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Author: Subject: ethyl formate - esterification by reactive distillation
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[*] posted on 20-5-2020 at 11:14
ethyl formate - esterification by reactive distillation


Surprisingly this esterification experiment is not yet posted in the SC forum although both reagents are readily available, formic acid is sold to beekeepers and ethanol is everywhere.

Esterification by reactive distillation is a process where low boiling ester distills directly from the reaction mixture which shifts equilibrium towards the product. If it is not the ester itself (ethylformate) which has the lowest b.p. and is distilling, then its azeotrope with water (propylformate, ethylacetate, propylacetate).

Formic acid is strong enough to catalyze its own esterification. Adding conc. H2SO4 is bad idea as it catalyzes formic acid decomposing into highly poisonous odorless CO.

This experiment was a study of the ester formation (limited by capabilities of home lab) and focused on squeezing as much of the product from the formic acid as possible.

Initially the reactants were calculated as:
2,5 mol ethanol M=46,07 g/mol, 115,2 g 100% ethanol = 121,2 g 95% ethanol
3,5 mol formic acid M=46,03 g/mol, 161,1 g 100% = 189,5 g 85% formic acid

Apparatus:

IMG_20200518_152646_6_sm.jpg - 37kB

The apparatus is shown on the picture - 500 ml 3-neck RBF, dropping funnel, 30 cm Vigreux colum, third neck with themometer to monitor the reaction temperature (later it revealed to be useless). Also the position of the dropping funnel revealed not to be optimal as dropps of ethanol to the flask wall could lead to its evaporation (dropping into reaction mixture instead to the flask wall could be better). Distillation head with thermometer, condenser, vacuum take-off adapter (necessary to reduce loses of the product due to its low b.p.), 250 ml 2-neck flask for collecting the distillate (its second arm allows to remove various fractions of the product during the reactive distillation). The column was thermally insulated by few layers (approx 4-5) of newspaper wrapped around hold on place by thin wire.
The flask was filled with 198,5 g of 85% formic acid (yes, I do not measure volumes of liquids, I weigh them instead) and the dropping funnel with 121,2 g of 95% ethanol (commercial ethanol denaturated with 1% methylethylketone was purified by refluxing with NaOH and then distilled through 500 mm Hempel column).
The reaction flask was heated on boiling water bath (the pot on the picture was later covered with aluminium foil to reduce heat loses and water evaporation, though it was necessary to replenish the missing water few times with fresh boiling water during few hours lasting experiment). Once the formic acid reached temperature 60 C, addition of ethanol was started dropwise at a rate of approx 1 drop per 1 second. After about 15 minutes the product started to distill and the rate of ethanol drops was adjusted to match the rate of the product (note that size of drops could be different due to diameters of tubes of dropping funnel and vacuum take-off adapter, later it was necessary to slightly increase the rate of ethanol drops as the rate of drops of the product decreased.
The distillation rate was 15 drops per 10 seconds intially (as the time passes, reactants consumed and H2O in reaction increases, the rate of distillate decreases so the rate of ethanol dropping due to lower level in the addition funnel, after half of the product distilled the rate was 12 drops per second).

IMG_20200518_165211_0_sm.jpg - 17kB

The T in the head recorded vapour temperature in range 51-56 C, most of the distillate in range 54-56 C (the low temp. was only at the beginning when the distillation started and at the end when reactants consumed). I collected approximatelly 170 ml of the product and then then rate of drops of distillate ceased and T in distillation head decreased. This product was removed from the collector flask (by rotating the vacuum take-off adapted to horizontal position) and kept aside. Fraction 1.
I decided to add extra 46 g of 95% ethanol (0,95 mol) into addition funnel with a hope to obtain more product and continued dropping. The reaction resumed and T in distillation head rose again to well known 54-56 C and dropping rate was the same as previously. I collected approx 60 ml of distillate. Fraction 2.
Then as a curiosity (to squeeze as much formic acid from the reaction mixture as possible) I added again 46 g of 95% ethanol (0,95 mol) and resumed the reaction. T rose again to well know good temperature, but after about half of the ethanol dropped it slowly rose further to 60 C and at the end it was 65 C and the rate of distillate 5 drops per 30 seconds. Fraction 3, approx 50 ml.
Although during the last fraction 3 the T in the head rose to 65 C it did yield some ester very likely with some ethanol (due to higher b.p.). Fraction 3 was washed 2 times with 50 ml of ice cold saturated NaCl solution to remove substantial content of ethanol and it surprisingly yielded approximatelly 40 ml of ester as upper layer in the separatory funnel.

IMG_20200518_201631_5_sm.jpg - 24kB

Fraction 2 was washed once with 50 ml of ice cold saturated NaCl solution as I expected much less ethanol in it than in fraction 3.
Fraction 1 was washed once with 100 ml of ice cold saturated NaCl solution as there was barely any ethanol.
All fractions were merged together (volume approx 260 ml). Note the solubility of ethylformate in water is 9% at 18 C (perhaps it could be economical to distill the ester from all the washings).
It was dried by 20 g of anhydrous CaCl2 (as it binds not only H2O but also ethanol) for 1 day at room temperature ocassionally shaking it in stoppered flask and breaking the drying agent lumps with a glass stick (perhaps traces of formic acid could be removed by addition of of CaCO3 ?). Then the flask was put into a freezer to -18 C to decrease the amount of Ca salts dissolved in the ester.
Vacuum filtration is unsuitable because of low b.p. so it was just poured (almost all CaCl2 stayed sticked to the bottom) into 500 ml RBF from which the product was distilled on a steam bath using 500 mm Hempel collum packed with Raschig rings and insulated by newspaper (no distillation head for taking reflux as I do not have it yet so the reflux rate was unknown and quite poor and was just slightly improved by wrapping 3 turns of rubber hose around column top end).
Fraction collected 53,5-54,5 C (most of it distilled at 54 C). Yield 170,7 g. Forerun 15 ml. A lot of CaCl2 stayed in the flask as a distillation residuum (so maybe it is not too much suitable drying agent although it binds ethanol too) from which after adding water 20 ml of ester obtained. From the drying agent after adding H2O 10 ml of ester obtained. Approx 25 ml of ester could be lost during washing in separatory funnel from which it was not recovered (distillation could easily recover it). Maybe there is no need to rectify Fraction 1 (maybe it is already pure enough) and only Fractions 2, 3 should be processed further?
b.p. 54,0 C (lit.)
The scent of the product is solvent-like.




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[*] posted on 20-5-2020 at 13:22


Really nice prep, thanks for posting!

I also plan in future make some ethyl formate. Can be this synthesis done just with simple distillation setup?

Solvent-like scent of product surprise me, because ethyl formate is used as rum essence. But scent may be different in low concentrations than in pure form...





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[*] posted on 20-5-2020 at 19:32


epic x)

low MW esters such as ethyl acetate, ethyl formate, are more challenging to isolate b/c of their relatively high solubility in water. typically you do a normal fischer esterification reflux for several hours. then the first workup step is fractional distillation. The azeotrope of the ester with water is lower-boiling than other components. which isolates mostly ester with a smaller amount of water which can be removed with dessicants.

[Edited on 21-5-2020 by Cou]




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[*] posted on 20-5-2020 at 22:24


Hi Bedlasky, of course it could be done using simple distillation apparatus, in that case just don't forget to use some excess of formic acid. When dropping ethanol into reactor the amount of ethanol in the reactor is smaller so less contamination of the product with etanol than if it is present in the reactor from the beginnig at the full amount. If you use excess of ethanol it would contaminate the product especially at the end of distillation (only 20 C difference in boiling points).
The scent of ethylformate is not pleasant for me, just solvent like scent. In the rum essence there must be at least another ester with it, not only this one. I synthesized this ester because expected the same as you, rum essence (my father bought rum essence in grocery shop half a year ago wanted to prepare home made rum from food or pharma grade ethanol but unluckily bought rum essence dissolved in vegetable oil which was for cooking not for beverages - "microscopic" letters on small flask claimed its usage for baking).
Cou ethyl acetate could be done the same way but there are huge loses of the ester during workup for removing ethanol contamination. If you want I can post here a scanned page from my old organic chemistry book which would require my translation into english as it is in my language. Ethyl acetate is readily available everywhere (if you are unlucky and cannot buy it from chemical supplier then you can certainly get it as nonacetone nail polish remover) and very cheap so no need to synthesize it in home lab. I suggest rather formic acid as the product is much purer and loses of the ester are smaller than for ethylacetate.
Back to my experiment - I realized the setup was not optimal as dropping ethanol from side arm of the flask to flask's wall could lead to ethanol evaporation before reaching reaction mixture. Better setup would be mixing ethanol with formic acid e.g. at 1 mol : 1 mol and put it into dropping funnel (so less ethanol evaporation if dropping to flask wall) and having 0,5 mol of the acid in the reactor flask. Then using dropping funnel with so long stem that the end of the stem is positioned below the level of reaction mixture - optimally close to the bottom of the reaction flask (if ethanol just drops to the top of the reaction mixture it is more prone to evaporation than when it enters the bottom of the reactor). That could improve the purity of the product so maybe fraction 1 could be already pure enough so no need to rectify it further (reducing loses of the product).
The surprising yield of the ester from the fraction 3 when theoretically all formic acid should be already consumed (fraction 1 should consume 2,5 mol of acid and fraction 2 should consume 0,95 mol of the acid which was initially 3,5 mols, but fraction 3 still yielded approx 40 ml of the ester (0,5 mol) so that could be explained only that some ethanol escaped and contaminated product (mostly in fraction 2 and maybe at smaller extent also fraction 1). Other explanation is that may acid claimed to be 85% was more concentrated and that number is only granted lowest minimal limit of the concentration (in fact the label has 85% with big letters and on the right side of the label with small letters is minimal concentration 95,0% so I do not know whether 95,0% is not only typo mistake...).

[Edited on 21-5-2020 by Fery]




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[*] posted on 22-5-2020 at 11:04


Nice work, I have often wondered whether it was possible to making ethyl formate from 85% formic acid. I had assumed that the yield would be very poor and had therefor looked into ungrading the formic acid feed to say >97%. I have checked out a lot of references and patents but few are very DIY friendly. I tried azeotropic distillation but the gains from 85% acid are small and numerous attempts gave a tritration max of about 93%. The freezing method works and this looked like the best amateur method. But now you have shown that it not necessary to up grade the formic acid.

Well done!
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[*] posted on 22-5-2020 at 13:05


Quote: Originally posted by turd  
Check out: E. C. Wagner, Journal of Chemical Education, 27, 245-247 (1950)




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[*] posted on 22-5-2020 at 19:39


I will be using this procedure to make ethyl formate for my ester collection. I'm thinking about if this will work for methyl formate too. BP of methyl formate: 32 C. BP of methanol: 64.7 C



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[*] posted on 23-5-2020 at 03:37


Cou methyl formate should be produced the same way too, just use efficient condenser (Graham, not Liebig) and ice cold liquid for circulation through the condenser. For ethyl formate the Liebig and 10 C tap water was efficient enough. Also use vacuum takeoff adapted and another ground glass flask to reduce the ester evaporation during distillation (do not let it to just drip from the end of condenser through open air). Very likely it will be much more soluble in water so perhaps worth of recovering it from wash water by distillation of the wash water to increase yield. Maybe do not wash at all and just column distillation to remove small amount of alcohol and acid from the ester?
Because of dropping the alcohol into excess of acid the amount of alcohol evaporated is reduced (just try to drop it into the reaction mixture, not to the flask wall, or the best use some dropping funnel with long stem which ends just above the bottom of the distillation/reactor flask deep enough below the reaction mixture surface - that will further reduce the alcohol evaporation/distillation). A column also increases efficiency.
I forgot to tell that the scent of ethyl formate I recognized was at quite high concentration, from the flask. Maybe diluting the ester, e.g. 1% in dipropylene glycol would give more pleasant scent? Yesterday I synthesized n-propyl acetate and I recognized the scent from the hose connector of the vacuum takeoff adapter while it was slowly distilling from reactor flask as pleasant pear scent but opening the side arm of the flask by removing the stopper the scent was much worse, solvent-like again and no more even traces of pears at all. I will post the synthesis when everything done (currently drying it and scheduled to do final distillation).
Also for methyl formate heat the reactor/distillation flask on boiling water bath as was enough for ethyl formate too. For n-propyl acetate I used oil bath as it was necessary to heat the oil to 140-150 C to made the ester to distill which is somewhat surprising because b.p. of acetic acid 118 C but the ester started to distill only after oil bath reached 140 C (surely the temperature in the reactor/distillation flask was below 118 C, just the oil bath had to be more hot). For methyl formate the T of bath using boiling water will be enough.
If you do not have dropping funnel then just mix e.g. 2,5 mol of methanol + 3,5 mol of formic acid, I suppose more methanol will enter the crude ester than if dropping it into acid (reducing yield and more work to remove it from the product).
Best pear ester for me is n-pentylacetate but I won't synthesize it because I've bought 1 L of this ester already. I wanted to try another pear ester so that's why I tried n-propyl acetate. Ethyl formate I tried because my dad was sad a disappointed with rum essence bought in grocery shop which was for baking, not for making alcoholic beverages (my father ended up with vegetable oil floating on the surface of 40% alcohol).
Btw this is the label from my canister of formic acid, I think it is 85% and the minimal content 95,0% is just a typo, but who knows?

IMG_20200518_194029_1_sm.jpg - 40kB




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[*] posted on 23-5-2020 at 16:01


Instead of buying a Graham condenser, can you circulate dry ice cold alcohol/acetone through a liebig condenser? Seems fine to me, unless the extreme cold would cause heat shock.

Question 2: this method doesn't work for heavier formate esters, such as octyl formate, because their boiling points are higher than formic acid. That means the formic acid should have as little water as possible. Ebay has 99% formic acid, dudadiesel has 90% formic acid for even cheaper. Is it possible to remove water from 90% formic acid using calcium chloride or magnesium sulfate?


[Edited on 24-5-2020 by Cou]




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[*] posted on 23-5-2020 at 23:55


Cou, if you don't have Graham condenser I would use Liebig through which I would circulate a water from a source where some ice added so the T of circulating water would be 0 and distill slowly, it is still a chance that Liebig manages it + use takeoff vacuum adapter and receiving flask surrounded with ice+water or better ice + salt.
Exactly, for heavier esters this does not work. It works for formic acid + methanol/ethanol/propanol and for acetic acid + methanol/ethanol/propanol.
For heavier esters you can still distill the water out of reaction using hexane or benzene and thus shift equilibrium towards products.
Here in reactive distillation the equilibrium is shifted by distilling out the ester (methyl/ethyl formates) or azeotrope of water + ester (ethyl/propyl acetates).
Attached an article where explained (wagner1950.pdf) and how difficult to separate ethylacetate obtained by esterification at industrial scale (bock1997.pdf), I can post a laboratory preparation method of ethylacetate where yield 44% due to loses of the ester during workup/purification - as I wrote for ethylacetate better to buy than synthesize in home lab (industrially produced not by esterification but by alkylation of acetic acid with ethylene).
There is no need to use such concentrated formic acid. 85% works fine. At the end a quite diluted formic acid stays in the reaction/distillation flask from which the ester distilled out. No need to concentrate the formic acid entering the reaction, because at the end you still have diluted acid in the reaction flask.
Also for formic acid do not use any catalyst as it is strong enough to catalyze its own esterification and H2SO4 would catalyze its decomp to CO. For acetic acid esters adding the H2SO4 is necessary.

Attachment: wagner1950.pdf (3.7MB)
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Attachment: bock1997.pdf (918kB)
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[*] posted on 25-5-2020 at 01:29


Actually you don't even need to use benzene for the dean-stark method. if you use toluene, just monitor the temperature of the reaction mixture and make sure it stays between the boiling points of the toluene-water azeotrope and water. The BP of the toluene-water azeotrope is low enough for all alcohols, even 1-propanol, if you're careful.

though i'm still tempted to order some benzene lol. nice carcinogen. i mean who wants to live to be 65 years old, lol. i'm gonna go out like Walter White

[Edited on 25-5-2020 by Cou]




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[*] posted on 25-5-2020 at 02:10


Cou that's why I prefer hexane instead of benzene though the efficiency of hexane is much lower. Toluene is even more efficient and xylene is ultimate but their high boiling points distract me somewhat.
The benzene is not so bad carcinogen, it is unreactive in human body, does not produce any radicals, any oxidation products etc. Of course it could readily dissolve in fat tissue and lipids (maybe cell membranes too) so it perhaps could accumulate there. Gasoline we tank into cars contain a little of benzene too.
But the best route is when the ester distills out with water so then no need to add hexane/benzene/toluene/xylene. Or when the ester does not form azeotrope with water and distills at low temperature (methyl/ethylformate).




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[*] posted on 25-5-2020 at 02:14


Wikipedia says hexane doesn't form an azeotrope with water. https://en.wikipedia.org/wiki/Azeotrope_tables'

Aromatic compounds like benzene and toluene are good because many compounds are soluble in them. salicylic acid has a much higher solubility in toluene than in hexane.




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[*] posted on 1-6-2020 at 18:01


Fantastic.
Thanks for taking the time to write this up.

I think I might give it a try, but I only have 54% formic acid.
Do you think it would work?

I tried today to distil my acid to get the 77% azeotrope, but I was unsuccessful.

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[*] posted on 1-6-2020 at 21:58


SplendidAcylation it is necessary to try it. I think you'll get some product, but very likely less yield. 85% formic acid is sold to beekeepers, so perhaps you can find such a shop.



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[*] posted on 2-6-2020 at 03:33


Cou here you have tables and graphs of hexane-water vapour system.
http://www.ddbst.com/en/EED/VLE/VLE%20Hexane%3BWater.php
There is always a mixture of not only hexane-water but also ester, acid, alcohol. Heptane is more powerful, cyclohexane is more powerful, but hexane is very cheap at the cost of less water in vapour phase (it lasts longer to remove water from the reaction). Also the advantage of hexane is bigger difference in boiling points for medium/long chain esters (for short chain you can distill the ester directly from the reaction mixture to shift the equilibrium towards product, for medium and longer chain esters you need to drive out water and these esters have much higher b.p. than hexane).




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[*] posted on 2-6-2020 at 05:25


Quote: Originally posted by Cou  
Wikipedia says hexane doesn't form an azeotrope with water. https://en.wikipedia.org/wiki/Azeotrope_tables'


Wikipedia doesn't say hexane and water doesn't form an azeotrope, Wikipedia just doesn't give a value for the azeotrope formed. You are doing your search the wrong way around...

Google: hexane azeotrope with water, fourth hit.
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[*] posted on 18-6-2020 at 18:37


Quote: Originally posted by Fery  

For heavier esters you can still distill the water out of reaction using hexane or benzene and thus shift equilibrium towards products.


Unfortunately I found out from "Azeotropic data" that formic acid and hexane form an azeotrope at 60.6 C, composed of 28% formic acid. So I don't know if you can use the dean-stark apparatus with hexane to remove water from formic acid.

Drying with anhydrous copper ii sulfate will be a pain b/c formic acid must be vacuum distilled at 25 C. i hate going out and buying dry ice just to set up a vacuum trap, but i will do it if i must.

95% formic acid is cheap, so I would just use a large excess in fisher esterification, instead of trying to remove the 5% water. Sometimes in organic chemistry, you accept low yields because it would be equally expensive to improve yields, e.g. buying big bags of dry ice at kroger when a 1 Liter bottle of formic acid is only $10

EDIT: here is a sample calculation, assuming that the equilibrium constant of fischer esterification is 1

using a 5 fold molar excess of 100% formic acid would give a theoretical yield of 83%, while using same mass of 95% w/w formic acid would give 74% yield. Not too bad of a decrease. Again formic acid is so cheap per mole that maybe you could use a 10 fold excess.

in that case, 100% formic acid would give theoretical yield of 91%, while 95% formic acid gives theoretical yield of 81%

if you were using 10.07 mL isopentyl alcohol, then a 10 fold molar excess of formic acid would be only 34.87 mL.

[Edited on 19-6-2020 by Cou]




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