Lesson Notes By Weeks and Term v4 - SHS 3
CHARACTERISZATION OF ORGANIC COMPOUNDS
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CHARACTERISZATION OF ORGANIC COMPOUNDS
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Subject: Chemistry
Class: SHS 3
Term: 2nd Term
Week: 11
Grade code: 1.3.1.LI.2
Strand code: 3
Sub-strand code: 1
Content standard code: 1.3.1.CS.1
Indicator code: 1.3.1.LI.2
Theme: CHEMISTRY OF CARBON COMPOUNDS
Subtheme: CHARACTERISZATION OF ORGANIC COMPOUNDS
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Organic compounds are the building blocks of life and many materials we use daily. From the food we eat, like *kenkey* and groundnuts, to the medicines that keep us healthy, and the fuel that powers our cars and *trotros*. But how do chemists know what these compounds are made of? How can they be sure that a new drug extracted from a local plant like *Nibima* (*Cryptolepis sanguinolenta*) has the correct elements? This lesson introduces the fundamental techniques used to "interrogate" an organic compound to reveal the elements it contains (qualitative analysis) and the amount of each element present (quantitative analysis).
The characterization of an organic compound involves two main stages: Qualitative Analysis: Detecting which elements are present. Quantitative Analysis: Determining the mass or percentage of each element present. Part A: Qualitative Analysis (Detecting the Elements) Test for Carbon and Hydrogen Almost all organic compounds contain carbon and hydrogen. Their presence can be confirmed in a single experiment. Principle: The organic compound is heated strongly with an oxidizing agent, typically dry copper(II) oxide (CuO). The carbon in the compound is oxidized to carbon dioxide (CO₂), and the hydrogen is oxidized to water (H₂O). C (in compound) + 2CuO(s) → 2Cu(s) + CO₂(g) 2H (in compound) + CuO(s) → Cu(s) + H₂O(l) Procedure: Mix a small amount of the organic sample with about three times its weight of dry copper(II) oxide in a dry hard-glass test tube. Set up the apparatus as shown below, with a delivery tube leading from the test tube to another test tube containing limewater (calcium hydroxide solution, Ca(OH)₂). Place some anhydrous copper(II) sulphate powder (which is white) in the cooler part of the reaction tube or in a U-tube before the limewater. Heat the mixture strongly. Observations and Conclusion: Observation 1: The white anhydrous copper(II) sulphate turns blue. Conclusion: This confirms the presence of water (H₂O), which must have come from the hydrogen in the organic compound. CuSO₄(s) (white) + 5H₂O(l) → CuSO₄·5H₂O(s) (blue) Observation 2: The colourless limewater turns milky or cloudy. Conclusion: This confirms the presence of carbon dioxide (CO₂), which must have come from the carbon in the organic compound. Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) (white precipitate) + H₂O(l) Test for Nitrogen, Sulphur, and Halogens (Lassaigne's Test) Nitrogen, sulphur, and halogens are covalently bonded within the organic molecule. These bonds must be broken first to convert the elements into ionic forms, which can then be detected using simple inorganic tests. Principle (Sodium Fusion): A small piece of clean, dry sodium metal is heated strongly with the organic compound in a fusion tube. The intense heat breaks the covalent bonds, and the elements combine with sodium to form ionic salts. C, N (from compound) + Na → NaCN (Sodium cyanide) S (from compound) + 2Na → Na₂S (Sodium sulphide) X (from compound) + Na → NaX (Sodium halide, where X = Cl, Br, I)
This mixture is then plunged into distilled water, breaking the tube and dissolving the ionic salts to form the sodium fusion extract or Lassaigne's filtrate, which is used for the tests. SAFETY FIRST! Sodium metal is extremely reactive with water and must be handled with tongs. Wear safety goggles. The fusion process should be done in a fume cupboard. Procedure for the Tests (using the filtrate): a) Test for Nitrogen: To about 2 cm³ of the filtrate, add a few drops of freshly prepared iron(II) sulphate (FeSO₄) solution and warm gently. A greenish precipitate of Fe(OH)₂ may form. Acidify the solution with dilute sulphuric acid (H₂SO₄). Observation: The formation of a deep blue colour or precipitate (Prussian blue). Conclusion: Nitrogen is present. Chemistry: Fe²⁺(aq) + 6CN⁻(aq) → [Fe(CN)₆]⁴⁻(aq) (hexacyanoferrate(II) ion) The acid dissolves the Fe(OH)₂ and some Fe²⁺ is oxidized to Fe³⁺. 4Fe³⁺(aq) + 3[Fe(CN)₆]⁴⁻(aq) → Fe₄[Fe(CN)₆]₃(s) (Prussian blue) b) Test for Sulphur: Method 1: Lead(II) Ethanoate Test To about 2 cm³ of the filtrate, add a few drops of lead(II) ethanoate solution. Acidify with dilute ethanoic acid. Observation: A black precipitate is formed. Conclusion: Sulphur is present. Chemistry: S²⁻(aq) + Pb²⁺(aq) → PbS(s) (black precipitate) Method 2: Sodium Nitroprusside Test To about 2 cm³ of the filtrate, add a few drops of sodium nitroprusside solution. Observation: An intense purple or violet colouration appears. Conclusion: Sulphur is present. c) Test for Halogens (Cl, Br, I): To about 2 cm³ of the filtrate, add dilute nitric acid (HNO₃) and boil gently for a few minutes. (This is crucial to expel any cyanide or sulphide ions, which would interfere by also forming precipitates with silver ions). Cool the solution and add a few drops of silver nitrate (AgNO₃) solution. Observations and Conclusions: A white precipitate, soluble in aqueous ammonia (NH₃(aq)), indicates the presence of Chlorine. (AgCl) A pale cream precipitate, sparingly soluble in aqueous ammonia, indicates the presence of Bromine. (AgBr) A yellow precipitate, insoluble in aqueous ammonia, indicates the presence of Iodine. (AgI) Chemistry: Ag⁺(aq) + X⁻(aq) → AgX(s)
Part B: Quantitative Analysis (Determining Mass Composition)
Once we know which elements are present, we can determine how much of each is present. This is usually expressed as a percentage by mass.