanol to a
minimum so that there isn't a lot of it around to react with the butyric acid formed.
perhaps if you slowly dripped in butanol into a flask of oxone,NaCl and water,you could reduce the amount of ester formed. |
I don't really like using oxone all that much, since it's a bit expensive and doesn't give you all that much oxygen per unit mass. I was hoping to
use peroxide somehow, and actually found an interesting presentation describing how to make acids from alcohols using peroxide, an alkali tungstate,
and a phase-transfer catalyst. I actually have a lot of alkali tungstate from my experiments from extracting thorium oxide from 4% thorium, 96%
tungsten TIG welding rods, so this would be a good option for me:
https://www2.chemistry.msu.edu/courses/cem958/FS08_SS09%5CMi...
Tunstates aren't expensive, at least not if you buy tungsten and dissolve it electrolytically, or dissolve it in H2O2. And since it's just a
catalyst, you don't need that much. Molybdenum (which is directly above tungsten on the periodic table) can also do this, but will stop at the
aldehyde, and will not oxidize that further. That seems like it'd be a useful reaction as well. Supposedly > 95% conversion and selectivity of
benzyl alcohol to benzaldehyde. It even works with benzyl chloride, converting that to an aldehyde as well.
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