Revision Notes: Purification and Characterisation of Organic Compounds | Chemistry Class 12 - NEET PDF Download

A. Purification Techniques

Crystallization

Principle: Crystallization is based on the principle that solubility of a compound in a solvent decreases as the solution cools, causing the solute to form crystals. Impurities generally remain dissolved in the solvent.

Procedure:

1. Selection of solvent: Choose a solvent in which the compound has high solubility at high temperature and low solubility at low temperature.
2. Dissolution: Dissolve the impure solid compound in the hot solvent.
3. Filtration: Filter out any insoluble impurities from the solution.
4. Cooling: Allow the solution to cool slowly, during which pure crystals of the compound will form.
5. Separation: Filter the crystals and wash them with a small amount of cold solvent to remove soluble impurities.
6. Drying: Dry the crystals in a desiccator or under reduced pressure.

Applications: Used for purifying solid compounds such as salts, organic compounds (like sugar), and antibiotics.

Sublimation

Principle: Sublimation occurs when a solid changes directly into a vapor and then back to a solid without passing through the liquid phase. This happens when the vapor pressure of a substance is sufficiently high at a given temperature.Sublimation Diagram

Procedure:

1. Heat the impure substance in a sublimation apparatus.
2. The substance sublimates into a vapor.
3. The vapor condenses on a cooler surface to form crystals of the purified substance.

Applications: Used to purify volatile solids like iodine, camphor, and naphthalene.

Distillation

Principle: Distillation is based on the differences in the boiling points of components in a mixture. When a liquid mixture is heated, the component with the lowest boiling point vaporizes first, followed by condensation into a liquid.Distillation Process

Types of Distillation:

• Simple Distillation: Used when the boiling points of components are significantly different.
• Fractional Distillation: Used for mixtures where components have closer boiling points.
• Vacuum Distillation: Used when components decompose at high temperatures; reduces the boiling point by applying a vacuum.

Procedure:

1. Heat the liquid mixture in a distillation apparatus.
2. The component with the lowest boiling point vaporizes.
3. The vapor is condensed and collected in a separate container.

Applications: Separation of volatile liquids (e.g., separating water and ethanol), purification of solvents, essential oils, and crude oil.

Differential Extraction

Principle: Differential extraction separates components of a mixture based on their differential solubility in immiscible solvents.

Procedure:

1. Add an immiscible solvent to the mixture.
2. Shake the mixture and allow the layers to separate.
3. Separate the desired component from one of the layers.

Applications: Used for separating organic compounds from aqueous solutions, such as in the extraction of alkaloids, acids, and phenols.

Chromatography

Principle: Chromatography separates the components of a mixture based on the differential distribution between a stationary phase and a mobile phase.

Types of Chromatography:

• Paper Chromatography: Uses paper as the stationary phase.
• Thin-Layer Chromatography (TLC): Uses a thin layer of adsorbent material as the stationary phase.
• Column Chromatography: Uses a column packed with adsorbent material.
• Gas Chromatography (GC): Uses a gas as the mobile phase for separating volatile compounds.
• High-Performance Liquid Chromatography (HPLC): Uses a liquid mobile phase to separate compounds based on their polarity and interactions with the stationary phase.

Applications: Separation of complex mixtures (e.g., amino acids, proteins, pesticides), and analysis of organic compounds in environmental and pharmaceutical samples.

B. Qualitative Analysis

Detection of Nitrogen

Test: Lassaigne's test for Nitrogen

Procedure:

1. Heat the organic compound with sodium metal in a fusion tube.
2. Add water to the fusion product and filter the solution.
3. Add ferric chloride (FeCl₃) to the filtrate.
4. If a prussian blue color appears, nitrogen is present.

Reaction:

NaCN + FeCl₃ → Fe(CN)₆³⁻ (Prussian blue formation)

Detection of Sulphur

Test: Lassaigne's test for Sulphur

Procedure:

1. Heat the organic compound with sodium metal in a fusion tube.
2. Add water to the fusion product and filter the solution.
3. Add lead acetate solution (Pb(CH₃COO)₂) to the filtrate.
4. If a black precipitate of PbS forms, sulphur is present.

Reaction:

Na₂S + Pb(CH₃COO)₂ → PbS (black precipitate) + 2NaCH₃COO

Detection of Phosphorus

Test: Lassaigne's test for Phosphorus

Procedure:

1. Heat the organic compound with sodium metal in a fusion tube.
2. Add water to the fusion product and filter the solution.
3. Add ammonium molybdate (NH₄)₆Mo₇O₂₄ to the filtrate.
4. If a yellow precipitate forms, phosphorus is present.

Reaction:

Na₂HPO₄ + (NH₄)₆Mo₇O₂₄ → MoO₃.P₂O₅ (yellow precipitate)

Detection of Halogens

Test: Lassaigne's test for Halogens

Procedure:

1. Heat the organic compound with sodium metal in a fusion tube.
2. Add water to the fusion product and filter the solution.
3. Add silver nitrate solution (AgNO₃) to the filtrate.
4. A white precipitate (AgCl) indicates chlorine, yellow precipitate (AgBr) indicates bromine, and yellowish-white precipitate (AgI) indicates iodine.

Reactions:

NaCl + AgNO₃ → AgCl (white precipitate)NaBr + AgNO₃ → AgBr (yellow precipitate)NaI + AgNO₃ → AgI (yellowish-white precipitate)