Purification and Characterisation of Organic Compounds

Master purification techniques, qualitative and quantitative analysis for JEE Chemistry.

This chapter covers methods for purifying organic compounds and analyzing their composition.

Overview

graph TD
    A[Purification] --> B[Physical Methods]
    A --> C[Analysis]
    B --> B1[Crystallization]
    B --> B2[Distillation]
    B --> B3[Chromatography]
    C --> C1[Qualitative]
    C --> C2[Quantitative]

Purification Methods

Crystallization

For solid compounds with different solubilities at different temperatures.

Steps:

  1. Dissolve in hot solvent
  2. Filter hot solution
  3. Cool slowly
  4. Collect crystals

Distillation

Simple Distillation: For liquids with BP difference > 30°C

Fractional Distillation: For liquids with BP difference < 30°C (uses fractionating column)

Steam Distillation: For water-immiscible compounds

Vacuum Distillation: For high-BP compounds that decompose

Differential Extraction

Separates compounds based on solubility in immiscible solvents.

Chromatography

Separates based on differential adsorption or partition.

Types:

  • Column chromatography: Uses adsorbent column
  • TLC: Thin layer on glass plate
  • Paper chromatography: Paper as stationary phase
  • HPLC: High pressure, fine particles

Qualitative Analysis

Detection of Elements

Nitrogen: Lassaigne’s test (sodium fusion → NaCN)

  • Test with FeSO₄ + FeCl₃ → Prussian blue

Sulfur: Lead acetate test (black PbS)

Halogens: Silver nitrate test after fusion

  • AgCl: white, soluble in NH₃
  • AgBr: pale yellow, partially soluble
  • AgI: yellow, insoluble

Phosphorus: Ammonium molybdate test (yellow precipitate)

Quantitative Analysis

Carbon and Hydrogen (Liebig’s Method)

Compound + CuO → CO₂ + H₂O

  • CO₂ absorbed by KOH (weigh increase)
  • H₂O absorbed by anhydrous CaCl₂
$$\% C = \frac{12 \times \text{mass of CO}_2}{44 \times \text{mass of compound}} \times 100$$ $$\% H = \frac{2 \times \text{mass of H}_2\text{O}}{18 \times \text{mass of compound}} \times 100$$

Nitrogen (Dumas/Kjeldahl)

Dumas: Measure N₂ volume

Kjeldahl: Convert to NH₃, titrate

Halogens (Carius Method)

Heat with HNO₃ + AgNO₃, weigh AgX

Sulfur

Heat with HNO₃, precipitate as BaSO₄

Molecular Formula Determination

  1. Find percentage composition
  2. Calculate empirical formula
  3. Find molecular mass (vapor density method, etc.)
  4. Determine molecular formula
JEE Tip
Molecular formula = n × Empirical formula, where n = Molar mass / Empirical formula mass

Practice Problems

  1. A compound contains 40% C, 6.7% H, and 53.3% O. Find its empirical formula.

  2. Explain how you would separate a mixture of benzoic acid and naphthalene.

Quick Check
Why is Lassaigne’s test preferred over direct tests for element detection?

Further Reading