Qualitative Analysis of Organic Compounds: Class 11 Chemistry Notes on Element and Functional Group Detection
1. Introduction
Qualitative analysis of organic compounds involves the detection of different elements (such as carbon, hydrogen, oxygen, nitrogen, sulfur, halogens, and phosphorus) present in a compound, along with identifying the functional groups (such as alcohols, aldehydes, amines, etc.).
2. Detection of Elements
The elements in organic compounds can be detected using specific tests:
2.1 Detection of Carbon and Hydrogen
Test: Heat the organic compound with copper(II) oxide (CuO). If carbon and hydrogen are present, they will oxidize to carbon dioxide (CO₂) and water (H₂O).
CO₂ Test: Pass the evolved gas through lime water (Ca(OH)₂). It turns milky due to the formation of calcium carbonate (CaCO₃).
H₂O Test: Anhydrous copper sulfate (CuSO₄), when exposed to water vapor, turns blue from its white anhydrous form.
2.2 Detection of Nitrogen
Lassaigne’s Test (Sodium Fusion Test): The organic compound is fused with sodium metal, converting nitrogen to sodium cyanide (NaCN). The cyanide ion is then detected.
Ferric chloride test: The fusion extract is boiled with ferrous sulfate (FeSO₄), followed by acidification with dilute sulfuric acid (H₂SO₄). A Prussian blue precipitate (ferric ferrocyanide) confirms the presence of nitrogen.
2.3 Detection of Sulfur
Lassaigne’s Test: When the organic compound containing sulfur is fused with sodium, sodium sulfide (Na₂S) is formed.
Lead acetate test: The sodium fusion extract is acidified with acetic acid and lead acetate solution is added. A black precipitate of lead sulfide (PbS) indicates sulfur's presence.
2.4 Detection of Halogens (Cl, Br, I)
Lassaigne’s Test: Organic compounds containing halogens, when fused with sodium, produce sodium halides (NaCl, NaBr, NaI).
- The sodium fusion extract is acidified with nitric acid (HNO₃) and treated with silver nitrate (AgNO₃).
- White precipitate: Chloride (Cl⁻) present.
- Pale yellow precipitate: Bromide (Br⁻) present.
- Yellow precipitate: Iodide (I⁻) present.
2.5 Detection of Phosphorus
- The organic compound is heated with an oxidizing agent like nitric acid (HNO₃). Phosphorus is oxidized to phosphate (PO₄³⁻).
- Ammonium molybdate test: The solution is treated with ammonium molybdate and concentrated nitric acid. The formation of a yellow precipitate indicates the presence of phosphorus.
3. Detection of Functional Groups
3.1 Alcohols (–OH group)
Lucas Test: A mixture of zinc chloride (ZnCl₂) and concentrated hydrochloric acid (HCl) is used. Tertiary alcohols give an immediate turbidity, secondary alcohols take time, while primary alcohols do not react at room temperature.
Sodium Metal Test: Alcohols react with sodium to release hydrogen gas.
3.2 Aldehydes (–CHO group)
Fehling's Test: Aldehydes reduce Fehling’s solution (a mixture of copper(II) sulfate and sodium potassium tartrate) to red copper(I) oxide (Cu₂O).
Tollen’s Test (Silver Mirror Test): Aldehydes reduce ammoniacal silver nitrate to metallic silver, which deposits on the inside of the test tube as a silver mirror.
3.3 Carboxylic Acids (–COOH group)
- Sodium Bicarbonate Test: Carboxylic acids react with sodium bicarbonate (NaHCO₃) to release carbon dioxide (CO₂) gas, which can be identified by the effervescence and lime water test.
3.4 Phenols (–OH attached to benzene ring)
Ferric Chloride Test: Phenols give a violet coloration when treated with neutral ferric chloride (FeCl₃).
3.5 Amines (–NH₂ group)
Carbylamine Test: When primary amines are heated with chloroform (CHCl₃) and alcoholic potassium hydroxide (KOH), they produce an unpleasant smell of isocyanide (carbylamine).
Hinsberg’s Test: Different amines react distinctly with benzene sulfonyl chloride. Primary amines form soluble sulfonamides, secondary amines form insoluble sulfonamides, while tertiary amines do not react.
Read Also: Fundamental Concepts in Organic Reaction Mechanism
4. Solubility Tests
The solubility of organic compounds can also give clues about the nature of functional groups present:
Soluble in Water: Compounds containing –OH, –NH₂, –COOH, etc.
Soluble in Ether: Non-polar compounds such as hydrocarbons.
5. Summary of Important Tests
This overview provides the key concepts for qualitative analysis of organic compounds in Class 11 Chemistry, including elemental detection and functional group identification.