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Purification
Basics

Master the three essential techniques: extraction, recrystallization, and distillation to isolate pure compounds from mixtures.

Organic Chemistry 1

Why Do We Purify?

Organic reactions rarely produce a single product. The crude mixture contains unreacted starting materials, by-products, solvents, and catalysts. Purification isolates the desired compound so we can characterize it and confirm its identity.

The Big Three Techniques

Extraction
Separates compounds between two immiscible solvents based on solubility and acid-base properties.
Recrystallization
Dissolves a solid in hot solvent, then cools slowly so only the pure compound crystallizes out.
Distillation
Separates liquids by exploiting differences in boiling point: heat, vaporize, condense, collect.

The Governing Principle: "Like Dissolves Like"

Polar solutes dissolve in polar solvents (water, methanol). Nonpolar solutes dissolve in nonpolar solvents (hexane, dichloromethane). This single principle underlies all three purification methods.

Polar Phase Water, MeOH, EtOH NaCl Sugar RCOOH Nonpolar Phase Hexane, DCM, Ether Oil Wax Grease ↔ Immiscible layers in a separatory funnel

Liquid-Liquid Extraction

Organic Layer (less dense) Aqueous Layer (more dense) Interface Nonpolar solutes Polar solutes

Acid-Base Extraction

We can force an organic acid into the aqueous layer by converting it to its ionic conjugate base. Adding a base like NaOH deprotonates the acid, making it water-soluble:

Wash with NaOH
Removes acidic compounds (carboxylic acids, phenols) into the aqueous layer.
Wash with HCl
Removes basic compounds (amines) by protonating them to water-soluble salts.

How Recrystallization Works

Temperature Solubility Desired Impurity Hot Cold Crystals form as solution cools

Recrystallization Step by Step

1
Dissolve the impure solid in a minimum volume of hot solvent.
2
Hot filter (gravity filtration) to remove insoluble impurities while solution is still hot.
3
Cool slowly: allow the solution to reach room temperature, then ice-bath. Slow cooling produces larger, purer crystals.
4
Vacuum filter the crystals. Wash with a small amount of ice-cold solvent.
5
Dry the crystals and check purity via melting point analysis.

Simple Distillation

Mixture Heat Thermometer Condenser
\( \text{H}_2\text{O} \) in
\( \text{H}_2\text{O} \) out
 Distillate Vapor rises

Simple vs. Fractional Distillation

Simple Fractional
BP Difference \( > 25^\circ\text{C} \) \( < 25^\circ\text{C} \)
Key Equipment Condenser only Fractionating column + condenser
Separation Power Low (one vaporization) High (many vaporizations)
Example Removing solvent from product Separating ethanol from water
Speed Faster Slower

Choosing the Right Technique

Extraction Recrystallization Distillation
Mixture Type Liquid-liquid Solid with impurities Liquid-liquid
Property Used Solubility / acid-base Solubility vs. temperature Boiling point
Product Recovered Solute in one layer Pure crystals Distillate or residue
When to Use Separate by polarity Solid product purification Separate volatile liquids

The Partition Coefficient

The partition (distribution) coefficient tells us how a solute distributes between two immiscible solvents at equilibrium:

Example: Single Extraction

A compound has \( K_D = 3 \) between ether and water. You dissolve \( 12 \text{ g} \) in \( 100 \text{ mL} \) water, then extract with \( 100 \text{ mL} \) ether. How much is extracted?

1
Let \( x \) = grams in ether, \( 12 - x \) = grams remaining in water.
2
Set up: \( K_D = \dfrac{x / 100}{(12 - x) / 100} = \dfrac{x}{12 - x} = 3 \)
3
Solve: \( x = 3(12 - x) = 36 - 3x \), so \( 4x = 36 \), \( x = 9 \text{ g} \)

Example: Two Extractions

Same setup (\( K_D = 3 \), \( 12 \text{ g} \) in \( 100 \text{ mL} \) water), but now use two \( 50 \text{ mL} \) portions of ether instead of one \( 100 \text{ mL} \) portion.

1
First wash (\( 50 \text{ mL} \) ether): \( \dfrac{x/50}{(12-x)/100} = 3 \implies \dfrac{2x}{12-x} = 3 \implies x = 7.2 \text{ g} \)
2
\( 4.8 \text{ g} \) remains. Second wash: \( \dfrac{y/50}{(4.8-y)/100} = 3 \implies y = 2.88 \text{ g} \)
3
Total extracted: \( 7.2 + 2.88 = 10.08 \text{ g} \) (\( 84\% \))

Example: Choosing a Recrystallization Solvent

Your product is a polar solid. You test three solvents:

Solvent Cold Solubility Hot Solubility Verdict
Water Low High ✓ Ideal
Ethanol High High ✕ Too soluble
Hexane Low Low ✕ Won't dissolve

Common Mistakes

✕ Too Much Solvent
Using excess solvent for recrystallization keeps product dissolved when cold, reducing yield dramatically.
✕ Cooling Too Fast
Rapid cooling produces tiny crystals that trap impurities. Slow cooling gives large, pure crystals.
✕ Shaking Too Hard
Vigorous shaking of the separatory funnel creates emulsions: stubborn mixtures that won't separate into layers.

Question 1 of 5

Which purification technique is best for separating two liquids with boiling points of \( 78^\circ\text{C} \) and \( 110^\circ\text{C} \)?

Question 2 of 5

In acid-base extraction, adding NaOH to an organic layer containing benzoic acid would:

Question 3 of 5

A compound has \( K_D = 5 \) between ether and water. If \( 10 \text{ g} \) is dissolved in \( 100 \text{ mL} \) water and extracted once with \( 100 \text{ mL} \) ether, how much is extracted?

Question 4 of 5

What property makes a solvent ideal for recrystallization of a given compound?

Question 5 of 5

Why are multiple small extractions more efficient than a single large extraction with the same total volume of solvent?