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blogfast25
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Quote: Originally posted by aga  |
Had to make Acetic Anhydride first, so it took a while.
( 30ml Ethanol close to anhydrous, 30ml Acetic Anhydride, 6ml 98% H2SO4, reflux 30 mins)
Must have worked as the smell is now that of NVR instead of vinegar, however it won't separate from the 'neutralised' reaction mixture.
Some questions :-
If you're going to reflux with heating, is there any point cooling the mixture as you add the acid ?
Is it necessary to neutralise the acid after the reaction ? |
You meant anhydrous acetic acid, aka glacial acetic acid, 'acetic anhydride' is something quite different:
http://en.wikipedia.org/wiki/Acetic_anhydride
If the ester didn't separate out that's most likely because there was still too much water in your reagents. The catalyst is concentrated sulphuric
acid: dilute it too much and it won't work.
google for a decent prep of ethyl acetate, there are oodles of them.
Normally you separate the reaction water from the ester, then neutralise any remaining acid in the ester and distil it for purity.
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Texium
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I would highly recommend methyl salicylate rather than ethyl acetate as a first esterification, because unless you have pure glacial acetic acid or
acetic anhydride, it's unlikely that you will have much success with ethyl acetate. With esterifications, you need to do whatever you can do to keep
water out of the system. Otherwise, you'll have virtually no yield. Methyl salicylate is much easier. You can make salicylic acid from aspirin
(acetylsalicylic acid) by hydrolyzing it using hydrochloric acid, and then reflux that with methanol and a bit of sulfuric acid for a few hours. If
you don't have access to methanol, you could try making ethyl salicylate. It's supposed to work exactly the same, although it didn't for me, so I
can't personally vouch for it.
Here's a couple of useful links from our very own wiki:
Methyl Salicylate
Synthesis of Salicylic Acid
Edit- I also just want to say, that I only recently started getting into organic chemistry myself, but I am really enjoying it, and think that I will
shift my main focus towards it.
[Edited on 3-30-2015 by zts16]
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Darkstar
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| Quote: Originally posted by zts16 | | I also just want to say, that I only recently started getting into organic chemistry myself, but I am really enjoying it, and think that I will shift
my main focus towards it. |
*evil grin*
Yes, yes. Excellent. You, j_sum, Cou, aga...all of you just come on over to the dark side. That's right. That's right...
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kavu
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| Quote: Originally posted by aga |
Some questions :-
If you're going to reflux with heating, is there any point cooling the mixture as you add the acid ?
Is it necessary to neutralise the acid after the reaction ? |
The point of cooling in the first place is to prevent an overrun reaction. Sulfuric acid can readily protonate a variety of species in such a reaction
mixture, releasing a lot of heat. There is also the possibility of conc. sulfuric acid forming a dense lower layer at the bottom of the flask leading
to all sorts of nasty side reactions including dehydration reactions, so stirring is in place here.
Neutralization of the acid (so-called quench) is needed, the reaction mixture will contain water from the esterification (check the mechanism). If
left like this it will undergo an acid catalyzed ester hydrolysis and eventually reach an equilibrium where your product is partially hydrolyzed back
to starting materials. Such an equilibrium exists in the reaction as well, but excess of alcohol is usually used to drive it towards completion. Upon
neutralization the reaction effectively stops as the catalytic proton source is depleted.
Most every organic synthetic procedure calls for a quench of some type. This is typically a neutralization/acidification/hydrolysis procedure to kill
one or few of the reacting species.
As for starting with organic chemistry I can highly recommend the famous NotVoodoo-site (experimental shenanigans) as well as the very clearly written theoretical resource MasterOrganicChemistry.
[Edited on 30-3-2015 by kavu]
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aga
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Wow !
Thanks to all for the interesting and useful info/links.
The 'overrun' sounds perfectly sensible, and i think can see the point now : if it gets too hot Other evil darkside reactions happen.
Once it's reacted to as much of the target product the catalyst needs removing to prevent further Other darkside reactions happening.
The excess ethanol to push the equilibrium to the side you desire is also making some sense, although i'm unclear as to why it'd be the ethanol in
excess rather than the glacial acetic acid.
I definitely had too much water in the mix as the acid didn't seem to want to be neutralised, so i kept adding and adding ...
So, in a nutshell, it's all a Balance, in that there isn't 1 reaction at all, there are many.
What you Sith Masters are doing is carefully balancing reaction conditions to achieve as high a veild of the target compound as possible, at as high a
purity as possible.
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Darkstar
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| Quote: Originally posted by aga |
The excess ethanol to push the equilibrium to the side you desire is also making some sense, although i'm unclear as to why it'd be the ethanol in
excess rather than the glacial acetic acid. |
Excess GAA would indeed drive the equilibrium to the right as well. Since the reaction is reversible, the ethanol and acetic acid are in equilibrium
with ethyl acetate and water:
EtOH + AcOH ⇌ EtOAc + H2O
An excess of something on the left side shifts the equilibrium over to the right. An excess of something on the right side shifts it over to the left.
This is also why you want as little water present in the reaction mixture as possible.
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morganbw
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| Quote: Originally posted by aga | Thanks for the pointers.
Making ethyl acetate looks do-able, and has some indicator of success (smell) so i'll give that a go. |
Here is a file which I snatched from the web. It gives a fairly nice overview
and does go into the purification.
Attachment: A2 Expt13.5(3) The Synthesis of Ethyl Ethanoate Teacher guide.doc (42kB)
This file has been downloaded 26769 times
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kavu
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| Quote: Originally posted by aga |
What you Sith Masters are doing is carefully balancing reaction conditions to achieve as high a veild of the target compound as possible, at as high a
purity as possible. |
A good starting point is usually to refer to the reaction mechanism, a stepwise depiction of how the reaction (likely) will take place. If you notice
key intermediates and points where side reactions can take place you can adjust temperature/solvent/reagents and so on accordingly. Also checking the
literature gives you a good picture of how a reaction is typically carried out.
Key to a successful synthesis is monitoring. You need to be constantly checking on the reaction: has the starting material been consumed, are there
products forming, how many products, is the reaction clean and so on. Typically this is achieved using a combination of TLC, 1H NMR and GC/LC-MS.
Optimizing for high yields calls for numerous experiments carefully changing reaction parameters one at a time. This will paint a parameter landscape
of how the reaction selectivity/yield behaves allowing to pinpoint the highest yielding combination. Typically the parameter space is too huge to
cover exhaustively so compromises are in place.
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aga
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@morganbw: Thanks ! In light of 'pushing the equation to the right' info, that paper makes more sense than the other process i tried.
So by distilling off the EtOAc as it forms, and adding more reagents, the equilibrium is biased over towards the right hand side (more EtOAc). I think
i just learned something !
@kavu: I'll seek out the reaction mechanism and see if i can make sense of it.
TLC is do-able, but i can't remember seeing any NMR or GC/LC-MS equipment in my shed ...
Would it be the case that OC can be so immensely complex that the Only way to really know what the Product(s) are is to analyse them using seriously
high-tech kit ?
Edit:
Just stumbled on this explanation of that particular mechanism:-
http://www.chemguide.co.uk/physical/catalysis/esterify.html
[Edited on 30-3-2015 by aga]
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brubei
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| Quote: Originally posted by aga | @morganbw: Thanks ! In light of 'pushing the equation to the right' info, that paper makes more sense than the other process i tried.
So by distilling off the EtOAc as it forms, and adding more reagents, the equilibrium is biased over towards the right hand side (more EtOAc). I think
i just learned something ! |
You cant distil AcOEt while your reaction isnt finished, because the boilling point of AcOEt and EtOH are too close 77°C ans 75°C
Catching the water as far as it is produced by adding dessicant (like moleculare sieve) is the best way do displace the equilibrium.
| Quote: Originally posted by aga | Would it be the case that OC can be so immensely complex that the Only way to really know what the Product(s) are is to analyse them using seriously
high-tech kit ?
[Edited on 30-3-2015 by aga] |
TLC is not so high tech
OC is not as simplex as you can read on paper.
Doing a reaction is often followed by secondary product and you have to deal with the unreacted products. The purity of the compound that you aim to
produce is far to be guaranteed so you have to check it.
Checking the content of your mixture is also the way to learn how to handle chemistry. It allow you to see the influence of your experiments when you
decide to take initiatives by changing or creating your protocol.
[Edited on 30-3-2015 by brubei]
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Loptr
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| Quote: Originally posted by Darkstar | | Quote: Originally posted by zts16 | | I also just want to say, that I only recently started getting into organic chemistry myself, but I am really enjoying it, and think that I will shift
my main focus towards it. |
*evil grin*
Yes, yes. Excellent. You, j_sum, Cou, aga...all of you just come on over to the dark side. That's right. That's right...
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I have been rereading through the inorganic sections of my college chemistry books in hopes of gleaning more about potential catalysts, etc., for the
time I intend to spend in amateur organic chemistry.
Inorganic just bores me for some reason--I tend to be attracted to complexity for some reason. Yes, I know inorganic chemistry is what we first learn
to gain some fundamentals in the various theories and techniques, but are there books out there that start from zero and go from there while providing
a focus on Organic Chemistry? The books I have been reading tend to stay on the inorganic side of things.
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kavu
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| Quote: Originally posted by Loptr |
Yes, I know inorganic chemistry is what we first learn to gain some fundamentals in the various theories and techniques, but are there books out there
that start from zero and go from there while providing a focus on Organic Chemistry? The books I have been reading tend to stay on the inorganic side
of things. |
I can highly recommend Organic Chemistry by Clayden Jonathan, Greeves Nick, Warren Stuart and Wothers Peter. This book, widely used in universities
all around the world, starts from molecular orbital theory (effectively "zero") and relates that to organic reactivity.
[Edited on 31-3-2015 by kavu]
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aga
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Inorganic, Organic - it's all great.
The basic thing i get from exploring OC is that nobody really knows what's going on (compared to IOC) which is wonderful.
Temperatures are also generally lower, which is helpful.
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Loptr
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You can have a very good idea, but it starts to get math and computationally intensive.
When I go back for my masters in the next year or so, I am toying around with the idea of computational chemistry. I have the computer science and
math background (rusty), but it should be fun.
Not that I would actually use it in my job...
[Edited on 3-4-2015 by Loptr]
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