Organic Chemistry – Some Basic Principles and Techniques

Methods of Purification

The methods of purification of organic compounds are:

1. Crystallization: Used for purifying solid compounds based on their solubility differences in a solvent. Impurities remain dissolved in the solvent.

2. Sublimation: Separates volatile solids from non-volatile impurities by direct conversion from solid to gas upon heating.

3. Distillation: Purifies liquids by heating them to their boiling points and condensing the vapors. Fractional distillation is used for mixtures with closer boiling points.

4. Extraction: Utilizes differences in solubility in two immiscible solvents to separate components.

5. Chromatography: Separates compounds based on their affinity for a stationary phase and a mobile phase. Common types include paper, thin-layer, and gas chromatography.

Feature	Column Chromatography	TLC	Paper Chromatography
Principle	Differential adsorption	Differential adsorption	Partition chromatography
Stationary Phase	Silica/Alumina	Silica/Alumina layer	Water in Paper Fibers
Mobile Phase	Solvent (liquid)	Solvent (liquid)	Solvent (liquid)
Sample Size	Large	Small	Very small
Speed	Slow	Fast	Moderate
Application	Purification	Monitoring Reactions	Analysing Polar Compounds

    

Qualitative Analysis of Organic Compounds

The qualitative analysis of organic compounds involves identifying elements present in a compound. It focuses on detecting carbon, hydrogen, oxygen, nitrogen, sulfur, halogens, and phosphorus using specific tests.

1. Detection of Carbon and Hydrogen

• Test: Heating the compound with cupric oxide.
• Observation: Formation of carbon dioxide (turns lime water milky) and water (condensation on cooler surfaces).

2. Detection of Nitrogen

• Lassaigne’s Test: Sodium is fused with the compound. The extract is tested with FeSO₄ and NaOH.
• Observation: Formation of Prussian blue color (ferric ferrocyanide) confirms nitrogen.

3. Detection of Sulfur

• Lassaigne’s Test: Sodium fusion extract is tested with lead acetate or sodium nitroprusside.
• Observation: Black precipitate (PbS) or purple color confirms sulfur.

4. Detection of Halogens (Cl, Br, I)

• Lassaigne’s Test: Sodium fusion extract is tested with AgNO₃ after acidifying with HNO₃.
• Observation: White precipitate (Cl⁻), pale yellow (Br⁻), or yellow (I⁻).

5. Detection of Oxygen

• Test: No direct test, but its presence is inferred if the compound does not respond to halogen, nitrogen, or sulfur tests.

6. Detection of Phosphorus

• Test: The compound is fused with Na₂O₂ and the extract is tested with ammonium molybdate.
• Observation: Yellow precipitate confirms phosphorus.

These tests help confirm the elemental composition of the organic compound.

Quantitative Analysis of Organic Compounds

The quantitative analysis of organic compounds involves determining the percentage composition of various elements (C, H, O, N, S, halogens, etc.) in a compound. Key methods include:

1. Estimation of Carbon and Hydrogen

• Method: Combustion in excess oxygen.
• Principle: Carbon is converted to CO₂, and hydrogen to H₂O, which are absorbed in KOH and anhydrous CaCl₂, respectively.
• Calculation:
  • %C = $\frac{\text{Mass of CO₂} \times 12}$
  • %H = $\frac{\text{Mass of H₂O} \times 2}$

2. Estimation of Nitrogen (Kjeldahl’s Method)

• Method: Compound is digested with concentrated H₂SO₄ to form (NH₄)₂SO₄, which is distilled with NaOH to release NH₃. NH₃ is absorbed in H₂SO₄ and titrated.
• Calculation:
  • %N = $\frac{\text{Equivalent of NH₃ in H₂SO₄} \times 14}$

3. Estimation of Sulfur

• Method: Combustion in oxygen to form SO₂, absorbed in H₂O₂ to form H₂SO₄, titrated with standard NaOH. Alternatively, gravimetric analysis is performed using BaCl₂ (BaSO₄ precipitate).
• Calculation:
  • %S = $\frac{\text{Mass of BaSO₄} \times 32}$

4. Estimation of Halogens (Carius Method)

• Method: The compound is heated with fuming HNO₃ in a sealed tube to convert halogens into AgX (precipitate with AgNO₃).
• Calculation:
  • %Halogen = $\frac{\text{Mass of AgX} \times \text{Atomic weight of halogen}}$

5. Estimation of Oxygen

• Method: By difference:
  • %O = 100 - (%C + %H + %N + %S + %Halogens).

Quantitative analysis provides precise elemental composition, aiding in molecular formula determination.