Difficulty: Beginner to Intermediate | Prerequisites: Basic lab safety, understanding of polarity and solubility
These techniques, separatory funnel extraction, chemical drying, and simple distillation, form the backbone of organic chemistry laboratory work. Nearly every synthesis you run in this course will require at least one of them to isolate and purify your product. Understanding the theory behind each step (not just the procedure) is what separates a student who can troubleshoot problems from one who blindly follows instructions. If you are behind on polarity, solubility, and acid-base chemistry, review those first.
Liquid-liquid extraction uses a separatory funnel to separate compounds between immiscible layers based on solubility. Drying agents remove residual water from organic layers. Simple distillation separates liquids by boiling point differences and is used to purify the final product.
Separatory Funnel (Sep Funnel)
A pear-shaped glass vessel with a stopcock at the bottom, used to separate immiscible liquid layers. Think of it as a specialised container that lets you drain one layer out the bottom while keeping the other inside.
Liquid-Liquid Extraction
A purification technique that exploits different solubilities of compounds in two immiscible solvents (typically an organic solvent and water). Impurities partition into the layer where they are more soluble.
Brine Wash (Aqueous NaCl)
A wash with saturated sodium chloride solution. Its purpose is to reduce emulsions and to "salt out" residual water from the organic layer. The high ionic strength of brine decreases the solubility of the organic compound in the aqueous phase.
Sodium Bicarbonate Wash (NaHCO₃)
An aqueous wash with 10% NaHCO₃ used to neutralise and remove acidic impurities (such as residual HCl) from the organic layer. The bicarbonate reacts with acids to produce CO₂ gas, water, and a sodium salt.
Drying Agent
An anhydrous inorganic salt added to an organic liquid to absorb dissolved water. Common examples include anhydrous calcium chloride (CaCl₂), magnesium sulphate (MgSO₄), and sodium sulphate (Na₂SO₄).
Simple Distillation
A purification method that separates liquids based on differences in boiling point. The liquid is heated until it vaporises, the vapour travels through a condenser, and the purified liquid is collected. Effective when the boiling points of the components differ by at least 25 °C.
Boiling Stones (Boiling Chips)
Small, porous stones added to a liquid before heating to promote smooth, even boiling and prevent dangerous superheating (bumping). In simple terms, they provide nucleation sites for bubble formation.
Theoretical Yield
The maximum amount of product possible from a given amount of limiting reagent, calculated from stoichiometry.
Percent Yield
Actual yield divided by theoretical yield, times 100. A measure of how much product you recovered versus the theoretical maximum.
A plain water wash is done first to remove the bulk of water-soluble impurities (unreacted HCl, water-soluble by-products) before the sodium bicarbonate wash. This prevents excessive fizzing and potential emulsion formation when the bicarbonate reacts with large amounts of acid. Doing it in the wrong order risks foaming over.
The 10% aqueous NaHCO₃ wash neutralises any remaining acid (HCl) in the organic layer. The reaction:
NaHCO₃ + HCl → NaCl + H₂O + CO₂↑
The CO₂ gas produced causes pressure build-up, so the separatory funnel must be vented frequently during shaking.
An aqueous NaCl (brine) wash reduces the amount of water dissolved in the organic layer
Theory: the high concentration of dissolved ions in brine decreases the solubility of organic compounds in the aqueous phase (salting-out effect) and simultaneously draws water out of the organic layer
Also helps break emulsions by increasing the density difference between layers
An Erlenmeyer flask has a narrow neck that reduces evaporation of volatile organic solvents. A beaker's wide opening allows solvent to evaporate, leading to product loss and inaccurate yields.
The top (organic) layer should be poured out through the top of the separatory funnel, not drained through the stopcock. Draining through the bottom would contaminate the organic layer with residual aqueous phase clinging to the walls and stopcock of the funnel.
Anhydrous inorganic salts coordinate with water molecules, incorporating them into a hydrated crystal lattice. For example, anhydrous calcium chloride reacts with water:
CaCl₂ + 6H₂O → CaCl₂·6H₂O
The product (the hydrate) is a solid that settles to the bottom and is removed by filtration or decanting.
According to Mohrig, anhydrous calcium chloride (CaCl₂) is commonly recommended for drying alkyl halides. Its properties:
Capacity: High (absorbs a large amount of water relative to its own mass)
Efficiency: Moderate (leaves some residual water, roughly 1-2 mg/L)
Speed: Fast
Magnesium sulphate (MgSO₄) is another common choice: high capacity, good efficiency, and fast. Sodium sulphate (Na₂SO₄) has high capacity but is slower.
Two observations that indicate the organic liquid is adequately dry:
The drying agent granules move freely in the liquid (they do not clump together)
The liquid appears clear, not cloudy. Cloudiness indicates dissolved water remains.
Distilling to dryness (boiling off all liquid) is dangerous. Residual high-boiling impurities or peroxides can concentrate in the flask and potentially decompose violently when overheated. The flask can also crack from thermal shock. Always leave a small amount of liquid in the distilling flask.
If you forgot to add boiling stones or a stir bar before heating began:
Do not add them to a hot liquid. Dropping boiling stones into superheated liquid can cause violent, sudden boiling (bumping)
Instead, remove the heat source, allow the liquid to cool to well below its boiling point, then add the boiling stones before resuming heating
Liquid-liquid extraction is one of the most widely used separation techniques in the chemical and pharmaceutical industries, from purifying drug candidates to isolating natural products from plant extracts. Simple distillation underpins everything from petroleum refining to whisky production. The same drying principles apply whenever water contamination would ruin a product or a subsequent reaction step.
"The organic layer is always on top." Usually true for common organic solvents less dense than water, but chlorinated solvents (dichloromethane, chloroform) are denser than water and sink to the bottom. Always check which layer is which by adding a drop of water and seeing which layer it joins.
"More drying agent is always better." Excess drying agent can adsorb your product along with the water, lowering your yield. Use enough to see free-flowing granules, then stop.
"Boiling stones can be added at any time." Adding boiling stones to a superheated liquid can trigger violent bumping. They must be added before heating begins, or after the liquid has cooled.
"A cloudy organic layer just needs more time to settle." Cloudiness usually indicates dissolved water, not an emulsion. The fix is a drying agent, not more waiting.
⚠️ Know the purpose of each wash step in order: water wash removes bulk impurities, bicarbonate wash neutralises acid, brine wash removes residual water. Expect to be asked why this order matters.
⚠️ Be ready to write the chemical equation for the sodium bicarbonate wash: NaHCO₃ + HCl → NaCl + H₂O + CO₂.
⚠️ Know the reaction of a drying agent with water (e.g. CaCl₂ + 6H₂O → CaCl₂·6H₂O) and be able to name at least one drying agent with its capacity, efficiency, and speed.
⚠️ The question "why never distill to dryness?" appears frequently. Safety (risk of decomposition, thermal shock) is the answer.
⚠️ Expect a practical troubleshooting question about forgetting boiling stones. The answer is always: cool down first, then add them.
True or False: A brine wash is used to neutralise acid in the organic layer. (False, that is the bicarbonate wash. Brine removes residual water.)
Fill in the blank: You should never add boiling stones to a ________ liquid. (hot / superheated)
True or False: Dichloromethane (an organic solvent) floats on top of water. (False, it is denser than water and sinks to the bottom.)
Fill in the blank: The purpose of a drying agent is to remove dissolved ________ from the organic layer. (water)
True or False: When using a separatory funnel, the top organic layer should be drained out through the stopcock. (False, pour it out through the top to avoid contamination.)
Q: What is the purpose of the aqueous sodium bicarbonate wash, and what is the chemical reaction involved?
A: It neutralises residual acid (HCl) in the organic layer. The reaction is NaHCO₃ + HCl → NaCl + H₂O + CO₂. The CO₂ gas must be vented from the separatory funnel during the wash.
Q: Why is a water wash performed before the sodium bicarbonate wash?
A: To remove the bulk of water-soluble impurities and excess HCl first. This reduces the amount of acid the bicarbonate has to neutralise, preventing excessive foaming and potential emulsion formation.
Q: What function does a brine wash serve, and what is the underlying theory?
A: Brine (saturated NaCl) draws residual water out of the organic layer through the salting-out effect. The high ionic strength decreases the solubility of organic compounds in the aqueous phase and reduces the water content of the organic layer.
Q: Write the reaction of anhydrous calcium chloride (a drying agent) with water.
A: CaCl₂ + 6H₂O → CaCl₂·6H₂O. The product is a hydrated solid.
Q: How can you tell when the organic layer is adequately dry?
A: The drying agent granules move freely (they are not clumped), and the liquid appears clear rather than cloudy.
Q: Why should you never distill to dryness?
A: Residual impurities or peroxides may concentrate and decompose violently when overheated. The flask can also crack from thermal shock. Always leave some liquid in the flask.
Q: You have begun heating your distillation flask and realise you forgot the boiling stones. What do you do?
A: Remove the heat source and allow the liquid to cool well below its boiling point before adding the boiling stones. Never add them to a hot liquid.
Q: Why is an Erlenmeyer flask preferred over a beaker for collecting organic layers?
A: The narrow neck of an Erlenmeyer flask minimises evaporation of volatile organic solvents, reducing product loss.
These techniques connect to virtually every experiment in the CHEM 2311 lab sequence. Extraction and washing principles reappear whenever you synthesise and purify an organic compound. Drying and distillation are used in the Grignard reaction, Fischer esterification, and aldol condensation labs, among others. The concept of salting out (brine wash) also connects to biochemistry, where ammonium sulphate precipitation is used to purify proteins by the same principle.
Related Terms / Search Tags
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