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Author Topic: How to know whether it's in a organic or aqueous phase?  (Read 19317 times)

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neurones

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How to know whether it's in a organic or aqueous phase?
« on: September 15, 2011, 03:16:34 PM »

Hello,

First post on this forum, I'm pretty sure we'll be very good friends ;) :P
I'm taking a organic chemistry lab this semester and I have a question please.

During the lab, my TA (Teacher Assistant - the one who supervises the lab) kept saying that if you mix this with this, it will be immisible so two phases will be formed: aqueous and organic. The problem is with a seperation funnel which will eventually have two phases, A and B and we have to identify which one is aqueous and which one is organic.

How did he know so easily just by looking at it? He sometimes added water aswell to see if they seperate or not (which I have no idea what that means.)

Thanks so so much for all your help !!!! :)


EDIT: In case it helps explaining my question, here's the class question:
"A students adds the following to a separatory funnel: 30 mL of water, 100 mL diethylether, 15 mL 0.1 M HCl, 0.25 g cholesterol and 0.25 g tyrosine, and proceeds to shake the funnel. Indicate the contents of phase A and phase B."

I don't just want to know the answer, I would love to understand this so when I get tested on it at the end of the year, I'll know how to answer. Anything could help.

Thanks so much again ! :)
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asd34

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Re: How to know whether it's in a organic or aqueous phase?
« Reply #1 on: September 15, 2011, 05:01:40 PM »

It's pretty easy, the organic phase is heavier than the aqueous, so the organic will be the lower phase.
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voidSetup

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Re: How to know whether it's in a organic or aqueous phase?
« Reply #2 on: September 15, 2011, 05:37:15 PM »

First think about the solvents, water is very polar and the HCl solution is aqueous. Diethyl ether is non-polar so what will be miscible with what?

Then think about the solutes. If a molecule has a functional group on it that can be protonated so that the molecule bears a charge, it will make it more soluble in a polar solvent. A non polar solute will be more soluble in a nonpolar solvent.

In order to determine which phase is which you look at the densities of the solvents (diethyl ether is about 0.71 g/mL and water is 1 g/mL). The denser solvent will be the bottom layer.
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neurones

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Re: How to know whether it's in a organic or aqueous phase?
« Reply #3 on: September 15, 2011, 05:53:53 PM »

Oh and I forgot to add in the question that phase A is on the top and phase B is on the bottom.

In order to determine which phase is which you look at the densities of the solvents. The denser solvent will be the bottom layer.

But in this case there's 5 solvents/solutes to look at. How can I divide those into only two groups? I could rank them from lowest to highest density but I can't figure out a way to divide them into two phases.

I Googled up the densities and came out with the following:
Diethyl ether: 0.71
Water: 1
HCl: 1.4
Couldn't find neither cholesterol nor Tyrosine but I'm guessing they're pretty dense.

Going by the functional groups, all but Diethyl ether have an OH which means all but Diethyl ether could be protonated. Hence all will be soluble in water except for Diethyl ether. So everything is phase B except for Diethyl ether which is phase A.

Is my reasoning right? If yes, is there a better reasoning? If not, :( , what's the better reasoning?

Thank you :)
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voidSetup

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Re: How to know whether it's in a organic or aqueous phase?
« Reply #4 on: September 15, 2011, 06:38:22 PM »

Water, aqueous HCl, and diethyl ether are the solvents.  Tyrosine and cholesterol are the solutes so don't worry about the densities of those. I made a slight mistake thinking tyrosine had an amine but forgot it's an amino acid so its an amide...it is fairly polar though.
« Last Edit: September 15, 2011, 06:53:57 PM by voidSetup »
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Borek

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Re: How to know whether it's in a organic or aqueous phase?
« Reply #5 on: September 15, 2011, 08:55:35 PM »

It's pretty easy, the organic phase is heavier than the aqueous

No. Some solvents have higher density, some have lower. Dichloromethane will be below, but diethyl ether will be above.

Diethyl ether: 0.71
Water: 1
HCl: 1.4
Couldn't find neither cholesterol nor Tyrosine but I'm guessing they're pretty dense.

Densities of HCl, cholesterol and tyrosine don't matter - it is density of the solution that matters, and this is usually close to that of water - unless we are talking of concentrated solutions.

As far as I know there is no simple way of determining which layer is which - with some experience you can see how they behave and determine the water (it has a specific combination of viscosity and surface tension, add to that the way it wets glass surface) jut by looking. If in doubt - add some water and watch which layer volume grows.
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Dan

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Re: How to know whether it's in a organic or aqueous phase?
« Reply #6 on: September 15, 2011, 10:02:26 PM »

I made a slight mistake thinking tyrosine had an amine but forgot it's an amino acid so its an amide...it is fairly polar though.

Tyrosine has no amides. Amino acids have an amine and a carboxylic acid.

neurones,

To answer the questions, think about the densities of the solvents used - you can probably ignore the solutes when considering density. The solutions are likely to be dilute enough that the relatively small amount of solute does not dramatically change the density.

Once you've determined which layer is on top and which is on the bottom, you need to think about which solutes are dissolved in which phase. Think about what you know about solubility. More polar compounds are more readily dissolved in polar solvents, and the less polar dissolve more readily in low polarity solvents.

Also be aware of the role of acids and bases in organic aqueous extraction - acids are used to protonate bases (such as amines) to form polar water-soluble salts. Bases are used to deprotonate acids to form more polar salts.

Perhaps you could post the other questions involving five solvents, because it's difficult to give guidance without seeing the questions. In general, to group five solvents into two phases you just need to think about miscibility of solvents. So, if you mixed water, brine, diethyl ether, ethyl acetate and hexane you would get two layers. Brine (saturated aqueous NaCl) is miscible with water (but not the organic solvents), and the three organic solvents are miscible with each other,but not with aqueous solutions. The two groups are [water, brine] and [diethyl ether, ethyl acetate, hexane].
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discodermolide

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Re: How to know whether it's in a organic or aqueous phase?
« Reply #7 on: September 15, 2011, 11:40:35 PM »

Hello,

First post on this forum, I'm pretty sure we'll be very good friends ;) :P
I'm taking a organic chemistry lab this semester and I have a question please.

During the lab, my TA (Teacher Assistant - the one who supervises the lab) kept saying that if you mix this with this, it will be immisible so two phases will be formed: aqueous and organic. The problem is with a seperation funnel which will eventually have two phases, A and B and we have to identify which one is aqueous and which one is organic.

How did he know so easily just by looking at it? He sometimes added water aswell to see if

they seperate or not (which I have no idea what that means.)
Thanks so so much for all your help !!!! :)
EDIT: In case it helps explaining my question, here's the class question:
"A students adds the following to a separatory funnel: 30 mL of water, 100 mL diethylether, 15 mL 0.1 M HCl, 0.25 g cholesterol and 0.25 g tyrosine, and proceeds to shake the funnel. Indicate the contents of phase A and phase B."
I don't just want to know the answer, I would love to understand this so when I get tested on it at the end of the year, I'll know how to answer. Anything could hel
Thanks so much again ! :)

Ether on top water on bottom
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fledarmus

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Re: How to know whether it's in a organic or aqueous phase?
« Reply #8 on: September 16, 2011, 01:47:09 AM »


During the lab, my TA (Teacher Assistant - the one who supervises the lab) kept saying that if you mix this with this, it will be immisible so two phases will be formed: aqueous and organic. The problem is with a seperation funnel which will eventually have two phases, A and B and we have to identify which one is aqueous and which one is organic.

How did he know so easily just by looking at it? He sometimes added water aswell to see if they seperate or not (which I have no idea what that means.)

If you have a separatory funnel with two layers, and you add water, one of the two layers will get bigger - that is the aqueous layer. Another way to distinguish the two is to take a small vial or test tube full of water and add a few drops of each layer to the center of the water with a pipette. If you are adding the organic layer, cohesive drops will come out of the pipette and either rise to the surface or drop to the bottom of the vial (depending on the density of the organic solvent). If you are adding the aqueous layer, it will mix with the water in the vial and you will not see cohesive drops in the water.

Quote

EDIT: In case it helps explaining my question, here's the class question:
"A students adds the following to a separatory funnel: 30 mL of water, 100 mL diethylether, 15 mL 0.1 M HCl, 0.25 g cholesterol and 0.25 g tyrosine, and proceeds to shake the funnel. Indicate the contents of phase A and phase B."


In this case, it's easy to pick out the solvents - they are the ones in the largest amounts. (Your 0.1 M HCl solution is a solution in water so it will be part of the water layer).

Inorganic salts, acids, and bases are almost never soluble in organic solvents. If they are soluble at all, they will be soluble in the water layer.

It takes some experience and some understanding of the structure of your molecules to determine which layers the organic components will be in. In your case, you want to know whether the cholesterol and tyrosine would dissolve best in ether or in a dilute acid. Cholesterol is a large carbohydrate with almost no polarity - the hydrophilic OH group is very small compared to the hydrophobic rest of the molecule. It could be ionized in a strong base which would give it a negative charge and make it more polar, but since you have an acidic solution, it will not be. It will most likely dissolve in the ether.

Tyrosine is an amino acid (has both an -NH2 and a carboxylic acid attached to the same carbon) that also contains a phenolic hydroxy group. All three of those functional groups are polar and hydrophilic, while only the benzene ring of the phenol is hydrophobic. In addition, in dilute acid, the amine is protonated (-NH3+), making it even more polar. This compound will most likely dissolve in the water layer. If you had a basic solution instead, the amine wouldn't be protonated and the carboxylic acid would be deprotonated, and if the base was strong enough, the phenol would be deprotonated as well. Even at neutral pH, the carboxylic acid would be deprotonated and the amine protonated to give a zwitterion. All of these variations are very polar, and amino acids are almost always water soluble at any pH.
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voidSetup

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Re: How to know whether it's in a organic or aqueous phase?
« Reply #9 on: September 16, 2011, 02:49:11 AM »


Tyrosine has no amides. Amino acids have an amine and a carboxylic acid.


Right you are, too much beach and not enough chemistry this summer for me :-\
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neurones

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Re: How to know whether it's in a organic or aqueous phase?
« Reply #10 on: September 16, 2011, 03:02:32 PM »

Thanks so so much everyone !! You really helped a lot :)
So many things got cleared up; thank you ! :D
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