Separation help!

[735513]

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Hi,

I'm having a little confusion with separation of compounds and the different solvents. I seem to get different info everywhere. What is the general rule with separation...

Strong acid- add weak base? or strong base to make the salt?
etc....

I had a question that you had to separate 3 different compounds (ranging in basicity) and say what types of solvents you would use. THe answer said that if you use too strong of a solvent you would protonate both compounds instead of separating them one by one. I 've looked to my ochem lab text and it's really general. Can anyone help clarify? Thanks so much!

 

[croco]

For separation techniques, which I had about 3-4 questions on in the actual exam is very simple and is composed of general rules. There is a theory governed by Raoult's Law in Organic Chemistry called the distribution coefficient labled K=C1/C2 where c1 is typically ether and c2 is water. On the DAT, separation techniques do not apply to this formula, because C stands for solubility and the compounds they give will be very soluble in one solvent, which allows for a great deal of separation. In general if you are given an organic acid (ie. R-COOH) or an organic base (R-NH2), these are both MORE soluble on organic solvents such as ether than in water. However the salts of the compunds (RCOONa) and (R-NH3CL) will be more soluble in a solvent such as water since they are ionic in nature. Now for example imagine trying to separate an acid and a nuetral compound with the two layer:ether and water (water layer at bottom being more dense). The nuetral and acidic mixture is separated in the ether layer and is EXTRACTRED by the addition of a base such as Sodium Bicarbonate which is a base that deprotanates the acid and converts it to a salt! What now? Well as I mentioned the salt is more soluble in the water layer and hence you have separated the nuetral compound which is now found in the ether layer and the acidic compound which is found in the water layer. When you extract the water layer you can RE-protanate this salt by use of an acid substance such as Sulfuric acid or HCL

 

[PERFECT3434]

wowo croco,

you had 3-4 questions on seperation? i was told by a lot of people not to worry about any questions on lab techniques. i was told that at most you'll have one lab question so i didn't even study those in depth. thank god i had none on my DAT.

that again confirms my view that we always make a common mistake of going by what others say what will be on DAT. its really true that you need to know everything and younever know what are gonna get.

 

[735513]

Croco,

Thank you for the extensive explanation. It does help. I get how you're supposed to make the salt by deprotonating/protonating it, but when you have multiple things to separate how do you know what to use? I saw a question that showed 3 different organic compounds, increasing in basicity. The question asked what was the best way to extract each compound in order. The answer choices consisted of different combinations of weak aqueous acid, followed by stronger aq acid....or strong aq base, followed by strong aq acid, and every combination in between. I know I'm not explaining this question very well, but the answer was very specific in the type of acid/base to use because if you used too strong of a solution, it would protonate everything and you couldn't distinguish between the two compounds.

I had thought that if you have a weak organic base you would need a strong enough acid to protonate it. But the answer said that you would need to use an acid that is not so strong cause then it would protonate all basic compounds.

Am I making any sense, or am I just on the wrong track? Maybe that's more detail than I need to worry about...? Sorry if you can't follow me. I'll try to find the exact question, cause I'm sure that would help. You've already spent so much time helping me with your last post. Thank you again, I really appreciate it.


Thanks to everyone for your input!

(p.s. sorry if you can't understand what I'm saying, all this DAT studying has made me lose all communication skills... )😕

 

[croco]

First thing is RELAX!,
I think you are looking to much into the separation/extraction thing, especially for this test. I could explain what the answer meant but it would take me atleast a couple of paragraphs to explain the situation. Just focus I what I already wrote and should do fine.