Difficulty: Intermediate | Prerequisites: General Chemistry, basic lab technique (filtration, evaporation), introductory organic functional groups
Natural product isolation is a core skill in organic chemistry labs: you take a raw biological source, pull out a target compound with a solvent, then purify it. This experiment uses lichen as the source and acetone as the extraction solvent to isolate usnic acid, a yellow pigment with antimicrobial properties. Along the way you practise gravity filtration, drying with a desiccant, and two-solvent recrystallisation. If you are comfortable with solution preparation and basic lab glassware, you have what you need to follow along.
Usnic acid is extracted from lichen by soaking it in acetone, filtering, drying, and recrystallising from an acetone-ethanol solvent pair. A typical yield is around 1.2% by mass of dry lichen. Purity is confirmed by melting point and optical rotation measurements.
Usnic acid
A yellow, crystalline dibenzofuran derivative (C₁₈H₁₆O₇) produced by many lichen species. Think of it as the lichen's own antibiotic pigment.
Extraction (solid-liquid)
Dissolving a target compound out of a solid matrix using a solvent. In simple terms, you soak the raw material in a liquid that pulls out what you want.
Gravity filtration
Separating a liquid from solid residue by pouring through filter paper under gravity alone, without vacuum assistance. Think of it as the gentle, no-suction version of filtering.
Drying agent (sodium sulfate, Na₂SO₄)
An anhydrous salt added to a solution to absorb dissolved water. In simple terms, it soaks up water the way silica gel packets do in a shoebox.
Recrystallisation (two-solvent method)
A purification technique in which a crude solid is dissolved in a hot solvent, then a second solvent (in which the compound is less soluble) is added until the solution turns cloudy. On cooling, the target compound crystallises out while impurities stay in solution. Think of it as "dissolve, crowd out, and let the pure stuff crash back out."
Vacuum filtration
Collecting crystals on filter paper using reduced pressure beneath the funnel. This is the faster, suction-assisted version of gravity filtration.
Percent recovery (% extracted)
The mass of purified product divided by the mass of starting material, multiplied by 100. It tells you how much of what was available you managed to isolate.
Melting point / decomposition point
The temperature at which a solid turns to liquid, or breaks down chemically. A sharp melting point indicates high purity; decomposition (the sample darkens or chars rather than melting cleanly) means the molecule falls apart before it melts.
Fill a vial with dry lichen and add 15 mL of acetone plus a stir bar.
Lichen is green and grass-like; acetone is clear and colourless.
The acetone dissolves organic compounds from the lichen, including usnic acid, while leaving behind cellulose and other insoluble material.
Pour the extract through filter paper by gravity to remove lichen solids.
The filtrate (liquid that passes through) is pale yellow, indicating dissolved organic material.
Place anhydrous sodium sulfate in the filter paper or add it to the filtrate to absorb residual water.
Water interferes with recrystallisation, so removing it here is essential.
Collect the dried filtrate in a clean 25 mL flask.
Dry the filtrate using gentle heat and an air stream.
The acetone evaporates in a few minutes, leaving behind a yellow crude extract.
Do not overheat: usnic acid decomposes above 150 °C.
Dissolve the crude extract in a minimum volume of hot acetone (the "good" solvent).
Slowly add hot ethanol (the "poor" solvent, 95%) until the solution just turns cloudy.
Cloudiness signals the onset of crystallisation, the point at which the solution is saturated.
This is the trickiest step: too much ethanol crashes out impurities along with the product; too little means lower yield.
Allow the mixture to cool slowly to room temperature, then place in an ice bath.
Slow cooling promotes large, pure crystals; rapid cooling gives smaller, less pure ones.
Collect the yellow, needle-like crystals by vacuum filtration.
Wash the crystals with a small portion of cold acetone to remove surface impurities.
Cold solvent minimises re-dissolving the product.
Weigh the empty vial (8.293 g) and the vial plus product (8.305 g).
Recovered mass = 8.305 g minus 8.293 g = 0.012 g (12 mg).
Percent extracted = (0.012 g / 1 g lichen) x 100% = 1.2%.
A 1.2% yield is low but within the expected range for a student extraction from raw lichen.
Percent recovery
\% \text{ recovery} = \frac{\text{mass of purified product}}{\text{mass of starting material}} \times 100\%In this experiment: (0.012 g / 1.000 g) x 100% = 1.2%
Recrystallisation solvent-pair logic
The target compound must be very soluble in the "good" solvent when hot and much less soluble when cold.
The "poor" solvent must be miscible with the good solvent but dissolve the target compound poorly.
For usnic acid: acetone is the good solvent, 95% ethanol is the poor solvent.
Add the poor solvent to the hot solution until cloudiness appears, then cool slowly.
Usnic acid has genuine antimicrobial and UV-absorbing properties, which is why it appears in some cosmetics and traditional remedies. The same extraction and recrystallisation workflow you practise here scales up to pharmaceutical natural-product isolation, where chemists pull active ingredients from plants, fungi, and marine organisms.
Students often think "cloudy" means the solution should be opaque. It should not. You want the very first hint of cloudiness, a faint haze, not a milky suspension.
Students sometimes assume a low percent recovery means the experiment failed. It does not. A 1 to 2% yield from raw lichen is normal; most of the lichen mass is cellulose and other structural material, not usnic acid.
Students confuse decomposition with melting. Decomposition means the compound breaks apart chemically (darkening, charring) before it reaches a true liquid state. Usnic acid decomposes at roughly 150 °C rather than melting cleanly.
Students occasionally skip the drying step. Residual water in the extract interferes with recrystallisation by changing the solvent polarity and promoting oiling out instead of crystal formation.
⚠️ Know the purpose of each step in the extraction-recrystallisation sequence. Exams often ask "why do we add sodium sulfate?" or "why use hot solvent?"
⚠️ Be able to calculate percent recovery from raw mass data (empty vial, vial plus product, starting material mass).
⚠️ Understand why slow cooling produces purer crystals than rapid cooling.
⚠️ Be prepared to explain how adding too much or too little of the poor solvent affects yield and purity.
True or false: Sodium sulfate is added to remove organic impurities from the extract. (False, it removes water.)
Fill in the blank: In two-solvent recrystallisation, the poor solvent is added until the solution becomes ______. (cloudy)
True or false: A decomposition point and a melting point are the same thing. (False, decomposition involves chemical breakdown, not a clean solid-to-liquid transition.)
Fill in the blank: Percent recovery = (mass of product / mass of ______) x 100%. (starting material)
True or false: Washing crystals with cold solvent minimises product loss. (True, the product is less soluble in cold solvent.)
Q: Why is anhydrous sodium sulfate added to the filtered extract?
A: To absorb residual water. Water in the extract would interfere with recrystallisation by altering the solvent polarity and potentially causing the product to oil out rather than crystallise.
Q: In the two-solvent recrystallisation, what happens if you add too much 95% ethanol?
A: The solution becomes too saturated too quickly, causing both the product and impurities to precipitate together. This lowers purity. You may also get smaller, less well-formed crystals.
Q: The crude extract was dissolved in hot acetone. Why hot?
A: Solubility increases with temperature. Using hot acetone dissolves the maximum amount of crude product in a minimum volume, which is essential for effective recrystallisation.
Q: A student recovers 12 mg of usnic acid from 1.000 g of dry lichen. Calculate the percent recovery.
A: (0.012 g / 1.000 g) x 100% = 1.2%.
Q: The sample decomposed at 150 °C rather than showing a clean melting point. What does this indicate?
A: Decomposition means the usnic acid molecules break apart chemically before reaching a true liquid state. This is a characteristic property of usnic acid, not a sign of impurity.
This material connects directly to polarimetry and optical activity (covered in the companion study notes), since the extracted usnic acid is optically active and its specific rotation is used to confirm identity and calculate concentration. The recrystallisation technique applies to any purification scenario in organic chemistry, from drug synthesis to food-additive isolation. If you are studying spectroscopy methods (IR, NMR, mass spec), note that melting point and optical rotation are older, complementary characterisation tools still used alongside modern instruments.
Usnic acid, lichen extraction, natural product isolation, solid-liquid extraction, gravity filtration, sodium sulfate drying agent, two-solvent recrystallisation, recrystallization, acetone-ethanol solvent pair, vacuum filtration, percent recovery, yield calculation, melting point, decomposition point, crude extract, needle-like crystals, CHM 210, organic chemistry lab techniques