Cou
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Synthesis of pure esters from long-chain alcohols using Fischer esterification
Synthesis of pure esters from long-chain alcohols using Fischer esterification
Fischer esterification is straightforward when making esters from small alcohols, up to 5-carbon alcohols, because water washes can remove any
remaining alcohol in workup.
Once you get to 6 carbons and higher, water washes are unable to remove the alcohol from the crude ester, and this affects the smell.
Larger esters, such as octyl acetate and nonyl acetate, have interesting smells. octyl acetate is citrus, orange, earthy, while nonyl acetate is
mushroom, but the presence of residual 1-octanol and 1-nonanol can contaminate the smell.
I tried making octyl acetate and nonyl acetate, but even after washing with large amounts of water, the crude ester consistently boiled at a too low
temperature, likely due to an azeotrope of ester and residual alcohol. I also smelled some of the alcohol scent in the distilled ester. It's apparent
that you must have the reaction consume all the alcohol before working up.
One way is to have the reaction go to completion, so that all the alcohol is used up. Use an excess of carboxylic acid, since it can be easily
neutralized to its carboxylate salt and extracted with water. Instead of using acid chlorides or anhydrides, which are harder to get, you can use the
Dean-Stark method for fischer esterification, which dissolves all reactants and catalysts in toluene, and uses a Dean-stark trap to capture formed
water. Removing water from the reaction will drive it nearly to completion, and an excess of carboxylic acid should ensure all alcohol is reacted. A
nice thing about this method is that you can track reaction progress by looking at the volume of water in the trap, and once water stops forming, the
reaction is done.
Also do vacuum distillation so that heat doesn't hydrolyze the ester.
Source: vogel's practical organic chemistry
[Edited on 24-3-2020 by Cou]
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Fery
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Cou this works, I did it for allylvalerate synthesis and used hexane to drive out water so the reaction proceeded to the point where 1 reactant was
consumed (in my case it was acid, in your case it is alcohol), though there are certainly better solvents than hexane (the lower b.p. of the solvent
the lower effectiveness in dehydratation using Dean-Stark trap adapter).
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Dr.Bob
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My solution has been to use an excess of the acid if possible, and then just work up and push the ester through a small plug of silica gel, where the
acid is trapped, along with any traces of catalytic acid from the synthesis (HCl, sulfuric, or pTsOH). That works OK. A friend is doing this on
kilograms of material for work, and just simply heating (I think they use a Dean Stark as well) in pTsOH with slightly more acid than alcohol drives
the reaction to near completion, then they simply cool it and work it up quickly. But for good purity, they need to finish it with a plug (his
compound is too high MW to distill easily, but that is the best way to purify esters on scale).
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Cou
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Can I dissolve sulfuric acid in toluene or should I use p-TsOH for dean stark technique?
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Amos
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Quote: Originally posted by Cou  | | Can I dissolve sulfuric acid in toluene or should I use p-TsOH for dean stark technique? |
this generates the tosylic acid and water, if you're using a Dean-Stark and adding the toluene back this will have the same result as adding p-TsOH to
toluene in the first place.
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Cou
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I am currently trying out the fischer esterification of 1-nonanol and acetic acid using this method. 10 mL 1-nonanol, 6.6 mL acetic acid (double molar
excess), 1 mL sulfuric acid, dissolved in 66 mL toluene (10 times the volume of acetic acid)
theoretically if the reaction goes to completion, i should collect 1 mL of water in the dean stark trap, but I got 2 mL.
Most of the water separated out within 30 minutes of starting reflux, with a much smaller volume forming in the subsequent hours. I'm keeping it
going for a few more hours to see if more comes out.
i then found out that toluene and acetic acid form a lower boiling azeotrope, and once the reaction slows down and water formation slows, this
azeotrope drips into the dean stark trap, which messes with water separation b/c water is miscible with acetic acid. But there's already at least 10
mL of toluene in the trap so I don't think the acetic acid dissolving into the toluene will hinder the separation of water too much.
I don't know if this method works too well for forming acetate and formate esters. In that case, the best option would be using acetic anhydride and
formic anhydride.
Does anyone know if toluene and propionic acid form an azeotrope below the boiling point of toluene?
[Edited on 16-4-2020 by Cou]
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Cou
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i don't think this works, stick with acetic anhydride for making acetate esters of long alcohols.
I just distilled off the toluene, I have yet to distill the remaining liquid to judge purity by its smell and boiling point. I definitely smell the
mushroom smell of nonyl acetate, but theres still toluene in the flask so I can't tell yet if it's contaminated with 1-nonanol by smell.
[Edited on 16-4-2020 by Cou]
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Cou
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While acetic anhydride is a solution for acetate esters, formic anhydride is not practical for formate esters.
I found a paper that says you can use a mixture of choline chloride and glycerol (a deep eutectic solvent) to separate alcohols from their
corresponding esters. it seems you could use this to purify the ester formed from fischer esterification. https://www.sciencedirect.com/science/article/pii/S004040391...
choline chloride and glycerol are both readily available from amazon. I hope this is worth the money.
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Tsjerk
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If you use xylene, you could do a simple reflux with your acid, alcohol and sulfuric acid and still push the reaction to completion. With xylene, I
have seen a water layer forming at the bottom.
So the water separates in the reaction vessel. I was running a Dean Stark with xylene for the first time some time ago and couldn't collect any water
in the trap, but even while boiling I got two layers and after work-up a near quantitative yield (diketal of a ketone with ethylene glycol).
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Cou
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| Quote: Originally posted by Tsjerk | If you use xylene, you could do a simple reflux with your acid, alcohol and sulfuric acid and still push the reaction to completion. With xylene, I
have seen a water layer forming at the bottom.
So the water separates in the reaction vessel. I was running a Dean Stark with xylene for the first time some time ago and couldn't collect any water
in the trap, but even while boiling I got two layers and after work-up a near quantitative yield (diketal of a ketone with ethylene glycol).
|
I thought about this, but thought it wouldn't work b/c acid catalyst would transfer over to the newly formed water layer, stopping the reaction. I
can't find data on solubility of p-TsOH in toluene to compare it with the water solubility, i guess it would partition between both layers enough for
the reaction to continue, since only a tiny amount in the organic layer is needed.
Also the carboxylic acid would be soluble in the new water layer. But maybe when the carboxylic acid is used up in the organic layer, some new acetic
acid will migrate over from the water layer to establish equilbrium.
I just ordered some choline chloride and glycerol, to try out the separation of alcohols in esters in that paper. It seems easy. I'll try this before
I try your suggestion of xylene as a solvent.
[Edited on 17-4-2020 by Cou]
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Cou
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something funny:
https://www.youtube.com/watch?v=R6UugHqpu2Y
a video where nile red makes nonyl acetate using acetic anhydride instead of fischer esterification. Acetic anhydride reaction with alcohols goes to
completion, so all the alcohol can be used up instead of contaminating the ester. however, i prefer to try making esters from carboxylic acids since
they are more readily available and easy to make. Acetic anhydride is much more expensive than acetic acid too. And formate esters have interesting
smells, but formic anhydride is not stable, so i might as well master synthesis of esters using acetic acid so I can use the same methods for formic
acid.
I would only start doing things like steglich esterification, acid chlorides, azolides, etc, if you are trying to make an ester of phenol or tertiary
alcohols. Industry sticks with fischer esterification where possible, because the lower yield is better than using the expensive reagents in
esterification reactions that go to completion. And if you're using high MW alcohols and/or acids (which are more expensive than the low MW ones such
as acetic, formic acid), you can just use the dean stark method to drive the reaction to completion.
in the comments of that video, nile red says you can use acetic acid, but you can't get a pure ester that way. Even with excess of acetic acid,
there's a significant amount of unreacted 1-nonanol that contaminates the smell and can't be removed with water extraction or distillation.
It seems to me, the best way to do this is just a regular fischer esterification, no toluene solvent or dean-stark (doesn't work with acetic acid b/c
it forms a low-boiling azeotrope w/ toluene), and use the method in this paper to isolate the pure ester https://www.sciencedirect.com/science/article/pii/S004040391... even though the reaction won't go to completion, acetic acid and formic acid are
cheap. the more expensive acids, such as valeric acid, do need to be used as much as possible, and luckily they work with dean stark method, nice how
nature works out like that.
choline chloride and glycerin are both cheap, commonly used chemicals, available on amazon.
[Edited on 17-4-2020 by Cou]
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Cou
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I tried purifying hexyl acetate with a DES, dried with CaCl2 to avoid water azeotropes, but no matter what it always boils at 155 C, instead of the
171 C literature value. I don't know. don't have access to instruments b/c of covid-19
whatever is causing the reduced boiling point, i notice my esters smell a lot purer after the DES extraction.
I am thinking about trying a TLC plate
[Edited on 5-5-2020 by Cou]
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Cou
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oh derp. the reason i was getting low temperatures is because i had my thermocouple set to measure a T-type, when the attachment is a K-type. now i'm
actually getting correct boiliing points.
you know how computer scientists joke about spending hours debugging and going insane, only to realize it was a missing semicolon? that happens in
chemistry too.
i am working on a lab report for synthesis of benzyl acetate now.
[Edited on 9-5-2020 by Cou]
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