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Fery
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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:
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).
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.
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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Bedlasky
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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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Cou
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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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Fery
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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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Boffis
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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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S.C. Wack
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Cou
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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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Fery
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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?
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Cou
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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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Fery
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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)
This file has been downloaded 376 times
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Cou
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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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Fery
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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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Cou
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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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SplendidAcylation
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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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Fery
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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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Fery
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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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Tsjerk
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Quote: Originally posted by Cou  | Wikipedia says hexane doesn't form an azeotrope with water. https://en.wikipedia.org/wiki/Azeotrope_tables'
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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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Cou
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| 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.
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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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Boffis
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Hi Fery, I am currently trying your technique for the preparation of ethyl formate and things are going fine. Its beeen running now for about 1.5
hours. The column head thermometer is reading 52.5 C and has been for the last half hour and the distillation is now in a steady state but the
distillate is cloudy and slightly acidic. Do you know if if ethyl formate forms an azetrope with water that boils at about 52-53?
Well the cloudy distillate soon stopped and the reaction is still going, 700ml of crude product recovered so far (from 1L of 85% formic acid) and
stillhead temperature still only 54 C.
[Edited on 4-8-2020 by Boffis]
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Alucard
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| Quote: Originally posted by Fery |
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.
The scent of the product is solvent-like. |
It's possible to make ethyl formate by using H2SO4 (conc., 90%) taken in amounts to bind water, and this gave quantitative yelds to me.
Another way is to use CaCl2 taken in amounts enough to bind water, with no catalyst at all. MgSO4 also worked for this process.
BTW, ethyl formate is stored badly and slowly decomposes, even when well dried.
The aroma of this one is IMHO not solvent-like but smth awesome, but also I think it is no way is like rum.
[Edited on 5-8-2020 by Alucard]
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Dr.Bob
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This is a great writeup. Not sure if it works for alcohols to make esters, but for formamides, you can mix formic acid with acetic anhydride or
better yet acetyl chloride to make the mixed anhydride, and it will react with amines to give primarily the formamide, at least in my experience. I
suspect it is mostly steric direction, but there may be other causes of the selectivity. But it worked well for me.
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Fery
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Hi Boffis, there is no azeotrope water+ester in this case. Vapour pressure of formic acid at 54 C is very low but nonzero so a little of acid must
distill too, also a little of H2O for the same reason (nonzero vapour pressure at 54 C).
Attached a pdf where the info that no azeotrope.
Attachment: wagner1950.pdf (3.7MB)
This file has been downloaded 298 times
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Fery
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Alucard, do not add H2SO4, formic acid is strong enough to catalyze its own esterification. H2SO4 would produce toxic CO from formic acid...
CaCl2 is good to bind water produced in the reaction and water present in 85% formic acid. But a distillation does the same job and shifts equilibrium
towards product by distilling off the ester from the reaction mixture.
I sniffed the aroma from a flask with the final product (and also from apparatus and all glass used during the experiment). Perhaps diluting the ester
in propane-1,2-diol or anything else would produce better scent ?
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Alucard
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| Quote: Originally posted by Fery | Alucard, do not add H2SO4, formic acid is strong enough to catalyze its own esterification. H2SO4 would produce toxic CO from formic acid...
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No, you add H2SO4 and you do not receive CO back under such a conditions because H2SO4 is mixed with formic acid and ethanol too and thus is diluted,
if not mixed then it has low contact while being an separate layer. Also it catches water and it's concentration goes lower. Also formic acid will be
mixed with ethanol already and it will drive reaction to favour ester preparation but not preparation of CO. Of course you will have to mix formic
acid with ethanol before adding H2SO4.
To get CO you will want to mix exactly 99% (or maybe 80%) formic acid and 90% H2SO4 it will definitely give CO, I tried this and it really works.
I have a prepation writeup from the book, but it is written in Cyrillics and despite I can see your language is Czech and not Slovak, but I am not
sure you can read Cyrillics, and anyway I also think the best policy will be to avoid Cyrillics at this forum because it will probably disturb some
people here and to do the best I will not post this writeup here.
But a synthesis consists from exactly :
Formic acid = 50 grams (80% or more, % by weight)
Ethanol = 92 grams (abs.)
Sulphuric acid = 41 grams (90%, % by weight)
You mix all of this and do like usual, with Vigreux column and collect distillate about 52-54 Celsius.
I tried both H2SO4 and MgSO4 and every synthesis worked OK for me.
And about aroma, it's damn bad if one will sniff old long stored ethylformate, contaminated with formic acid due to degradation of ester, only
neutralized from formic acid and freshly prepared and a bit dried ester is OK, well it smells appropriate as I think.
[Edited on 6-8-2020 by Alucard]
[Edited on 6-8-2020 by Alucard]
[Edited on 6-8-2020 by Alucard]
[Edited on 6-8-2020 by Alucard]
[Edited on 6-8-2020 by Alucard]
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Boffis
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Well I ended up with 1060ml of crude ethyl formate from 800ml of 99% ethanol (denatured with a little ketone by the smell), I dried it with anhydrous
sodium sulphate 75g and a little solid bicarbonate of soda (5g) to neutralise any formic acid for an hour, filtered it and then carefully rectified
it. It is currently distilling at exactly 54 C from the first milliliter to the current 700ml! The curious thing is there is obviously a little water
left in it and a small but constant amount is distilling over with the ethyl formate. Does ethyl formate form an azeotrope with water? and how soluble
is water in ethyl formate?
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