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Short Notes: Purification And Characterisation Of Organic Compounds
1 Methods of Purification of Organic Compounds
1.1 Sublimation
- Principle: Substance changes from solid to vapor without passing through liquid state
- Used for: Purification of volatile solids
- Examples: Benzoic acid, Naphthalene, Camphor, Anthracene, Iodine
- Method: Heat impure substance → vapors condense on cool surface → pure crystals obtained
1.2 Crystallization
- Principle: Based on difference in solubility of compound and impurities in a solvent
- Steps:
- Dissolve impure compound in minimum amount of hot solvent
- Filter hot solution to remove insoluble impurities
- Cool the filtrate → crystals separate out
- Filter and dry the crystals
- Solvent Selection: Compound should be sparingly soluble in cold solvent, highly soluble in hot solvent
1.3 Distillation
| Type | Use | Examples |
|---|---|---|
| Simple Distillation | Liquids with b.p. difference > 25°C; volatile liquid from non-volatile impurities | Purification of acetone, chloroform, toluene |
| Fractional Distillation | Liquids with b.p. difference < 25°C | Separation of petroleum fractions, mixture of benzene and toluene |
| Distillation under Reduced Pressure | Liquids with high b.p. or decompose at normal b.p. | Glycerol, concentrated sugar solution |
| Steam Distillation | Immiscible with water, volatile in steam, impurities non-volatile | Aniline, nitrobenzene, essential oils |
1.4 Differential Extraction
- Principle: Based on difference in solubility in two immiscible solvents
- Apparatus: Separating funnel
- Method: Shake mixture with solvent → organic compound dissolves in organic layer → separate layers
- Example: Extraction of organic compound from aqueous solution using ether/chloroform
1.5 Chromatography
| Type | Principle | Application |
|---|---|---|
| Adsorption Chromatography | Based on differential adsorption on solid adsorbent (alumina, silica gel) | Column chromatography, Thin layer chromatography (TLC) |
| Partition Chromatography | Based on differential partitioning between stationary and mobile phase | Paper chromatography |
Rf Value (Retention Factor):
Rf = Distance traveled by substance / Distance traveled by solvent
• Always less than 1
• Characteristic for a particular compound under specific conditions
Rf = Distance traveled by substance / Distance traveled by solvent
• Always less than 1
• Characteristic for a particular compound under specific conditions
2 Qualitative Analysis
2.1 Detection of Carbon and Hydrogen
- Test: Heat organic compound with CuO
- Carbon detection: CO2 evolved turns lime water milky
- Hydrogen detection: H2O formed turns anhydrous CuSO4 blue
2.2 Detection of Other Elements (Lassaigne's Test)
Sodium Fusion: Organic compound + Na metal (heat) → converts covalent compounds to ionic form
| Element | Reagent Added to Extract | Observation | Inference |
|---|---|---|---|
| Nitrogen | FeSO4, then H2SO4, boil and cool | Prussian blue color | N present (forms Na4[Fe(CN)6] → Fe4[Fe(CN)6]3) |
| Sulphur | Lead acetate or Sodium nitroprusside | Black ppt. of PbS or violet color | S present (forms Na2S) |
| Chlorine | Acidify with HNO3, add AgNO3 | White ppt. soluble in NH4OH | Cl present (forms NaCl → AgCl) |
| Bromine | Acidify with HNO3, add AgNO3 | Pale yellow ppt. sparingly soluble in NH4OH | Br present (forms NaBr → AgBr) |
| Iodine | Acidify with HNO3, add AgNO3 | Yellow ppt. insoluble in NH4OH | I present (forms NaI → AgI) |
Note: If N or S is present along with halogens, extract must be boiled with HNO3 to decompose NaCN and Na2S before testing for halogens (otherwise AgCN and Ag2S will interfere)
3 Quantitative Analysis
3.1 Estimation of Carbon and Hydrogen (Liebig's Method)
- Principle: Organic compound combusted with excess CuO in stream of oxygen
- Products: C → CO2 (absorbed in KOH); H → H2O (absorbed in anhydrous CaCl2)
- Formulae:
- % of C = (12 × mass of CO2 × 100) / (44 × mass of substance)
- % of H = (2 × mass of H2O × 100) / (18 × mass of substance)
3.2 Estimation of Nitrogen
Dumas Method
- Principle: Nitrogenous compound + CuO → CO2, H2O, N2
- CO2 absorbed by KOH; N2 volume measured
- Formula: % N = (28 × Volume of N2 at STP × 100) / (22400 × mass of substance)
Kjeldahl's Method
- Suitable for: Compounds with N directly linked to C (not for nitro, azo, diazo compounds)
- Steps:
- Heat compound with conc. H2SO4 → (NH4)2SO4
- Add excess NaOH and boil → NH3 evolved
- NH3 absorbed in known volume of standard H2SO4
- Unused acid titrated back with standard NaOH
- Formula: % N = (1.4 × Molarity of acid × Volume of acid used) / (mass of substance)
3.3 Estimation of Halogens (Carius Method)
- Principle: Heat compound with fuming HNO3 in presence of AgNO3 in sealed Carius tube
- Halogen → HX → AgX (precipitate)
- Weigh AgX precipitate
- Formula: % X = (Atomic mass of X × mass of AgX × 100) / (Molecular mass of AgX × mass of substance)
3.4 Estimation of Sulphur (Carius Method)
- Principle: Heat with fuming HNO3 → S → H2SO4
- Add BaCl2 → BaSO4 precipitate
- Weigh BaSO4
- Formula: % S = (32 × mass of BaSO4 × 100) / (233 × mass of substance)
3.5 Estimation of Phosphorus
- Heat with HNO3 → H3PO4
- Add magnesia mixture → (NH4)3PO4.12MgO yellow ppt.
- Ignite → Mg2P2O7
- Formula: % P = (62 × mass of Mg2P2O7 × 100) / (222 × mass of substance)
4. Molecular Formula Determination
Steps to determine Molecular Formula:
- Find % composition of elements
- Calculate empirical formula (simplest whole number ratio)
- Determine molecular mass (by various methods)
- Molecular formula = n × Empirical formula
where n = (Molecular mass) / (Empirical formula mass)
| Method | Formula |
|---|---|
| Victor Meyer's Method | Molar mass = (w × Vm × Ts × Ps) / (Vs × Tm × Pm) |
| From Ideal Gas Equation | M = (m × R × T) / (P × V) |
| Elevation in B.P. | M = (Kb × w × 1000) / (ΔTb × W) |
| Depression in F.P. | M = (Kf × w × 1000) / (ΔTf × W) |
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FAQs on Short Notes: Purification And Characterisation Of Organic Compounds
| 1. What are some common methods for the purification of organic compounds? |  |
Ans. Common methods for the purification of organic compounds include recrystallisation, distillation, chromatography, and sublimation. Recrystallisation is effective for solid compounds, distillation is used for liquids with different boiling points, chromatography separates compounds based on their movement through a stationary phase, and sublimation is used for purifying solids that can transition directly from solid to gas.
| 2. How is recrystallisation performed, and what are its advantages? | |
Ans. Recrystallisation involves dissolving a solid compound in a suitable hot solvent and then allowing it to cool gradually, leading to the formation of purer crystals. The advantages include the ability to remove impurities effectively and the provision of high-purity products. The choice of solvent is crucial, as it must dissolve the compound at high temperatures but not at low temperatures.
| 3. What is the principle behind chromatography, and what are its types? | |
Ans. Chromatography operates on the principle of separation based on differential affinities of compounds to a stationary phase and a mobile phase. Major types include paper chromatography, thin-layer chromatography (TLC), column chromatography, and gas chromatography (GC). Each type varies in its application and separation efficiency, suitable for different compounds and mixtures.
| 4. How can the purity of an organic compound be characterised? | |
Ans. The purity of an organic compound can be characterised through various techniques, such as melting point determination, boiling point determination, thin-layer chromatography, and spectral analysis (like NMR or IR spectroscopy). A pure compound exhibits sharp and specific melting or boiling points, while impurities typically broaden these ranges and can be detected using chromatographic methods.
| 5. What role does sublimation play in the purification of organic compounds? | |
Ans. Sublimation is a purification method where a solid compound transitions directly into gas without passing through a liquid phase. This method is particularly useful for purifying volatile solids or compounds that decompose upon melting. It allows for the separation of the desired compound from non-volatile impurities, ensuring higher purity levels.
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