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Author: Subject: Azeotropic esterifications
Boffis
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[*] posted on 24-11-2019 at 05:06
Azeotropic esterifications


Having examined Pumukli's desription of his experiment into the production of ethyl benzoate by the azeotropic method using chloroform as the entrainer I decided to give this method a go with several esters. Because I have a bottom draining Dean & Stark water trap I decided that it would be better to use an organic phase that is lighter then water. Traditionally benzene was used for the removal of water from alcohol so this was my first choice in spite of its known problems.

My first experiment was with oxalic acid and the result was excellant particularly considering that I use oxalic acid dihydrate straight out of the bottle, no drying first, and rectified spirit (95% ethanol) The amount of oxalic acid recovered from the Na bicarbonate wash is small and I am now working up the ester.

Flush with success I decided to run a second batch with benzoic acid but because this is a weaker acid I added a small amount of conc sulphuric acid (5ml to 180ml of 95% ethanol). This reaction by contrast did not yield a single millilitre of immiscible aqueous phase in 5 hours of reflux!

So whats the problem? A few drops of the reaction mixture from the benzoic acid experiment mixed with a little dilute Na bicarbonate gave oil drops so at least some ester has formed but why then have I collected no water? I will work this mixture up and report my results in due coarse.

In the mean time I wonder if the problem is that 5ml of conc sulphuric is capable of sequestrating the water in the partially reacted mixture (9g from the ethanol and about the same from the ester formation if all 60g of benzoic acid reacted)?

The obvious extension to this line of thought is- would it be better to use a strong acid that has little tendency to hold water? If so does anyone have any ideas? HCl is often used but it is volatile and may complicate the procedure, then there are the sulphonic acids like toluene sulphonic acid and methanesulphonic acid; trichloro or trifluoroacetic etc.

A couple of final questions: 1) are benzene and water-free ethanol miscible? 2) Why is benzene the solvent of choice, I know that toluene can be used but is much less efficient and is still an aromatic liquid. Could an aliphatic solvent such as hexane, cyclohexane, 60-80C boiling range petroleum ether etc be used? They have similar boiling points, low water solubility and are less toxic.
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[*] posted on 24-11-2019 at 05:32


"are benzene and water-free ethanol miscible?"
Yes, and the ethanol can "carry" water into benzene and vice versa.
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Pumukli
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[*] posted on 24-11-2019 at 05:55


Dear Boffis,

I'm glad I could contribute something useful to the knowledgebase of the group. Here are a few more tid-bits regarding your questions:

First of all I don't think the problem is in the ability of the sulfuric acid to sequester the water. There is so much water (relatively) in the mixture that I don't "feel" it possible.

I checked my original lab-book I was working from and found in the fine print the following info:"Benzene is also appropriate (for making ethyl- or propyl-esters). In this case, however, the ratio of water to the volume of alcohol also distilling off is more unfavourable."

The general procedure in the book calls for 1 mol carboxylic acid, 1,75 mol alcohol (not necessarily anhydrous) and 5 g conc. sulfuric acid, or toluene-sulfonic-acid or naphthalene-sulfonic-acid or freshly prepared acidic ion exchange resin (5 g H-ion capacity), plus 100 ml chloroform (or other solvent). For hydroxy-acids or alpha,beta-unsaturated acids or in the case of secondary alcohols the preferred catalyst is sulfuric acid. Ion-exchange resins must be mechanically stirred otherwise severe bumping occurs.

The book explicitely states that the method can be used for the preparation of oxalic acid ethyl-ester. In this case oxalic acid hydrate can be used and no additional catalyst is necessary. But you already found out this yourself. :)

The book also lists benzoic-acid METHYL ester as a possible candidate for this azeotropic method. Curiously though, no other aromatic ester (either methyl or ethyl) is listed as suitable for this method. (Salicylic methyl, salicylic ethyl, toluic acid methyl and phthalic acid diethyl esters are in the book too.) I proved that at least benzoic acid ethyl ester can be made via this route, though I used chloroform entrainer.

On the other hand the method is appropriate for most aliphatic acids, even delicate ones like crotonic acid, lactic acid or tartaric acid. (The OH-acids require ion-exchange resin as a catalyst and washing with water should be omited due to the sensitivity of the esters.)

Benzene and anhydrous ethanol is most probably miscible. Anhydrous ethanol is miscible with petrol-ether (gasoline) as far as I know.
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Boffis
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[*] posted on 24-11-2019 at 07:38


Thanks Pumukli, I am just doing the work-up on the benzoate batch. I first distilled off 150ml of the volatile fraction and then mixed the flask residue (quite clear) with water (about 100ml) and then added saturated Na bicarbonate, about 200ml + 12ml of 50% NaOH solution too. When effervescence stopped I poured it into a separation funnel and ran the lower organic layer back into the beaker and added more sodium bicarbonate solution (150ml). I extracted the aqueous layer with DCM and then acidified it to recovery the unreacted benzoic acid, its chilling now in the fridge.

It looks like I have got about 40-50ml of crude ethyl benzoate anyway. I will do this again as I want about 200ml of ethyl benzoate but next time I think I will use the alcohol as the entrainer. My plan is to dry the alcohol azeotropically with benzene or similar first and then reflux the benzoic acid with the dried ethanol for a couple of hours and then distil of the alcohol through a fractionating column. The idea being that if the excess of alcohol is such that the esterification does not generate enough water to drop the ethanol content below 95% the column return will be drier than the vapour leaving the reaction mixture and so drive the equilibrium towards completion. With 100g of benzoic acid complete esterification would yield just less than 15g of water so as long as I have more about 300g of dry ethanol excess over that required for esterification the vapour at the top of fractionating column should contain more water than the liquid leaving at the bottom of the column. The column exit vapour should be close to 95% ethanol.

I also recovered the unreacted oxalic acid from the first ester prep and recrystallised it from water (nice glassy prisms). I am ready to distil the oxalate ester now, watch this space!
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S.C. Wack
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[*] posted on 24-11-2019 at 10:18


Maybe the question is what kinds of azeotropes may form in a mixture of benzene, ethanol, water, and ethyl benzoate, and what is the boiling point of this mixture. I note that Cumming et al. gives 90% with refluxing dry ethanol and benzoic and sulfuric acids, and 95% yield of the methyl ester has been claimed by refluxing with DCE; water separating in the flask. BTW there is a diethyl oxalate method where the water is removed by vacuum distillation.

[Edited on 24-11-2019 by S.C. Wack]




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Boffis
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[*] posted on 25-11-2019 at 05:03


Well I distilled the diethyl oxalate and got 38.09g boiling between 183-185 C which is about 55% so not brilliant but I did recover 17.83g of oxalic acid from the bicarbonate wash with any attempt to concentrate the residue after the first crop of crystals.

I am currently distilling the ethyl benzoate and drying the recovered benzoic acid.

By the way Pumukli does your "book" say anything about succinic acid? I have also dug out some Amberlite 120 H+ form and some toluene sulphonic acid for future runs.
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Pumukli
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[*] posted on 25-11-2019 at 06:55


Yes. Diethyl-succinate can be prepared this way, the book claims 90% yield. (While only 70% for diethyl-oxalate.)

[Edited on 25-11-2019 by Pumukli]
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Boffis
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[*] posted on 25-11-2019 at 07:21


Thanks Pumukli, interestingly I am currently trying to azeotropically dry 350ml of bought 95% ethanol and I can't get any water out!! I am using the classic benzene method so whether the alcohol is actually almost dry or something else is wrong I am not sure.

I distilled the ethyl benzoate and recovered 43.14g or about 35% but there was about 10ml left in the still. I will add this to the next batch. So it appears that benzene is not a very useful entrainer for ethyl benzoate preparation. The oxalate prep gave 55% and worked well so I think a higher proportion of alcohol would help too as a lot of ethanol comes of with the water and I distilled off most of the alcohol too.

I will try a 50g batch with succinic acid
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Boffis
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[*] posted on 28-11-2019 at 04:59


I ran a larger scale oxalate preparation as before and added a little toluene sulphonic acid to assist with catalysing the reaction when was approaching completion but otherwise just a scaled up version of the earlier one but the yield was less only 52.7g or about 45% of theory based on oxalic acid. I ran this experiment for 24 hours and extracted 160ml of aqueous phase too. So it looks like pre-drying of the oxalic acid is the way to go.

I found a paper that describes the drying of oxalic acid by distillation with carbon tetrachloride or chloroform but I wonder if you could use say toluene?

I am going to try ethyl succinate next and also try recovering the excess alcohol distilled off from the three runs I have done so far.
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[*] posted on 28-11-2019 at 07:03


Quote: Originally posted by Boffis  
I found a paper that describes the drying of oxalic acid by distillation with carbon tetrachloride or chloroform but I wonder if you could use say toluene?
Yes, both Doug and Nile demo-ed this on YT.



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S.C. Wack
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[*] posted on 28-11-2019 at 08:36


Chemistry existed before videos. https://www.sciencemadness.org/whisper/viewthread.php?tid=10...



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Pumukli
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[*] posted on 28-11-2019 at 09:14


I think there is some sort of "unforeseen" problem with benzene in these reactions. Maybe as S.C.Wack suggested, some sort of unpleasant azeotrope forms with the product and it lowers the yield, I don't know.
What I know from first hand experience is chloroform worked nicely in my ethyl-benzoate synth, I can't remember the exact numbers but the yield were around 80% I think.
Also, the book I reffered from, uses either chloroform or CCl4 as entrainers, and although mentions "benzene can be used" but actually never uses it. It may mean something - and probably not the unavailability or price of this chemical in those days (50+ years ago).

If you have the proper glassware you might try a synth with one of these high-density solvents. You can recover them in the end, I think.

The book explicitely states that drying the oxalic acid is not necessary, and I also used battery acid in my experiment - although not with benzene but with chloroform.
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[*] posted on 29-11-2019 at 12:19


Hi Pumukli, on the point of using a heavy entrainer in a soxhlet extract as you had done; why doesn't the liquid, heavy entrainer and aqueous phase together, periodically just syphon off as it does when it is being used in its proper function?

I have also made an interesting observation; in my earlier experiments I didn't use enough entrainer. I am currently running 50g of succinic acid in 100ml of recovered rectified spirit and 80ml of benzene. With 40ml of benzene as I added to start with the benzene refluxed but no water came over. I have collected about 30ml of aqueous phase so far but when working up the aqueous distillate from the earlier experiments I found that it consists mostly of alcohol so in spite of the fact I am looking for only 20ml of water from the reaction and alcohol I may need to collect much more "aqueous phase".
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