Difficulty: Intermediate | Prerequisites: Acid-base extraction theory (see companion notes), basic lab safety
Big picture: These notes cover the hands-on side of solvent extraction: the procedure, the equipment, the calculations and the quality checks. Where the theory doc explains why each step works, this doc explains how to do each step and how to interpret your results. You should already understand why NaHCO3 extracts carboxylic acids and why NaOH extracts phenols before reading this.
The lab procedure dissolves three compounds in ether, then uses sequential washes with NaHCO3 and NaOH to pull acidic compounds into aqueous layers one at a time. Each compound is recovered by acidifying its aqueous extract (or evaporating the ether), and purity is checked by comparing the melting point range against literature values.
Gravity filtration
Filtering a solution through filter paper using only gravity. Used when you want to keep the filtrate (the liquid that passes through) and discard the solid. Think of it as letting the liquid drip through a paper cone at its own pace.
Vacuum filtration
Filtering a solution by applying a vacuum beneath the filter to speed up the process. Used when you want to keep the solid (the crystals on the filter paper) and discard the filtrate. The suction pulls the liquid through quickly and helps dry the crystals.
Drying agent (e.g. anhydrous Na2SO4)
A substance added to an organic solution to absorb dissolved water. In simple terms, it soaks up any trace water that sneaked into your ether layer during the extraction.
Percent recovery
The mass of compound you collected divided by the mass you started with, expressed as a percentage. It tells you how much product you lost along the way. A perfect extraction with no losses would give 100%, but real experiments rarely exceed 80%.
Melting point range
The temperature span from when a solid first begins to melt (T1) to when it is completely liquid (T2). A pure compound has a sharp, narrow range (1 to 2 degrees). A wider range suggests impurities are present.
Recrystallisation
A purification technique where a solid is dissolved in a hot solvent and then allowed to cool slowly so that pure crystals form and impurities remain in solution. In this lab, cooling an ice bath was used to crystallise the oily phenol.
Centrifuge tube
A small, tapered tube used here instead of a separatory funnel. The tapered bottom helps you see and separate the two layers with a pipet.
Pasteur pipet (disposable pipet)
A thin glass dropper used to carefully remove one layer from the other. Getting clean layer separation with a Pasteur pipet takes practice and is a common source of error in this experiment.
Weigh out the three compounds: acetanilide (about 68 mg), p-toluic acid (about 100 mg) and p-tert-butylphenol (about 100 mg).
Dissolve all three in 5 mL of tert-butyl methyl ether (TBME) in a 15 mL centrifuge tube. Everything should go into solution.
Add 2 mL of 0.5 M NaHCO3 to the centrifuge tube. Shake gently at first (CO2 is produced, so pressure builds), then cap and shake more vigorously, venting periodically.
Let the layers separate. Remove the bottom aqueous layer with a Pasteur pipet and transfer it to a labelled beaker.
Repeat the NaHCO3 wash two more times (2 mL each) to ensure complete extraction of p-toluic acid.
Rinse the ether layer once with 1 mL of distilled water. Combine this wash with the NaHCO3 extracts.
Add 2 mL of 0.5 M NaOH to the remaining ether layer. Shake and separate as before.
Remove the aqueous layer to a second labelled beaker.
Repeat the NaOH wash two more times (2 mL each).
Save the aqueous NaOH layers and reserve the ether layer.
To the combined NaHCO3 extracts, add 3 M HCl dropwise until the solution is acidic and no more solid forms.
The p-toluic acid precipitates out of solution as a white solid.
Collect the crystals by vacuum filtration and leave them to dry.
Heat the combined NaOH extracts to about 60 degrees C to drive off any dissolved ether.
Cool the solution, then acidify with 3 M HCl to precipitate the phenol.
If the phenol comes out as an oil rather than crystals, cool the solution in an ice bath to induce crystallisation.
Filter and dry the crystals.
Add about 0.2 g of anhydrous Na2SO4 to the remaining ether layer to remove dissolved water. Swirl gently and let it sit.
Decant the dried ether into a clean beaker.
Evaporate the ether in a fume hood. The acetanilide crystallises as the solvent leaves.
Measure the melting point range of each recovered compound.
Compare to literature values: acetanilide 113 to 115 degrees C, p-tert-butylphenol 98 to 101 degrees C, p-toluic acid 180 to 182 degrees C.
A narrow melting range close to the literature value indicates high purity. A wide or depressed range indicates impurities.
Percent recovery = (collected mass / initial mass) x 100%
Example from this experiment:
Compound | Initial mass (g) | Collected mass (g) | Percent recovery |
|---|---|---|---|
Acetanilide | 0.068 | 0.043 | 63.2% |
p-tert-Butylphenol | 0.101 | 0.051 | 50.5% |
p-Toluic acid | 0.116 | 0.071 | 61.2% |
All three recoveries are below 65%, indicating significant losses during extraction and transfer.
Compound | Observed T1 | Observed T2 | Observed range | Literature range |
|---|---|---|---|---|
Acetanilide | 81.4 C | 83.2 C | 1.8 C | 113 to 115 C |
p-tert-Butylphenol | 109.4 C | 112.2 C | 2.8 C | 98 to 101 C |
p-Toluic acid | 159.3 C | 162.6 C | 3.3 C | 180 to 182 C |
The observed melting points differ from literature values. A melting point that is lower and broader than expected suggests impurities. A melting point that is higher than expected (as with p-tert-butylphenol here) may point to packing errors during measurement rather than super-purity.
Solvent extraction is used at industrial scale to purify pharmaceutical intermediates, separating the desired compound from reaction by-products. Water-treatment plants use a version of this technique to remove organic contaminants from drinking water. In the mining industry, liquid-liquid extraction recovers metals like copper and uranium from ore leach solutions.
"A melting point higher than the literature value means the sample is very pure." Not necessarily. In this experiment, p-tert-butylphenol showed a higher-than-expected melting point, which was attributed to poor crystal packing in the capillary tube, not to exceptional purity. Large, irregularly packed crystals conduct heat unevenly and can give artificially high readings. Pulverise crystals with a microspatula before packing.
"Low percent recovery means the experiment failed." Low recovery indicates losses, not failure. Compound may have been left behind in a layer, lost during pipetting, or lost during filtration. The separation itself may still have worked. Purity (melting point) is the better indicator of whether the separation was successful.
"You can skip the water rinse between the NaHCO3 and NaOH steps." The water rinse removes residual NaHCO3 from the ether layer. Skipping it may contaminate the NaOH extract with leftover bicarbonate solution, carrying traces of carboxylic acid into the phenol fraction.
"The aqueous layer is always on the bottom." With tert-butyl methyl ether (density around 0.74 g/mL), yes, the aqueous layer is denser and sinks. With dichloromethane (density around 1.33 g/mL), the organic layer is on the bottom. Always confirm by adding a drop of water and watching which layer it joins.
⚠️ Know the percent recovery formula and be ready to calculate it from given masses.
⚠️ Understand what melting point range tells you about purity. A pure compound has a sharp range matching the literature value. Impurities depress and broaden the melting point.
⚠️ Be able to identify common sources of error: drawing up the wrong layer with the pipet, incomplete extraction (not enough washes), losing product during filtration, poor crystal packing for melting point measurement.
⚠️ Know why anhydrous Na2SO4 is added to the ether layer. It removes dissolved water. If you skip this step, the remaining water can interfere with crystallisation and give inaccurate mass readings.
⚠️ Know the purpose of acidification with HCl. Adding acid to the aqueous extracts re-protonates the salt, converting it back to the neutral organic compound, which precipitates because it is no longer water-soluble.
True or false: A percent recovery above 100% is physically impossible and always indicates a weighing error. Answer: False. It can also mean the product is wet or contaminated with a heavier impurity.
Fill in the blank: The purpose of adding anhydrous Na2SO4 to the organic layer is to remove ______. Answer: dissolved water (moisture).
True or false: Vacuum filtration is used when you want to keep the solid on the filter paper. Answer: True.
Fill in the blank: To recover p-toluic acid from its sodium salt in the aqueous layer, you add ______ until the solution is acidic. Answer: 3 M HCl (hydrochloric acid).
True or false: A melting point range of 5 degrees C suggests the compound is very pure. Answer: False. A wide range indicates impurities.
Q: A student obtains a percent recovery of 50% for p-tert-butylphenol. List two plausible sources of product loss.
A: (1) Some phenol remained dissolved in the ether layer because the NaOH washes did not fully extract it. (2) Product was lost when the aqueous layer was removed with the Pasteur pipet, especially if ether was accidentally drawn up along with the aqueous layer.
Q: Explain why the ether layer is treated with anhydrous Na2SO4 before the solvent is evaporated.
A: Water dissolved in the ether would remain after evaporation and contaminate the acetanilide. Anhydrous Na2SO4 absorbs this water, ensuring a dry product and an accurate mass measurement.
Q: A student measures a melting point range of 75 to 80 degrees C for acetanilide (literature: 113 to 115 degrees C). What does this suggest?
A: The sample is significantly impure. Impurities depress and broaden the melting point. The student likely has a mixture of acetanilide with residual solvent, water or another compound that was not fully separated.
Q: Why is it important to vent the centrifuge tube during the NaHCO3 extraction?
A: The reaction of NaHCO3 with the carboxylic acid produces CO2 gas. If the tube is sealed, pressure builds up and can pop the cap or crack the tube. Venting releases the gas safely.
Q: After all extractions, a student finds that the ether layer still contains some p-toluic acid. What procedural change would improve the separation?
A: Perform additional NaHCO3 washes (more than three) or use a larger volume of NaHCO3 solution per wash to ensure complete deprotonation and extraction of the carboxylic acid.
Melting point determination connects to the broader concept of phase transitions and purity testing, which appears in nearly every organic chemistry lab going forward.
Percent recovery calculations are used in every synthesis and isolation experiment. Getting comfortable with the formula now saves time later.
The drying and crystallisation steps here are the same techniques you will use in recrystallisation labs and synthesis work-ups throughout the course.
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