Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: pandasgomeow on October 12, 2014, 11:33:40 AM
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I have a laboratory practical. We are given no help and must separate an unknown binary solid mixture into the two individual components, a carboxylic acid and a neutral organic compound. We must calculate % recovery from the original mass we used, so I do not think I can take two separate portions of the original unknown and separate one from each. We are given 1.5 lab periods for this experiment.
I need help figuring out what actions I should take.
Here is what I'm thinking (based on online procedures)
(1) Extraction
Dissolve unknown in 60mL ether, place it in a separatory funnel.
Add 15mL 5% NaOH and shake. This should make two layers, a top organic neutral in ether layer and a bottom carboxylic acid in NaOH layer (please correct me if I am wrong).
Extract the aqueous layer out into beaker 1 (B1).
Add 10mL NaOH to the seporatory funnel (of the organic neutral), swirl, shake, and drain the resulting aqueous layer into B1.
Add 20mL distilled water (to remove water soluble impurities) + discard this into the aqueous waste solution.
Place the organic neutral solution into a beaker, add drying agent (2g Na2SO4), swirl, store.
(2) Purify the Unknown Acid
B1 contains carboxylate anion and NaOH.
- carboxylic anions are water soluble, so add an excess of acid to ensure complete precipitation (since carboxylic acids are not water soluble)
Neutralization: add 10mL 6M hydrchloric acid (slowly, with stirring)
- Unknown acid will precipitate when the OH- ion is neutralized
- Keep adding HCl until the solution is acidic (test with pH paper)
Add 2mL excess HCl to ensure complete preciptiation of the acid.
Cool in ice bath to 5C (decreases solubility).
Collect the precipitated acid using vacuum filtration + wash with COLD distilled H2O.
(Can save the filtrate but do not combine this with the pure filter flas crystals).
Scrape acid from filter paper into beaker for recrystallization.
RECRYSTALLIZATION: +12mL H2O to the beaker, boil it, add 15mL 95% EtOH until all solid dissolves
- if it dissolves in 10min, cool it (room temp) so the unknown acid can recrystallize
- if not dissolved completely, +5mL EtOH dropwise, reheat. Add 5mL EtOH portions until the acid dissolves when the solution is hot. Cool slowly (rapid recrystallization will trap impurities).
- check pH @ room temp (<7, continue. >7, add 6M HCl drop wise until pH <7)
Cool in ice bath.
Vacuum filtration. Wash crystals in COLD distilled water.
Air dry, determine melting point of the acid crystals next week.
ORGANIC NEUTRAL
REMOVE WATER AND SOLVENT: the Na2SO4 (drying agent) absorbs water, so use gravilty filtration (funnel + wool or filter paper, slowly).
SIMPLE DISTILLATION of above solution:
set up the apparatus (this will remove most of the organic solvent, two round bottom flasks - one to heat and one to receive). Boil the solution (but never over the top). Record any temperature changes.
Continue distillation until the temperature starts dropping. Remove from heat and cool.
The receiving flask has distilled solvent, which you don't need.
Remove the remaining residual TBME using a rotary evaporator - I do not have access to this, so what can I do instead? Vacuum filtration?
The remaining liquid is the neutral compound.
QUESTION: my original mixture is a solid. That means my neutral compound should be a solid. How do I modify this procedure to get a solid organic neutral? If I vacuum filtrate it, will the crystals just be my organic neutral?
Thank you in advance for all your *delete me*
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That's basically fine.
A couple of points:
1. "carboxylic acids are not water soluble" is a sweeping generalisation with many exceptions. The acid you have been given probably won't be water soluble.
2. The identity and volume of solvent for recrystallisation depend on the solubility properties of the compound you want to purify and how much of it you have. 12 mL of water + 15 mL EtOH will not necessarily be appropriate for your compound. Add just enough EtOH to dissolve the material (not necessarily 15 mL).
3. Lack of a rotary evaporator is not ideal, but workable. I would distil the ether until you see precipitate begin to form, then remove the heat, slowly cool the solution and collect the crystals by filtration (this is a recrystallisation). You could just let the remaining residual solvent evaporate from an open flask or a dish instead of using a rotavap, but the material would be less pure.
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Hi Dan,
Thanks so much for looking this over! I really appreciate your advice. I'm already distilling the ether, one procedure just suggested a rotary evaporator after simple distillation. Since melting points are worth 40% of this lab, I need a pure compound, so I think I will use vacuum filtration after distillation to get the crystals (as you said). Would distilling it a second time (before vacuum filtration) or using vacuum evaporation be useful or just a waste of time?
Thanks again for all your *delete me*
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Would distilling it a second time (before vacuum filtration) or using vacuum evaporation be useful or just a waste of time?
The only reason to evaporate to dryness would be that you do not intend to recrystallise from ether. So you either don't intend to recrystallise at all (but it sounds like you do plan to), or you want to remove all the ether and recrystallise from a different solvent (plausible, since ether may not work well - you will not know if that's the case until you try it).
There is no practical difference between evaporating to dryness and then recrystallising from ether vs evaporating an ethereal solution to saturation point, cooling and collecting the crystals (except that the former could be a bit more controlled).
If the solubility of the compound in ether is too high, another option is a mixed solvent recyst, where you add a second solvent (usually an alkane, like hexane) with poor solvating ability. To do this, you can remove most of the ether by distillation and then (maintaining boiling) add hexane slowly until crystals appear, then cool and collect.