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Author Topic: extracting with aq. NaHCO3  (Read 13859 times)

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johnj7

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extracting with aq. NaHCO3
« on: February 11, 2008, 04:22:39 PM »

Hello, I'm struggling with an extraction concept:

There is a mixture of aniline and naphthalene in ether.
Aniline = 4g/100mL of water; miscible with ether
Naphthalene = insoluble in water; 30 g/100 mL in ether
Extract with aq. NaHCO3

Why would you extract with aq. NaHCO3?
What exactly would happen?

This is a part of my lecture guide, and in my notes I wrote that aniline became protonated.  Is this proton from NaHCO3 or from an acid?

Would adding an acid do anything?

Heres my guess as to whats going on:
so we add an acid such as HCL, which will protonate aniline and create a salt, which is soluble in NaHCO3.  Then aniline can be extracted in that salt water, leaving napthalene in the ether.  Then a base, like NaOH (would NaOH be too strong of a base?)  can be added to obtain aniline. 

Is this correct?  Any help would be much appreciated.

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camp

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Re: extracting with aq. NaHCO3
« Reply #1 on: February 11, 2008, 06:19:02 PM »

You pretty much got it.  Protonate the amide and it becomes a salt.  Nothing happens to the naphalene.  They separate into a polar and organic layer which then can be separated with a sep. funnel.  If you had a carboxilic acid you could put a base in and pull a hydrogen off it and get the same result.  Neutralize and evaporate ;)
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johnj7

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Re: extracting with aq. NaHCO3
« Reply #2 on: February 11, 2008, 06:39:03 PM »

hmm gotcha, thanks

so the only purpose of the NaHCO3 is to create a polar aqueous layer in which the aniline salt can be extracted into?  Why wouldn't you just use water, for instance?

 
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azmanam

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Re: extracting with aq. NaHCO3
« Reply #3 on: February 12, 2008, 04:03:55 AM »

***SAFETY WARNING***

Mix sodium bicarbonate (NaHCO3) and acid (especially HCl) SLOWLY!  Otherwise you'll get a science-fair worthy volcano - potentially in your face.  Wear safety goggles.  (What do you suppose the products are of mixing sodium bicarbonate with a strong acid?)

***SAFETY WARNING***

--------

This statement is not technically correct:

Quote
we add an acid such as HCL, which will protonate aniline and create a salt, which is soluble in NaHCO3.

Let's look more closely at the various acid/base equilibria in this problem.  We have several potential acids:


CompoundpKa
HCl~ -7
aniline-H+~ 4
(conj. acid of aniline)
NaHCO3~ 10.5
aniline~ 30
naphthalene~ 40

Remebering that acid/base equilibra favor the weaker acid (the compound with the highest pKa, do you predict the following equilibra favor the right side of the arrow, or the left side of the arrow?

HCl  +  aniline  --><--  aniline-H+  +  Cl-
NaHCO3  +  aniline  --><--  aniline-H+  +  NaCO3-
aniline-H+  +  NaHCO3  --><--  aniline  +  H2CO3

With those answers in mind:


--What happens when you add aqueous HCl to a mixture of naphthalene/aniline in ether? 
--Do you think your HCl/ether naphthalene/aniline mixture will be one phase or biphasic? 
--What happens if you add NaHCO3  to that resulting mixture of HCl/ether naphthalene/aniline, as your "will be soluble in NaHCO3" statement implies? 
--What happens after you separate your aniline from the naphthalene  (is the aniline in the water layer or the ether layer?  Is the aniline protonated or not?) 
--And what happens when you add NaHCO3 to that separated aniline?  What if you added HCl to that separated aniline?



Hopefully these questions will help you understand exactly what is going on in your acid/base extraction.  Enjoy the smell of mothballs
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johnj7

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Re: extracting with aq. NaHCO3
« Reply #4 on: February 12, 2008, 07:28:25 PM »

ahh okay, i think i understand now

HCl  +  aniline  --><--  aniline-H+  +  Cl-
NaHCO3  +  aniline  --><--  aniline-H+  +  NaCO3-
aniline-H+  +  NaHCO3  --><--  aniline  +  H2CO3

the second equilibrium stays to the left, as NaHCO3 is a weak acid.
We are concerned with only the first and last equilibrium equations.
So dilute aq HCl is added, protonating aniline. 
- The HCl/ether mixture will be biphasic.

I'm still confused as to what the NaHCO3 would do,
is this correct?
- extract aq protonated aniline layer,
- add NaHCO3 to depronate and get aniline back
- add fresh ether to get aniline in organic layer
- evaporate ether


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azmanam

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Re: extracting with aq. NaHCO3
« Reply #5 on: February 13, 2008, 01:53:43 AM »

Well done.

Just make sure you add that bicarb slowly.  How do you suppose you'll know when you've added enough bicarb?
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