Question

Many students found solids in the acidified NaHCO3 layer, acidified NaOH layer, and the evaporated organic layer. No starting mixtures contained three compounds. Explain how solids could have been found in each of those layers.

Background into the experiment:

Obtain approximately 3.0 g of your unknown mixture and dissolve it in 30 mL of diethyl ether (aka ether).

2. Wash (shake, vent, settle, drain) the ether solution with 15 mL of a 10% aqueous sodium bicarbonate solution. When shaken vigorously the mixture may develop pressure, so be sure to vent the funnel periodically by inverting the funnel and opening the stopcock. Separate the layers and extract the ether solution with an additional 5 mL of 10% aqueous sodium bicarbonate solution. Combine the aqueous layers. KEEP BOTH THE ORGANIC AND AQUEOUS LAYERS IN SEPARATE LABELED CONTAINERS!!!! (See below, step 5, for instructions for the aqueous layer).

3. Wash the organic (ether) layer from step 2 with 15 mL of a 5% aqueous sodium hydroxide solution. [PLEASE NOTE: If a solid dispersion forms in the aqueous layer, add about 10 mL of water, shake, and the solid should dissipate.] Separate the layers, making sure that this aqueous extract goes in a different flask from the one used in step 2, and extract the organic layer with an additional 5 mL of hydroxide solution. Combine the aqueous layers from this step only.

4. Wash the organic layer from step 3 with 10 mL saturated aqueous NaCl. Separate the aqueous layer. Dry the organic layer with MgSO4. Gravity filter the organic solution and rinse the solid filter cake with 10 mL of ether. Transfer the organic solution to a tared 100 mL round bottom flask and remove the solvent by using the rotary evaporator, demonstrated by your TA. Solvent removal using the rotovap is complete when there is only a solid left in the flask (weigh the flask again the amount of solids present) or if there is nothing left in the flask. Scrape solids out of flask and place in a pre-massed scintillation vial, cover with parafilm, puncture the parafilm, and store until week 3. If a solid was obtained, recrystallize 0.5 g of the collected crystals from ethanol. Once crystals have formed, filter, rinse, and let dry. In week 4 (during experiment 4) obtain a melting point, percent yield, and IR spectra of the compound collected from organic layer.

5. Backwashing: Extract the aqueous layer from step 2 with 5 mL of ether. TLC the organic layer and if nothing is present, discard the organic wash (top layer). Carefully acidify (pH 2) the aqueous solution by the drop-wise addition of 6 M HCl. Caution: The bicarbonate solution will vigorously liberate carbon dioxide when neutralized with HCl, so stir the solution as HCl is added to avoid the solution bubbling out of the flask. Collect any solid that precipitates by suction filtration, wetting the paper with water. Wash the filter cake with cold water and allow it to air dry. In week three, recrystallize 0.5 g of the collected crystals from water (rheostat setting 5 or 50), and then filter, rinse, and let dry. In week 4 (during experiment 4) obtain a melting point, percent yield, and IR spectra of the compound collected from the sodium bicarbonate aqueous layer.

6. Repeat the procedure from step 5 with aqueous solution from step 3. In the third week, if you have a solid, recrystallize as follows: Weigh about 0.5 grams of the solid into a clean and dry 50 mL Erlenmeyer flask. Add hot methylcyclohexane slowly with heating until all solids have dissolved. Cool the solution to room temperature and then cool in an ice bath. If crystallization does not occur spontaneously, scratch the sides and bottom of the flask with a glass stirring rod. In week 4 (during experiment 4), obtain a melting point, percent yield, and IR spectra of the compound collected from the sodium hydroxide aqueous layer.

Answer 1

Indeed the three solutions could result in the precipitation of even the single compound taken initially.

since the liquid-liquid extraction is a physical separation process and is not so much accurate Eg., the separation of the aqueous and organic phase may be different for different students. hence some of the sample may very well be left after the first separation using NaOH. therefore along with NaOH the sample can also go into the separated organic layer and then when it is dried, flakes may form.

for increasing the accuracy of separation, we carry out the separation using our solvent at least 3-4 times with vigorous shaking for at least 5 minutes, otherwise the sample may not get very well separated especially if it has marginal dipole. hence the aqueous layer may also very well have the compound and NaHCO3 layer on acidification can result in the sample.

the appearance of sample in layer other than the intended one indicates a lack of good experimental hand.