February 28, 2020, 04:48:07 PM
Forum Rules: Read This Before Posting


Topic: extraction... neutral component?  (Read 12250 times)

0 Members and 1 Guest are viewing this topic.

Offline jnimagine

  • Regular Member
  • ***
  • Posts: 41
  • Mole Snacks: +1/-1
extraction... neutral component?
« on: November 21, 2008, 01:55:58 AM »
We did an extraction with a mixture of carboxylic acid and neutral compound, benzophenone, in diethyl ether.  NaOH was added to separate the two components.  My question is, if in separation of ether solution from NaOH layer, some of the aq layer was left with ether solution, how would this affect the recovery of the neutral component???
Also, why do we we get a low recovery of the neutral component??
Thanks in advance! :)

Offline azmanam

  • Chemist
  • Sr. Member
  • *
  • Posts: 1417
  • Mole Snacks: +160/-24
  • Mediocrity is a handrail -Charles Louis d'Secondat
Re: extraction... neutral component?
« Reply #1 on: November 21, 2008, 08:17:01 AM »
What does NaOH do to each compound?  After adding base, which compound is in which layer?

If some aqueous layer is left with the ether layer during separation, how will that effect the mass of each compound after the solvent is removed?

http://en.wikipedia.org/wiki/Acid-base_extraction
Knowing why you got a question wrong is better than knowing that you got a question right.

Offline jnimagine

  • Regular Member
  • ***
  • Posts: 41
  • Mole Snacks: +1/-1
Re: extraction... neutral component?
« Reply #2 on: November 21, 2008, 03:22:15 PM »
What does NaOH do to each compound?  After adding base, which compound is in which layer?

If some aqueous layer is left with the ether layer during separation, how will that effect the mass of each compound after the solvent is removed?

http://en.wikipedia.org/wiki/Acid-base_extraction

NaOH will dissolve in each compound? when you add base, the acidic component would be in the aqueous layer at the bottom and the neutral one stays with ether at the top... and yeah.. exactly my question... ;; what Will happen happen when the aqueous layer is left with the ether layer..? X( i'm not getting it...

Offline azmanam

  • Chemist
  • Sr. Member
  • *
  • Posts: 1417
  • Mole Snacks: +160/-24
  • Mediocrity is a handrail -Charles Louis d'Secondat
Re: extraction... neutral component?
« Reply #3 on: November 21, 2008, 03:59:24 PM »
Quote
the acidic component would be in the aqueous layer at the bottom and the neutral one stays with ether at the top

Yes... except the acidic component will not be an acid anymore.  Do you know what it will be?

Quote
and yeah.. exactly my question

Ok, so we've established that the neutral component should be the only compound in the ether layer.  If you were perfect with your separation, you should be able to evaporate the solvent and recover 100% of the neutral component.

So describe to me what it physically means to have incomplete separation.  Think in terms of amounts of the neutral and 'acidic' components.  Where is the neutral component?  Where is the 'acidic' component?  Let's say you didn't realize that some of the aqueous layer was left behind with the ether layer, and you finished the experiment anyway.  How would your result deviate from the expected results?
Knowing why you got a question wrong is better than knowing that you got a question right.

Offline jnimagine

  • Regular Member
  • ***
  • Posts: 41
  • Mole Snacks: +1/-1
Re: extraction... neutral component?
« Reply #4 on: November 22, 2008, 01:10:38 AM »
Quote
the acidic component would be in the aqueous layer at the bottom and the neutral one stays with ether at the top

Yes... except the acidic component will not be an acid anymore.  Do you know what it will be?

Quote
and yeah.. exactly my question

Ok, so we've established that the neutral component should be the only compound in the ether layer.  If you were perfect with your separation, you should be able to evaporate the solvent and recover 100% of the neutral component.

So describe to me what it physically means to have incomplete separation.  Think in terms of amounts of the neutral and 'acidic' components.  Where is the neutral component?  Where is the 'acidic' component?  Let's say you didn't realize that some of the aqueous layer was left behind with the ether layer, and you finished the experiment anyway.  How would your result deviate from the expected results?

well the acidic component would be in a salt form in the aqueous layer. 

When the aqueous layer is still left behind... does the salt form of acidic component get transferred into the ether layer, so.... it reduces the amount of neutral component recovery?? cuz the neutral component gets acidified...?!?!?!?!?!

Offline azmanam

  • Chemist
  • Sr. Member
  • *
  • Posts: 1417
  • Mole Snacks: +160/-24
  • Mediocrity is a handrail -Charles Louis d'Secondat
Re: extraction... neutral component?
« Reply #5 on: November 22, 2008, 02:41:32 PM »
Quote
well the acidic component would be in a salt form in the aqueous layer. 

correct.

Quote
When the aqueous layer is still left behind... does the salt form of acidic component get transferred into the ether layer

correct.

Quote
so.... it reduces the amount of neutral component recovery??

no...  if you have all of your neutral component in the ether layer... then you accidentally add some 'non-neutral' impurity, what would that do to your final mass of neutral layer?

Let's think about it quantitatively.  Let's say you know you have 1 g of acid and 1 g of benzophenone.  You dissolve them in ether and add base.  You have 2 layers:  the aqueous layer should have all of the deprotonated acid salt, and the ether layer should have all 1 g of the benzophenone. 

In a perfect world, you would correctly separate the two layers.  You would remove the solvent from the organic layer and you would recover exactly 1 gram of material.  You would assume it is pure benzophenone, and you would record a 100% mass recovery.

In this imperfect scenario, you accidentally leave some of the aqueous layer behind in the separation, but you didn't know that it happened.  You continue with the procedure and remove the solvent.  You take the material over to the balance and record the mass of the recovered material - which you assume at this point to be pure benzophenone.  Do you think the mass you record will be more than, less than, or equal to 1 g? 
Knowing why you got a question wrong is better than knowing that you got a question right.

Offline jnimagine

  • Regular Member
  • ***
  • Posts: 41
  • Mole Snacks: +1/-1
Re: extraction... neutral component?
« Reply #6 on: November 22, 2008, 08:07:07 PM »
oh so because there is acidic salt in addition to benzophenone in the ether layer in this case, the recovered solid would be more than the pure benzophenone should have been in the layer??
if that is so,,, how can you explain the low percentage of recovery of benzophenone??
Some reasons I can come up is that some were lost when pipetting the aqueous layer out...
and does the washing the ether layer have anything to do with it??
for instance, it says that I should keep washing the combined aqueous extracts to remove impurities until the wash layer is close to neutral.  if it was still a bit basic due to NaOH that was added... would that affect the recovery of the neutral compound??

Offline azmanam

  • Chemist
  • Sr. Member
  • *
  • Posts: 1417
  • Mole Snacks: +160/-24
  • Mediocrity is a handrail -Charles Louis d'Secondat
Re: extraction... neutral component?
« Reply #7 on: November 22, 2008, 08:24:04 PM »
Quote
the recovered solid would be more than the pure benzophenone should have been in the layer??

yup.  Although I would make the following change:

the recovered solid would be more have a greater reported mass than...

It's not that you have more benzophenone, it's that you have an impurity.  Thus your mass recovery is greater than 100% - not because you created matter, but because the assumption that your material is one pure compound is flawed.

Quote
some were lost when pipetting the aqueous layer out

Sure.  there are a number of reasons.  Go through your proceedure as it is supposed to be done in perfect technique and check your assumptions.  If any of those assumptions are wrong, you can introduce error.  For one example, the procedure may begin by saying 'measure out 1 g of acid and 1 g of benzophenone and dissolve in ether.'  Maybe you only measured out 0.95 g of benzophenone.  You complete the procedure under the assumption that you are working with 1 g of benzophenone.  In the end, you only recover 0.95 g.  It would seem like you have a low yield, but your assumption was wrong.

Quote
I should keep washing the combined aqueous extracts to remove impurities

Are you sure it's the combined aqueous extract?  or the combined organic extracts?  If you wash the organic layer several times with water, that means you have to separate the layers each time, then add more water.  What assumption do you make about that physical act of separation?  How might a flawed assumption introduce error into the procedure which would cause the mass recovery of benzophenone to be low?
Knowing why you got a question wrong is better than knowing that you got a question right.

Offline jnimagine

  • Regular Member
  • ***
  • Posts: 41
  • Mole Snacks: +1/-1
Re: extraction... neutral component?
« Reply #8 on: November 23, 2008, 11:00:42 AM »
Quote
the recovered solid would be more than the pure benzophenone should have been in the layer??

yup.  Although I would make the following change:

the recovered solid would be more have a greater reported mass than...

It's not that you have more benzophenone, it's that you have an impurity.  Thus your mass recovery is greater than 100% - not because you created matter, but because the assumption that your material is one pure compound is flawed.

Quote
some were lost when pipetting the aqueous layer out

Sure.  there are a number of reasons.  Go through your proceedure as it is supposed to be done in perfect technique and check your assumptions.  If any of those assumptions are wrong, you can introduce error.  For one example, the procedure may begin by saying 'measure out 1 g of acid and 1 g of benzophenone and dissolve in ether.'  Maybe you only measured out 0.95 g of benzophenone.  You complete the procedure under the assumption that you are working with 1 g of benzophenone.  In the end, you only recover 0.95 g.  It would seem like you have a low yield, but your assumption was wrong.

Quote
I should keep washing the combined aqueous extracts to remove impurities

Are you sure it's the combined aqueous extract?  or the combined organic extracts?  If you wash the organic layer several times with water, that means you have to separate the layers each time, then add more water.  What assumption do you make about that physical act of separation?  How might a flawed assumption introduce error into the procedure which would cause the mass recovery of benzophenone to be low?

Well, the washing was done to the mixture containing diethyl ether, benzophenone and sodium hydroxide... prior to this the boottom layer, which had the acidic component was removed. and yes, we had to separate layers about 4-5 times... assumption here?? hmm.. i'm not sure....  ??? ???

Offline azmanam

  • Chemist
  • Sr. Member
  • *
  • Posts: 1417
  • Mole Snacks: +160/-24
  • Mediocrity is a handrail -Charles Louis d'Secondat
Re: extraction... neutral component?
« Reply #9 on: November 23, 2008, 11:12:46 AM »
we've been talking about how leaving aqueous layer behind during separation will cause the reported mass of benzophenone to be too high.  what else might happen during separation that might cause the reported mass of benzophenone to be too low?
Knowing why you got a question wrong is better than knowing that you got a question right.

Offline jnimagine

  • Regular Member
  • ***
  • Posts: 41
  • Mole Snacks: +1/-1
Re: extraction... neutral component?
« Reply #10 on: November 23, 2008, 08:03:39 PM »
we've been talking about how leaving aqueous layer behind during separation will cause the reported mass of benzophenone to be too high.  what else might happen during separation that might cause the reported mass of benzophenone to be too low?

all i can come up with is... taking too much of the ether layer out when you were only supposed to take out the aqueous layer....

Offline azmanam

  • Chemist
  • Sr. Member
  • *
  • Posts: 1417
  • Mole Snacks: +160/-24
  • Mediocrity is a handrail -Charles Louis d'Secondat
Re: extraction... neutral component?
« Reply #11 on: November 23, 2008, 08:06:49 PM »
Quote
taking too much of the ether layer out

sounds good to me.  good work.
Knowing why you got a question wrong is better than knowing that you got a question right.

Sponsored Links