In Organic Chemistry, we often prepare organic compounds through different reactions. But the product obtained after a reaction is rarely pure. It usually contains impurities like unreacted reactants, side products, catalysts, moisture, or inorganic salts. That’s why purification of organic compounds is an important topic in Class 12 Chemistry (and very useful for boards + competitive exams).
Purification means removing impurities from an organic compound to get it in a pure form. Pure compounds show fixed melting point, boiling point, and characteristic properties.
In this blog, you will learn Purification of Organic Compounds Class 12 Notes in a very detailed and step-by-step manner, including:
✅ Types of impurities
✅ Principles of purification
✅ Methods: crystallization, sublimation, distillation, chromatography, etc.
✅ How to choose the correct method
✅ Common repeated questions for exams
Purification of Organic Compounds Class 12 Notes PDF | Methods, Distillation, Crystallization
1. Why Purification of Organic Compounds is Needed?
Organic compounds after preparation may contain impurities due to:
(A) Reasons for Impurities
- Unreacted reactants remain mixed with product.
- Side reactions produce unwanted compounds.
- Catalysts (like acids, bases, metal catalysts) stay in the product.
- Solvents remain trapped.
- Moisture/water may be present.
- Inorganic impurities such as salts (NaCl, CaCl₂, etc.).
(B) Importance of Purification
Purification is important because:
- Pure compounds have sharp melting point / boiling point
- Pure samples give correct chemical tests
- Used in medicines, dyes, perfumes, polymers etc.
- Impure compounds give wrong results in analysis
2. Types of Impurities in Organic Compounds
Impurities can be:
(A) Soluble Impurities
These dissolve in the solvent along with the compound.
(B) Insoluble Impurities
These do not dissolve in the solvent and can be removed by filtration.
(C) Colored Impurities
Sometimes the compound becomes colored due to impurities. These are removed by activated charcoal.
(D) Inorganic Impurities
Such as salts, acids, bases, or metal catalysts.
3. General Principles of Purification
The method of purification depends on:
- Nature of compound (solid or liquid)
- Nature of impurities
- Difference in physical properties
- melting point
- boiling point
- solubility
- volatility
- adsorption behavior
4. Purification of Solids (Main Methods)
4.1 Crystallization (Most Important Method)
Crystallization is the process of obtaining a pure solid compound in the form of crystals from its solution.
Principle
A pure compound has different solubility in a solvent at different temperatures:
- It dissolves more in hot solvent
- It dissolves less in cold solvent
So, when the hot solution is cooled, pure compound separates as crystals while impurities remain in solution.
Step-by-Step Process of Crystallization
Step 1: Selection of Solvent
A good solvent should have these properties: ✅ The compound should be highly soluble in hot solvent
✅ The compound should be sparingly soluble in cold solvent
✅ Impurities should be either:
- completely soluble (remain in solution), or
- completely insoluble (removed by filtration)
✅ Solvent should not react chemically with compound
✅ Should be cheap and easily removable
Examples of solvents:
Water, ethanol, methanol, acetone, benzene, ether.
Step 2: Preparation of Hot Saturated Solution
- Take impure solid in a beaker
- Add minimum amount of solvent
- Heat until the compound dissolves completely
Step 3: Removal of Insoluble Impurities (Hot Filtration)
If insoluble impurities are present:
- Filter the hot solution using filter paper
- Insoluble impurities remain on filter paper
Step 4: Removal of Colored Impurities
If solution is colored:
- Add a small amount of activated charcoal
- Boil the solution
- Filter again
Activated charcoal absorbs colored impurities due to adsorption.
Step 5: Cooling the Solution
- Cool the hot solution slowly
- Pure crystals start forming
- Keep undisturbed for better crystal formation
Step 6: Filtration and Washing
- Filter crystals using Büchner funnel (vacuum filtration)
- Wash crystals with small amount of cold solvent
Step 7: Drying
Dry the crystals using:
- filter paper
- desiccator
- drying oven
Advantages of Crystallization
✅ Very effective for solids
✅ Produces highly pure crystals
✅ Easy and economical
4.2 Fractional Crystallization
This method is used to separate two solids present in a mixture based on their different solubilities.
Principle
If two solids have different solubility in a solvent, then:
- Less soluble solid crystallizes first
- More soluble solid remains in solution
Example
Separation of mixture of:
- KNO₃ and NaCl
- Benzoic acid and naphthalene
4.3 Sublimation
Sublimation is the process in which a solid changes directly into vapor without becoming liquid, and vapors condense back into solid.
Principle
Some solids are volatile and sublime on heating, while impurities do not sublime.
Used for
Compounds like:
- naphthalene
- camphor
- iodine
- ammonium chloride
- benzoic acid
Steps
- Heat the impure solid in a dish
- Vapors rise and deposit on a cold surface
- Collect pure solid crystals
5. Purification of Liquids (Main Methods)
5.1 Simple Distillation
Used for purification of a liquid from:
- non-volatile impurities
OR - another liquid having large difference in boiling points
Principle
A liquid boils at its boiling point, converts to vapor, then condenses back into pure liquid.
Used when
Boiling point difference is more than 25–30°C.
Example
Purification of:
- acetone from water
- benzene from non-volatile impurities
5.2 Fractional Distillation
Used when two or more liquids have close boiling points.
Principle
Fractionating column provides repeated condensation and vaporization, improving separation.
Used when
Boiling point difference is less than 25°C.
Example
Separation of:
- ethanol and water
- benzene and toluene
- crude oil fractions (industrial use)
5.3 Distillation Under Reduced Pressure (Vacuum Distillation)
Used for liquids that:
- decompose at their boiling point
- have very high boiling point
Principle
Reducing pressure lowers boiling point, so liquid distills without decomposition.
Example
Glycerol, aniline, and heavy oils.
5.4 Steam Distillation
Used for compounds that are: ✅ volatile in steam
✅ insoluble in water
✅ have high boiling point
Principle
When steam is passed through the mixture, organic compound vaporizes with steam at temperature below its boiling point.
Example
Purification of:
- aniline
- phenol
- essential oils (clove oil, eucalyptus oil)
6. Chromatography (Most Scoring Topic)
Chromatography is a technique used to separate and purify organic compounds based on their different adsorption or partition behavior.
Meaning
“Chroma” = color
“Graphy” = writing
Initially used to separate colored pigments.
6.1 Principle of Chromatography
Separation occurs because different components move at different speeds due to:
- different adsorption on stationary phase
- different solubility in mobile phase
Two Phases
- Stationary phase: solid or liquid fixed in place
- Mobile phase: liquid or gas that moves
6.2 Types of Chromatography
(A) Paper Chromatography
- Stationary phase: water trapped in paper
- Mobile phase: solvent mixture
Used for:
Ink pigments, amino acids, dyes.
(B) Thin Layer Chromatography (TLC)
- Stationary phase: silica gel/alumina coated plate
- Mobile phase: solvent
Advantages:
✅ fast
✅ requires small sample
✅ widely used in labs
(C) Column Chromatography
- Stationary phase: silica/alumina packed column
- Mobile phase: solvent
Used for separating mixtures like plant extracts.
Rf Value (Retention Factor)
Rf is a very important formula-based concept.
Rf = \frac{\text{Distance traveled by solute}}{\text{Distance traveled by solvent front}}
- Rf value is always less than 1
- Helps in identifying compounds
7. Differential Extraction (Solvent Extraction)
This method separates compounds based on their different solubility in two immiscible solvents.
Principle
A compound distributes itself between two immiscible solvents in a fixed ratio.
Example
Separation using:
- water and ether
- water and chloroform
Used in organic lab to separate:
- organic acids
- bases
- neutral compounds
8. Drying of Organic Compounds
Sometimes the compound contains water/moisture.
Drying Agents
Used to remove water:
- anhydrous CaCl₂
- anhydrous MgSO₄
- anhydrous Na₂SO₄
- P₂O₅ (strong drying agent)
Note: Drying agent must not react with compound.
9. How to Check Purity of Organic Compounds
Purity can be checked by:
(A) Melting Point (for solids)
- Pure solid has sharp melting point
- Impure solid melts over a range and lower melting point
(B) Boiling Point (for liquids)
- Pure liquid has fixed boiling point
- Impurities change boiling point range
(C) Chromatography
- Pure compound gives single spot
- Impure gives multiple spots
10. Quick Summary Table (Very Helpful for Exams)
| Method | Used for | Principle |
|---|---|---|
| Crystallization | Solids | Solubility difference |
| Fractional Crystallization | Two solids | Different solubilities |
| Sublimation | Volatile solids | Sublimable nature |
| Simple Distillation | Liquids | Boiling point separation |
| Fractional Distillation | Liquids close b.p. | Repeated vaporization |
| Steam Distillation | Steam volatile liquids | Volatile in steam |
| Vacuum Distillation | High b.p. liquids | Reduced pressure |
| Chromatography | Mixtures | Adsorption/partition |
Repeated / Important Questions (Purification of Organic Compounds Class 12 Notes Board + Competitive)
Very Frequently Asked (1 Mark / 2 Marks)
- What is purification of organic compounds?
- Why is purification necessary?
- Define crystallization.
- What is sublimation? Give two examples.
- What is chromatography?
- Write formula of Rf value.
- What is the difference between simple and fractional distillation?
- What is steam distillation used for?
- Name two drying agents.
- How do you check purity of a solid compound?
Repeated 3 Marks Questions
- Explain crystallization method step by step.
- Write characteristics of a good solvent for crystallization.
- Explain paper chromatography and its principle.
- Explain fractional distillation with diagram (if asked).
- Explain steam distillation and give examples.
Repeated 5 Marks Questions
- Explain different methods of purification of organic compounds in detail.
- Describe crystallization and fractional crystallization with examples.
- Explain chromatography, its principle, types, and Rf value.
- Differentiate between:
- simple distillation and fractional distillation
- sublimation and crystallization
- TLC and paper chromatography
Conclusion-Purification of Organic Compounds Class 12 Notes PDF
Purification of organic compounds is a very important and scoring topic for Class 12. It helps in obtaining pure organic substances by using methods like crystallization, sublimation, distillation, steam distillation, vacuum distillation, chromatography, and solvent extraction. Understanding the principles and step-by-step procedures will help you solve board questions easily and also strengthen your basic chemistry concepts.