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HgDinis25
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@ Etaoin Shrdlu
http://www.google.pt/url?sa=t&rct=j&q=&esrc=s&am...
Ammonia is present in large excess over hydroxide ions. Acetic acid is also present in very small quantities (let's say trace amounts). The aminolysis
of Ethyl Acetate should have a much lower rate than the acid/base reaction between Ammonium Hydroxide and Acetic Acid. Because the acid/base reaction
is faster than the aminolysis reaction, the equilibriums I mentioned in my previous posts should shift to the right. Of course, the aminolysis would
shift it to the left. However, the reaction that makes them shift to the right is faster.
At least, I would expect large contamination of Ammonium Acetate. Or is my assumption that the acid/base reaction is faster wrong?
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Etaoin Shrdlu
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Did you read the article? There is data on the contamination. I don't know exactly where your predictions went wrong.
EDIT: Oops, I missed that bananaman had posted the same document. Still relevant, though.
[Edited on 4-12-2015 by Etaoin Shrdlu]
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gdflp
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Quote: Originally posted by HgDinis25  |
Ammonia is present in large excess over hydroxide ions. Acetic acid is also present in very small quantities (let's say trace amounts). The aminolysis
of Ethyl Acetate should have a much lower rate than the acid/base reaction between Ammonium Hydroxide and Acetic Acid. Because the acid/base reaction
is faster than the aminolysis reaction, the equilibriums I mentioned in my previous posts should shift to the right. Of course, the aminolysis would
shift it to the left. However, the reaction that makes them shift to the right is faster.
At least, I would expect large contamination of Ammonium Acetate. Or is my assumption that the acid/base reaction is faster wrong?
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That's where you are getting confused. It isn't the difference in rate between the acid-base reaction, and the aminolysis of ethyl acetate. It is
the difference in rate between the hydrolysis of the ethyl acetate(EXTREMELY SLOW, it doesn't matter if the equilibrium is shifted
because the concentration of hydroxide ions is only 0.01M in 6M ammonia whereas the concentration of free ammonia 5.99M) and the
acid-base reaction, and the aminolysis of ethyl acetate.
[Edited on 4-12-2015 by gdflp]
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HgDinis25
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| Quote: Originally posted by gdflp | | Quote: Originally posted by HgDinis25 |
Ammonia is present in large excess over hydroxide ions. Acetic acid is also present in very small quantities (let's say trace amounts). The aminolysis
of Ethyl Acetate should have a much lower rate than the acid/base reaction between Ammonium Hydroxide and Acetic Acid. Because the acid/base reaction
is faster than the aminolysis reaction, the equilibriums I mentioned in my previous posts should shift to the right. Of course, the aminolysis would
shift it to the left. However, the reaction that makes them shift to the right is faster.
At least, I would expect large contamination of Ammonium Acetate. Or is my assumption that the acid/base reaction is faster wrong?
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That's where you are getting confused. It isn't the difference in rate between the acid-base reaction, and the aminolysis of ethyl acetate. It is
the difference in rate between the hydrolysis of the ethyl acetate(EXTREMELY SLOW, it doesn't matter if the equilibrium is shifted)
and the acid-base reaction, and the aminolysis of ethyl acetate. |
Of course it matters if the equilibrium is shifted! That's why you can easily hydrolyze Ethyl Acetate using, for instance, Sodium Hydroxide.
Now, what you could argue is that there is never enough Hydroxide ions in solution to actually accelerate the hydrolysis equilibrium enough to cause
problems.
From what I can see, this is more complex than simply stating "it happens because reaction 1 is faster than 2". Kinetics of all the equilibriums
should be taken into account. However, Nicodem's explanation makes sense.
I was planing a synthesis of this compound using dried Ammonia gas and dried Ethyl Acetate. I may resort to the much more simple method of adding the
two reagents together, though.
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DraconicAcid
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| Quote: | | Quote: Originally posted by HgDinis25 | | Of course it matters if the equilibrium is shifted! That's why you can easily hydrolyze Ethyl Acetate using, for instance, Sodium Hydroxide.
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No, it doesn't, because it's NOT at equilibrium!
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gdflp
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| Quote: Originally posted by HgDinis25 |
Of course it matters if the equilibrium is shifted! That's why you can easily hydrolyze Ethyl Acetate using, for instance, Sodium Hydroxide.
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No, the shifted equilibrium ensures that the ethyl acetate will be fully hydrolyzed. The high concentration of hydroxide ions increases the rate of
reaction so that it can occur at an appreciable rate in a laboratory experiment. These are two entirely different concepts!
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HgDinis25
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@gdflp
| Quote: |
No, the shifted equilibrium ensures that the ethyl acetate will be fully hydrolyzed. |
At which point did I state otherwise?
| Quote: |
The high concentration of hydroxide ions increases the rate of reaction so that it can occur at an appreciable rate in a laboratory experiment.
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Yes, that's why I stated:
Now, what you could argue is that there is never enough Hydroxide ions in solution to actually accelerate the hydrolysis equilibrium enough to
cause problems.
And that quote of yours gets in contradiction with a previous one you made:
| Quote: |
(EXTREMELY SLOW, it doesn't matter if the equilibrium is shifted) |
@DraconicAcid
I respect you and you are probably a very good chemist. But please don't do that. Don't just throw stuff into the air like you're adding fuel to the
fire. Don't state things like
No, it doesn't, because it's NOT at equilibrium!
without explaining better.
What exactly isn't at equilibrium? And why doesn't it matter if the equilibrium is shifted? I can't decode your comment into useful information. Could
you elaborate a little bit more, please?
What isn't at equilibrium?
[Edited on 12-4-2015 by HgDinis25]
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Etaoin Shrdlu
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The reaction doesn't ever reach equilibrium, because it's so slow.
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gdflp
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| Quote: Originally posted by HgDinis25 | @gdflp
| Quote: |
No, the shifted equilibrium ensures that the ethyl acetate will be fully hydrolyzed. |
At which point did I state otherwise?
| Quote: |
The high concentration of hydroxide ions increases the rate of reaction so that it can occur at an appreciable rate in a laboratory experiment.
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Yes, that's why I stated:
Now, what you could argue is that there is never enough Hydroxide ions in solution to actually accelerate the hydrolysis equilibrium enough to
cause problems.
And that quote of yours gets in contradiction with a previous one you made:
| Quote: |
(EXTREMELY SLOW, it doesn't matter if the equilibrium is shifted) |
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No, you're talking about accelerating the equilibrium and such, that's not what happens. And I am not contradicting myself, as those two quotes are
saying entirely different things. The equilibrium of a reaction and the rate of a reaction are two entirely different concepts. A hydrolysis
reaction could have an extraordinarily high equilibrium constant which favors entirely products, but that irrelevant if the kinetics of the reaction
show that it would take a century to reach equilibrium(extreme case obviously). An equilibrium determines what the concentration of the reactants
will be after a mixture has had ample time to react, but most equilibrium's don't occur instantly. Thus, the time it takes for a reaction to reach
equilibrium is determined by the kinetics of the reaction which are separate from the equilibrium of the reaction. There may be a change in the rate
of the reaction due to the kinetics being based off of the concentration of some reactants, but it is quite possible for the reaction rate to be
unaffected regardless of the concentration of any of the reactants and thus be a slow reaction even if there is a high or low equilibrium constant.
This is why a reaction can be extremely slow(unfavorable kinetics), but have a favorable equilibrium and will eventually reach equilibrium if
no other faster reactions are competing. This is the case here, the hydrolysis of ethyl acetate will eventually occur, but since the
concentration of free ammonia is so much higher than hydroxide in the reaction mixture, the aminolysis has much more favorable kinetics and will thus
be the predominant reaction.
The equation for the equilibrium of A + B <--> C + D is k = ([C]*[D])/([A]*[B]) whereas the equation for the kinetics of that reaction is Rate =
k[A]^n*[B]^m*[C]^p*[D]^q, but n m p and q could all be 0 and thus the concentration of any of them would not affect the rate of reaction.(Note that
the two k's are different)
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HgDinis25
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@gdflp
The presence of Hydroxide ions increases the hydrolysis rate. It also shifts the equilibrium to the right (product gets consumed). I don't understand
your persistence in trying to textbook me with kinetics.
This all comes down to:
Reaction A - Hydrolysis of Ethyl Acetate
Reaction B - Ionization of Ammonia
Reaction C - Aminolysis of Ethyl Acetate
Reaction D - NH4OH and Acetic Acid to form Ammonium Acetate
If Reaction C is faster than reaction A then Acetamide will be the major product. If reaction A is faster than reaction C then reaction D prevails and
Ammonium Acetate will be the major product.
Now, in the absence of Hydroxide ions, reaction A is very slow. However, in the presence of said ions, the rate of reaction A increases. Thus, if OH
ions are being consumed (reaction D), reaction B will ionize more ammonia. What you're all trying to sell is that reaction C still prevails. Some
actual data could help, though.
About your contradiction:
The high concentration of hydroxide ions increases the rate of reaction so that it can occur at an appreciable rate in a laboratory
experiment.
and
(EXTREMELY SLOW, it doesn't matter if the equilibrium is shifted)
Hydroxide ions increase the rate of reaction and shift the equilibrium to the right (by consuming one of the products).
In these conditions, if the rate is increased it is because of the Hydroxide ions. So, we can conclude that there must be hydroxide ions to increase
reaction rate. Then you said that reaction is extremely slow, not mattering if the equilibrium is shifted. For the equilibrium to be shifted there
must be hydroxide ions. And therefore, if there are hydroxide ions reaction rate increases. Now, what you might be saying is that the increase in
rate, with such small amount of hydroxide is negligible. I stated that a few posts ago:
Now, what you could argue is that there is never enough Hydroxide ions in solution to actually accelerate the hydrolysis equilibrium enough to
cause problems.
@Etaoin Shrdlu
Is reaction A that slow even in the presence of the Hydroxide ions? If so it must have a very low rate to allow acetamide formation free from Ammonium
Acetate.
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gdflp
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Yes, that is true. The reason that I didn't contradict myself is that in the two statements of mine, I was talking about a different equilibrium. It
was a poor choice of words, but my point was that it doesn't matter if the equilibrium concerning the ionization of ammonia is shifted, the
concentration of hydroxide ions will be miniscule and thus the reaction will be extremely slow. What it boils down to is that the aminolysis and
hydrolysis(in basic conditions) of ethyl acetate have similar reaction rates if the concentration of the nucleophile(NH3 and OH-
respectively) is the same. Even though the hydroxide is being constantly regenerated by the ionization of more ammonia molecules, it's concentration
at any one point is never very high, as I stated a few posts back the concentration in 6M ammonia is 0.01M OH-, whereas the concentration of the other
nucleophile, NH3 is very high throughout the entire reaction, thus it is the faster reaction. Some ammonium acetate will be formed, but as
I stated previously, this isn't much of an issue anyway since most of it should decompose to acetamide during the workup. In addition, in basic
conditions, reactions A and D occur simultaneously as different parts of the same mechanism.
[Edited on 4-12-2015 by gdflp]
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DraconicAcid
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| Quote: | @gdflp
The presence of Hydroxide ions increases the hydrolysis rate. It also shifts the equilibrium to the right (product gets consumed). I don't understand
your persistence in trying to textbook me with kinetics. |
Because kinetics are what's important in this particular case. You can't shift the equilibrium to the right or the the left unless the system has
reached equilibrium, and because the hydrolysis reaction with water is so slow, it doesn't get there over any reasonable time span.
Look- a wise man once said that there is no need to argue if an experiment can be done. Do you have any ethyl acetate? Take a mL of that, and a mL
of water, put them in a test tube, shake it up. Once equilibrium is reached for the hydrolysis reaction, there should be enough ethanol and acetic
acid present to give you a homogeneous mixture instead of a two-phase one. Shake it twice a day, and let us know when it stops separating. Do the
same thing with ethyl acetate and aqueous sodium carbonate solution (carbonate isn't very nucleophilic, unlike ammonia). See if the hydrolysis
reaches equilibrium any faster.
| Quote: |
This all comes down to:
Reaction A - Hydrolysis of Ethyl Acetate
Reaction B - Ionization of Ammonia
Reaction C - Aminolysis of Ethyl Acetate
Reaction D - NH4OH and Acetic Acid to form Ammonium Acetate
If Reaction C is faster than reaction A then Acetamide will be the major product. If reaction A is faster than reaction C then reaction D prevails and
Ammonium Acetate will be the major product.
Now, in the absence of Hydroxide ions, reaction A is very slow. However, in the presence of said ions, the rate of reaction A increases. Thus, if OH
ions are being consumed (reaction D), reaction B will ionize more ammonia. What you're all trying to sell is that reaction C still prevails.
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This is correct. Even in the presence of a low concentration of hydroxide ions, reaction A is extremely slow, so we basically don't have to worry
about it. If you were to use 1 M sodium hydroxide, you'd probably have appreciable hydrolysis quite quickly, but not in aqueous ammonia.
ETA: It seems that even a lower concentration of sodium hydroxide will hydrolyze ethyl acetate, at least according to several undergraduate
experiments that measure the rate, such as http://www.uni-ulm.de/physchem-praktikum/media/literatur/Kin...
[Edited on 12-4-2015 by DraconicAcid]
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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HgDinis25
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Well, this has been an interesting discussion so far. I'll make sure to attempt a synthesis on Acetamide from Ammonia and Ethyl Acetate
@gdflp
Again, this all comes down to both reaction rates. Some actual values would be good, though. Anyway, your arguments make sense. If we consider the
hydrolysis of Ethyl Acetate too slow in the presence of minimum amounts of Hydroxide ions then the most logical outcome would be the aminolysis of
Ethyl Acetate.
@DraconicAcid
| Quote: |
Because kinetics are what's important in this particular case. You can't shift the equilibrium to the right or the the left unless the system has
reached equilibrium, and because the hydrolysis reaction with water is so slow, it doesn't get there over any reasonable time span.
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You can't shift the equilibrium, that's correct. But you can dramatically increase the reaction rate by adding small amounts of hydroxide ions. That
would allow the system to reach equilibrium and then would allow it to be shifted.
| Quote: |
Look- a wise man once said that there is no need to argue if an experiment can be done. Do you have any ethyl acetate? Take a mL of that, and a mL of
water, put them in a test tube, shake it up. Once equilibrium is reached for the hydrolysis reaction, there should be enough ethanol and acetic acid
present to give you a homogeneous mixture instead of a two-phase one. Shake it twice a day, and let us know when it stops separating. Do the same
thing with ethyl acetate and aqueous sodium carbonate solution (carbonate isn't very nucleophilic, unlike ammonia). See if the hydrolysis reaches
equilibrium any faster. |
I couldn't disagree more. Of course there is a need to argue! The reaction you just mentioned can occur exactly like you said. However, it can be for
completely different reasons than the ones you stated. That's just like saying you can evaporate gold at ordinary temperatures. Just add it to mercury
and it will disappear! It evaporated! This is ridiculous. I can also think of he good old story about the earlier tests for benzene. The substance
wasn't actually testing for benzene.
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xfusion44
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Hi!
I've tried the Polesch's method of acetamide synthesis, but after 24h of stirring, when I turned off stirrer, there were still two separate layers.
Currently I'm waiting if some crystals will appear after some days. I used 25% NH4OH and probably pure enough EtOAc. Does someone know why this
doesn't work for me?
Thanks!
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xfusion44
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I'm back again, to tell you that the polesch's method worked! After a few days I did get some crystals (as on his pictures), even though solutions
still separated after 24h of mixing 
Best regards, xfusion44
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Hegi
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After 48 hours of stirring there were no more separate layers. BUT there was a small amount of white solid! After filtrating and reducing the volume
nothing came out. I will let the solution to evaporate freely and I´ll see. 7 mL of 27% ammonium solution was used.
Our webpage has been shut down forever cause nobody was willing to contribute. Shame on you all!!!
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Magpie
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I hope that DraconicAcid does not see this. 
[Edited on 26-4-2016 by Magpie]
The single most important condition for a successful synthesis is good mixing - Nicodem
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xfusion44
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| Quote: Originally posted by Hegi | | After 48 hours of stirring there were no more separate layers. BUT there was a small amount of white solid! After filtrating and reducing the volume
nothing came out. I will let the solution to evaporate freely and I´ll see. 7 mL of 27% ammonium solution was used. |
It looks like it takes a few days for crystals to start forming, just leave it to sit a few more days and I'm sure you'll see crystals soon 
  
[Edited on 26-4-2016 by xfusion44]
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Hegi
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| Quote: Originally posted by xfusion44 | | Quote: Originally posted by Hegi | | After 48 hours of stirring there were no more separate layers. BUT there was a small amount of white solid! After filtrating and reducing the volume
nothing came out. I will let the solution to evaporate freely and I´ll see. 7 mL of 27% ammonium solution was used. |
It looks like it takes a few days for crystals to start forming, just leave it to sit a few more days and I'm sure you'll see crystals soon
[Edited on 26-4-2016 by xfusion44] |
I´m not so sure. There is absolutely minimum amount of the solution. I would say max 2-3 mL.
[Edited on 27-4-2016 by Hegi]
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xfusion44
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| Quote: Originally posted by Hegi | | Quote: Originally posted by xfusion44 | | Quote: Originally posted by Hegi | | After 48 hours of stirring there were no more separate layers. BUT there was a small amount of white solid! After filtrating and reducing the volume
nothing came out. I will let the solution to evaporate freely and I´ll see. 7 mL of 27% ammonium solution was used. |
It looks like it takes a few days for crystals to start forming, just leave it to sit a few more days and I'm sure you'll see crystals soon
[Edited on 26-4-2016 by xfusion44] |
I´m not so sure. There is absolutely minimum amount of the solution. I would say max 2-3 mL.
[Edited on 27-4-2016 by Hegi] |
How much of ethyl acetate did you use? 2-3ml doesn't make sense to me, if you started with total of about 20ml (EtOAc + NH4OH) What about filtered
part of solution? You said that volume of solution was reduced significantly after filtering - maybe you filtered off your acetamide, since you also
mentioned that you could see some white stuff forming in solution (perhaps that was it). Although it's theoretically impossible to filter acetamide,
due to its very high solubility in water, you probably succeded with that, since crystals already started to form and if you didn't mix the solution
before filtering it, the crystals had no time to dissolve so they were probably stopped by the filter. Although, I could be wrong... How about the
purity of EtOAc? And are you sure your ammonia solution is 27%? If its old or if it was stored in warm place, it could be much less concentrated.
I'd suggest you to try again, but this time don't filter the solution, just leave it in wide container, at room temperature for 3-4 days and you
should see the crystals.
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Hegi
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I got back to the synthesis. I mixed 10 ml of p.a. ethyl acetate in a baker with 7 ml of 25-26% ammonia solution and stirred at 1000 rpms the mixture
for over 24 hours. The layers are still present. Can somebody explain this? Should I use excess of ammonia solution?
Thanks in advance.
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Hegi
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| Quote: Originally posted by Hegi | I got back to the synthesis. I mixed 10 ml of p.a. ethyl acetate in a baker with 7 ml of 25-26% ammonia solution and stirred at 1000 rpms the mixture
for over 24 hours. The layers are still present. Can somebody explain this? Should I use excess of ammonia solution?
Thanks in advance. |
I used more ammonia and layers separated. Then I got wrong by heating the solution and hydrolyzed my product... now I have acetic acid... I should
have left the solution freely to crystalize...
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Hegi
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| Quote: Originally posted by Hegi | | Quote: Originally posted by Hegi | I got back to the synthesis. I mixed 10 ml of p.a. ethyl acetate in a baker with 7 ml of 25-26% ammonia solution and stirred at 1000 rpms the mixture
for over 24 hours. The layers are still present. Can somebody explain this? Should I use excess of ammonia solution?
Thanks in advance. |
I used more ammonia and layers separated. Then I got wrong by heating the solution and hydrolyzed my product... now I have acetic acid... I should
have left the solution freely to crystalize... |
I repeated the procedure... After homogenization of solution I got final solution that was pretty basic (pH over 12). I poured this into Petri´s dish
and left freely to crystalllize. Next day I came to the lab the smell of ammonia was gone and replaced by the smell of acetic acid. However, small
crystals are on the surface of the liquid. I will wait till monday what is going to happen.
Our webpage has been shut down forever cause nobody was willing to contribute. Shame on you all!!!
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