Ester exchange
Chemistry discussion, revision, exam and homework help.
This thread is sponsored by:
| Announcements | Posted on | |
|---|---|---|
| Important: please read these guidelines before posting about exams on The Student Room | 28-04-2013 | |
-
- Reputation:
- Thread Starter
- Benevolent Member
- Posts: 730
Ester exchangePoly(ethanol) is produced in an ester exchange reaction between poly(ethenyl ethanoate) and methanol
How can this work, though? I thought that in an ester exchange reaction an alcohol group swaps with another alcohol group. Here the alcohol and carboxyllic acid group are swapping?? -
- Reputation:
- Thread Starter
- Benevolent Member
- Posts: 730
Re: Ester exchangeI don't think this is right, look:(Original post by illusionz)
The alcohol groups are swapping with each other. The original alcohol group is the polymer chain, and that is getting replaced with methanol, to leave the alcoholic polymer. -
- Reputation:
- Overlord in Training
- Posts: 2,560
Re: Ester exchangeIt is right. Do you know the mechanism for this reaction?(Original post by ilovemath)
I don't think this is right, look: -
- Reputation:
- Thread Starter
- Benevolent Member
- Posts: 730
Re: Ester exchangeno it is not in my book(Original post by illusionz)
It is right. Do you know the mechanism for this reaction?
i looked for another explanation online but could not find it
-
- Reputation:
- Overlord in Training
- Posts: 2,560
Re: Ester exchangeAn ester is analagous to an acyl chloride with an OR group instead of a Cl, and they are less reactive. What would the reaction between methanol and an acyl chloride be?(Original post by ilovemath)
no it is not in my book
i looked for another explanation online but could not find it
-
- Reputation:
- Thread Starter
- Benevolent Member
- Posts: 730
Re: Ester exchangewell I would eliminate the Cl from the acyl and the H from methanol so the link would be(Original post by illusionz)
An ester is analagous to an acyl chloride with an OR group instead of a Cl, and they are less reactive. What would the reaction between methanol and an acyl chloride be?
...C-O-C.. -
- Reputation:
- Overlord in Training
- Posts: 2,560
Re: Ester exchangeSo, draw out your polymer. The O-Polymer bit is your OR of the ester, attack the C=O with your methanol, push the arrows in exactly the same way and you should get the product.(Original post by ilovemath)
well I would eliminate the Cl from the acyl and the H from methanol so the link would be
...C-O-C.. -
- Reputation:
- Thread Starter
- Benevolent Member
- Posts: 730
Re: Ester exchangeso ester exchange and transesterfication are different then, right?(Original post by illusionz)
So, draw out your polymer. The O-Polymer bit is your OR of the ester, attack the C=O with your methanol, push the arrows in exactly the same way and you should get the product. -
- Reputation:
- Overlord in Training
- Posts: 2,560
Re: Ester exchangeI've never heard of the term ester exchange before, but they seem to be the same thing. This reaction is transesterification.(Original post by ilovemath)
so ester exchange and transesterfication are different then, right? -
- Reputation:
- Thread Starter
- Benevolent Member
- Posts: 730
Re: Ester exchangeI have now included arrows and it seems wrong, sorry(Original post by illusionz)
So, draw out your polymer. The O-Polymer bit is your OR of the ester, attack the C=O with your methanol, push the arrows in exactly the same way and you should get the product.
-
- Reputation:
- Thread Starter
- Benevolent Member
- Posts: 730
Re: Ester exchangeI am a little confused by the lower part of the diagram(Original post by illusionz)
If the R is your polymer chain, can you not see that they are the same?
NB some of your arrows are a little bit dodgy!
first where did the O+ come from? second what is the wierd shape to the left of the O+?? -
- Reputation:
- Overlord in Training
- Posts: 2,560
Re: Ester exchangeRedrawn more clearly.(Original post by ilovemath)
I am a little confused by the lower part of the diagram
first where did the O+ come from? second what is the wierd shape to the left of the O+??
The O becomes positive as it now has three bonds, and the 'weird shape' is a H3C-CO drawn in shorthand
-
- Reputation:
- Thread Starter
- Benevolent Member
- Posts: 730
Re: Ester exchangethis is hand drawn by me(Original post by illusionz)
Redrawn more clearly.
The O becomes positive as it now has three bonds, and the 'weird shape' is a H3C-CO drawn in shorthand
is this correct (It looks like it is but just wanna double check
-
- Reputation:
- Thread Starter
- Benevolent Member
- Posts: 730
Re: Ester exchangealso what would happen in a reaction like(Original post by illusionz)
Redrawn more clearly.
The O becomes positive as it now has three bonds, and the 'weird shape' is a H3C-CO drawn in shorthand
CH3COOC2H5 + HCOOH
how could I do this as I have TWO carbonyl groups? -
- Reputation:
- Overlord in Training
- Posts: 2,560
Re: Ester exchangeYes(Original post by ilovemath)
this is hand drawn by me
is this correct (It looks like it is but just wanna double check
although strictly in the step where you lose the H+ it would be removed by a solvent (probably water molecule), rather than just falling off.
The mechanism is probably a bit beyond your course, and in reality alcohols are not very good nucleophiles so you would use either an acid catalyst which protonates the C=O of the ester and makes it more reactive (like an acid chloride), or a base which deprotonates the alcohol and makes that more reactive (better nucleophile).
Still, the best way to understand the reaction is to understand the mechanism
-
- Reputation:
- Overlord in Training
- Posts: 2,560
Re: Ester exchangeI'm not quite sure what reaction you're trying to do! An ester will not react with a simple carboxylic acid.(Original post by ilovemath)
also what would happen in a reaction like
CH3COOC2H5 + HCOOH
how could I do this as I have TWO carbonyl groups?Last edited by illusionz; 09-04-2012 at 20:47. -
- Reputation:
- Thread Starter
- Benevolent Member
- Posts: 730
Re: Ester exchangeoh ok, my revision guide says it will but no mechanism made sense....(Original post by illusionz)
I'm not quite sure what reaction you're trying to do! An ester will not react with a simple carboxylic acid.
anhydries, I presume they just split at the O=C-C=O
e.g: (CH3CO)2O
Having never studied them before in depth I don't know if the two groups have to be the same
if not then how do I know which "part" takes the H bit of the alcohol and which bit takes the O-R bit from the alcohol -
- Reputation:
- Overlord in Training
- Posts: 2,560
Re: Ester exchangeWhat sort of product did the revision guide arrive at?(Original post by ilovemath)
oh ok, my revision guide says it will but no mechanism made sense....
anhydries, I presume they just split at the O=C-C=O
e.g: (CH3CO)2O
Having never studied them before in depth I don't know if the two groups have to be the same
if not then how do I know which "part" takes the H bit of the alcohol and which bit takes the O-R bit from the alcohol
For an anhydride:
Full size: http://i39.tinypic.com/2i97221.png
The bits in circles show you where the atoms in the products come from.
The attacking species ends up in the ester, part of the anhydride is lost to form the acid.Last edited by illusionz; 09-04-2012 at 21:05.
