Class 11 Organic Chemistry — Basic Principles and Techniques
Before organic chemistry gets into naming compounds and drawing mechanisms, NCERT's "Organic Chemistry — Some Basic Principles and Techniques" chapter covers something more practical: how an organic chemist actually purifies an unknown compound and finds out what elements it contains and in what proportion. This is a separate chapter from nomenclature and isomerism, and it is the one place in Class 11 where organic chemistry becomes genuinely numerical. Every method here comes with a real worked calculation.
Purification methods — which technique fits which situation
| Method | When it is used | Example |
|---|---|---|
| Crystallisation | Solids whose solubility differs sharply with temperature | Purifying impure alum or benzoic acid |
| Sublimation | Solids that pass directly from solid to vapour, leaving a non-sublimable impurity behind | Naphthalene, camphor, anthracene |
| Simple distillation | Liquids that do not decompose on heating and boil well below 300°C, with no other liquid close in boiling point | Separating water from a salt solution's solvent |
| Fractional distillation | Liquids whose boiling points are close together | Separating different fractions of crude petroleum |
| Steam distillation | Substances insoluble in water that have some appreciable vapour pressure at 373 K | Isolating essential oils, aniline |
| Distillation under reduced pressure | Liquids that decompose at or near their normal boiling point | Purifying glycerol |
| Differential extraction | Extracting an organic compound from an aqueous mixture using an immiscible solvent | Using a separating funnel with ether or another organic solvent |
| Chromatography | Separating very small amounts of closely related compounds | Column (adsorption) and paper (partition) chromatography |
Qualitative analysis — detecting which elements are present
Nitrogen, sulphur, halogens and phosphorus cannot be detected directly on an organic compound; the compound is first fused with sodium metal to convert these elements into water-soluble ionic compounds — this is Lassaigne's test, and the resulting solution is called the sodium fusion extract (Lassaigne's extract).
| Element | Test on the extract | Positive result |
|---|---|---|
| Nitrogen | FeSO₄, then acidified with dilute H₂SO₄ | Prussian blue colouration |
| Sulphur | Sodium nitroprusside | Violet/purple colouration |
| Sulphur (alternative) | Lead acetate | Black precipitate of PbS |
| Halogens | Dilute HNO₃, then AgNO₃ | White (Cl), pale yellow (Br), yellow (I) precipitate |
| Phosphorus | Boiled with conc. HNO₃, then ammonium molybdate | Yellow precipitate |
Quantitative analysis — Liebig's method for carbon and hydrogen
A known mass of an organic compound is burnt completely in a stream of oxygen; the CO₂ and H₂O produced are absorbed separately (in KOH and anhydrous CaCl₂ respectively) and weighed.
%H = (2 ÷ 18) × (mass of H₂O ÷ mass of compound) × 100
Worked example 1 — percentage of carbon and hydrogen
0.24 g of an organic compound gave 0.352 g of CO₂ and 0.144 g of H₂O on complete combustion. Find the percentage of carbon and hydrogen.
%C = (12 ÷ 44) × (0.352 ÷ 0.24) × 100 = 0.2727 × 1.4667 × 100 = 40.0%
%H = (2 ÷ 18) × (0.144 ÷ 0.24) × 100 = 0.1111 × 0.6 × 100 = 6.67%
If the compound is known to contain only carbon, hydrogen and oxygen, the remaining percentage is found by difference: 100 − 40.0 − 6.67 = 53.33% oxygen.
Duma's method and Kjeldahl's method for nitrogen
In Duma's method, the compound is heated with copper oxide in a stream of CO₂; the nitrogen released is collected over concentrated KOH solution (which absorbs the CO₂, leaving pure N₂) and its volume is measured at STP.
Kjeldahl's method instead converts the nitrogen to ammonium sulphate by heating with concentrated H₂SO₄; the liberated ammonia is absorbed in a known volume of a standard acid, and the unreacted acid is back-titrated.
Kjeldahl's method fails for compounds where nitrogen is present as a ring nitrogen (e.g. pyridine) or in a nitro or azo group, because that nitrogen does not convert to ammonium sulphate under the reaction conditions — Duma's method must be used for those instead.
Worked example 2 — percentage of nitrogen, both methods
Duma's method: 0.15 g of an organic compound gave 20 mL of N₂ gas collected at STP. Find %N.
%N = (28 × 20) ÷ (22,400 × 0.15) × 100 = 560 ÷ 3360 × 100 = 16.67%
Kjeldahl's method: 0.5 g of an organic compound required 20 mL of 0.1 N H₂SO₄ to neutralise the ammonia liberated. Find %N.
%N = (1.4 × 0.1 × 20) ÷ 0.5 = 2.8 ÷ 0.5 = 5.6%
Carius method for halogens, sulphur and phosphorus
The compound is heated with fuming nitric acid in the presence of silver nitrate (for halogens) inside a sealed Carius tube. The precipitate formed (AgX, BaSO₄ or Mg₂P₂O₇) is filtered, dried and weighed.
Worked example 3 — percentage of chlorine by Carius method
0.3 g of an organic compound gave 0.5 g of AgCl by the Carius method. Find the percentage of chlorine. (Atomic mass Cl = 35.5, molar mass AgCl = 143.5)
%Cl = (35.5 × 0.5) ÷ (143.5 × 0.3) × 100 = 17.75 ÷ 43.05 × 100 = 41.23%
Common mistakes that cost marks
- Testing an organic compound directly with AgNO₃ or FeSO₄ instead of first preparing the sodium fusion extract — the elements in the original compound are covalently bound and give no test until converted to ionic form.
- Mixing up the constants in the %N formulas. 28 = molar mass of N₂ (Duma's), 22,400 = molar volume at STP in mL, and 1.4 = 14/10 (Kjeldahl's) — writing the wrong constant is a common slip.
- Applying Kjeldahl's method to ring, nitro or azo nitrogen compounds, where it systematically underestimates %N.
- Forgetting the molar mass of the precipitate (AgCl = 143.5, BaSO₄ = 233, Mg₂P₂O₇ = 222) — these must be calculated correctly before the percentage formula can be applied.
Where this chapter appears in exams
| Exam | Typical use |
|---|---|
| CBSE Class 11 | Percentage composition numericals, qualitative test identification |
| JEE/NEET | Empirical formula problems built directly on Liebig's/Duma's/Carius results |
| IIT-JAM / CUET-PG | Applied as a routine step before structure-determination questions |
Check your elemental-analysis calculations. Use the free calculator suite's Molar Mass tool to verify the molar masses of AgCl, BaSO₄ and Mg₂P₂O₇ used in these formulas.
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