Organic Chemistry: Purification and Characterization of Carbon Compounds

This chapter forms the practical backbone of organic chemistry, detailing the essential techniques required to isolate, purify, and confirm the identity and composition of organic compounds. Mastery of these methods is crucial for laboratory work and understanding experimental data for NEET and JEE Main.

1. Introduction to Purification and Characterization

• Necessity of Purification: Organic compounds synthesized in the laboratory or isolated from natural sources are rarely pure. They are often contaminated with unreacted starting materials, by-products, or impurities from the isolation process. Impurities can significantly alter physical and chemical properties.
• Characterization: Once purified, a compound needs to be characterized to establish its identity (e.g., melting point, boiling point, density, refractive index, spectroscopic data) and determine its elemental composition and molecular formula.

2. Methods of Purification of Organic Compounds

The choice of purification method depends on the nature of the organic compound and the impurities present.

2.1. 1. Crystallisation

• Principle: Based on the difference in the solubilities of the organic compound and its impurities in a suitable solvent. The compound should be sparingly soluble at room temperature but appreciably soluble at higher temperatures. Impurities should either be highly soluble (remain in solution) or insoluble (removed by filtration).
• Procedure:
  1. Selection of Solvent: The ideal solvent dissolves the compound well when hot but poorly when cold. It should not react chemically with the compound.
  2. Preparation of Saturated Solution: Dissolve the impure compound in the minimum amount of hot solvent.
  3. Hot Filtration (Optional): If insoluble impurities are present, filter the hot solution.
  4. Cooling: Allow the hot, saturated solution to cool slowly. The pure compound crystallises out, while more soluble impurities remain in the mother liquor.
  5. Filtration: Separate the crystals by filtration.
  6. Washing and Drying: Wash the crystals with a small amount of cold solvent and dry them.
• Applications: Most effective for purifying solid organic compounds.

2.2. 2. Sublimation

• Principle: Used for solid organic compounds that can directly change from solid to vapor phase on heating without passing through the liquid phase (sublime). Impurities should be non-volatile.
• Procedure: Heat the impure solid in a china dish. The pure substance sublimes and deposits as a solid on a cool surface (e.g., an inverted funnel or watch glass with cold water/ice). Impurities remain in the dish.
• Applications: Used for compounds like Naphthalene, Camphor, Benzoic acid, Anthracene, p-Nitrophenol.

2.3. 3. Distillation

• Principle: Based on the difference in boiling points of the components of a liquid mixture. The more volatile component (lower boiling point) distils first.
• Types:
  • a. Simple Distillation: Used for mixtures of liquids having a large difference in boiling points (at least 25-30°C) and for non-volatile impurities.
    • Apparatus: Distillation flask, condenser, receiver, thermometer.
    • Procedure: Heat the mixture, vaporize the lower boiling point component, condense it, and collect it.
  • b. Fractional Distillation: Used for mixtures of liquids having boiling points close to each other (difference less than 25°C).
    • Apparatus: Similar to simple distillation, but with a fractionating column (packed with glass beads/rings or having trays) between the distillation flask and the condenser. The column provides a large surface area for repeated vaporization and condensation, leading to better separation.
    • Applications: Separation of crude oil fractions, alcohol-water mixture.
  • c. Distillation under Reduced Pressure (Vacuum Distillation): Used for liquids that decompose at or below their normal boiling points.
    • Principle: Lowering the external pressure (by using a vacuum pump) lowers the boiling point of the liquid, allowing it to distil at a much lower temperature, thus preventing decomposition.
    • Applications: Glycerol, sugar molasses (for alcohol production).
  • d. Steam Distillation: Used for compounds that are steam-volatile (volatile in steam), immiscible with water, and have high boiling points.
    • Principle: The liquid mixture boils when the sum of its vapor pressure and the vapor pressure of water equals the external pressure. Since water’s vapor pressure contributes, the compound distils at a temperature much lower than its normal boiling point.
    • Applications: Aniline, essential oils (e.g., eucalyptus oil, clove oil).

2.4. 4. Differential Extraction

• Principle: Used for separating an organic compound present in an aqueous solution by shaking it with an immiscible organic solvent in which the organic compound is more soluble than in water.
• Procedure: The organic compound preferentially dissolves in the organic solvent. The organic solvent layer is then separated using a separating funnel. This process is repeated multiple times (multiple extractions) to recover most of the compound. The solvent is then evaporated to obtain the pure compound.
• Applications: Extraction of organic compounds from aqueous extracts of plants/animals.

2.5. 5. Chromatography

• Principle: Based on the difference in the differential adsorption or differential partition of components of a mixture between two phases: a stationary phase (solid or liquid) and a mobile phase (liquid or gas).
• Types:
  • a. Adsorption Chromatography: Based on the principle of differential adsorption.
    • Column Chromatography: A mixture is applied to a stationary phase (e.g., Alumina, Silica gel) packed in a column. A mobile phase (eluent, liquid solvent) is passed through. Components with lower adsorption to the stationary phase move faster and are eluted first.
    • Thin Layer Chromatography (TLC): A thin layer of adsorbent (silica gel or alumina) is spread on a glass plate (stationary phase). The mixture is spotted on the base line. The plate is then placed in a chamber with a solvent (mobile phase). The solvent rises by capillary action, separating components. The relative adsorption is measured by the retardation factor (Rf value): Rf​=Distance travelled by substance/Distance travelled by solvent front
  • b. Partition Chromatography: Based on the principle of differential partition (distribution) of components between two immiscible liquid phases (one stationary, one mobile).
    • Paper Chromatography: A spot of the mixture is applied to a strip of chromatographic paper (stationary phase, which is water adsorbed on cellulose fibers). A suitable solvent (mobile phase) moves over the paper, separating components based on their different partition coefficients between stationary water and the mobile solvent.
    • Gas-Liquid Chromatography (GLC): The stationary phase is a non-volatile liquid coated on a solid support, packed in a column. The mobile phase is an inert gas (e.g., He, N2). Separates volatile organic compounds.

3. Qualitative Analysis of Organic Compounds (Detection of Elements)

This involves detecting the presence of specific elements (C, H, N, S, Halogens) in an organic compound.

3.1. 1. Detection of Carbon and Hydrogen

• Principle: When an organic compound is heated with copper(II) oxide (CuO), carbon is oxidized to carbon dioxide (CO2), and hydrogen is oxidized to water (H2O).
• Procedure:
  • Detection of Carbon (as CO2): Pass the gas through lime water (Ca(OH)2 solution). If CO2 is present, lime water turns milky due to the formation of insoluble calcium carbonate. CO2(g)+Ca(OH)2(aq)→CaCO3(s)+H2O(l)
  • Detection of Hydrogen (as H2O): Pass the gases over anhydrous copper(II) sulfate (white). If H2O is present, it turns blue due to the formation of hydrated copper(II) sulfate. CuSO4(anhydrous,white)+5H2O(l)→CuSO4.5H2O(blue)

3.2. 2. Detection of Nitrogen, Sulfur, and Halogens (Lassaigne’s Test)

• Principle: Organic compounds containing N, S, or halogens are fused with a small piece of sodium metal. This converts the elements into ionic inorganic salts (NaCN, Na2S, NaX, where X=Cl, Br, I). These ionic salts can then be easily detected in the aqueous extract (Lassaigne’s Extract or Sodium Fusion Extract).
• Sodium Fusion: Heat the organic compound strongly with sodium metal in a fusion tube.
  • Na+C+Nheat​NaCN
  • 2Na+Sheat​Na2S
  • Na+Xheat​NaX
• Preparation of Lassaigne’s Extract (L.E.): After fusion, plunge the hot tube into distilled water, boil, cool, and filter. The filtrate is the Lassaigne’s Extract.
• a. Detection of Nitrogen:
  • Test: To a portion of L.E., add freshly prepared ferrous sulfate solution (FeSO4). Boil, cool, and acidify with concentrated H2SO4.
  • Observation: Formation of a Prussian blue or green precipitate/color indicates nitrogen.
  • Reactions:
    • Fe2++2CN−→Fe(CN)2
    • Fe(CN)2+4CN−→[Fe(CN)6]4− (ferrocyanide ion)
    • 3[Fe(CN)6]4−+4Fe3+H+​Fe4[Fe(CN)6]3⋅xH2O (Prussian blue, ferric ferrocyanide)
    • (Note: Fe3+ is formed by oxidation of Fe2+ by traces of air or by adding FeCl3).
• b. Detection of Sulfur:
  • Test 1 (Sodium Nitroprusside Test): To a portion of L.E., add a few drops of sodium nitroprusside solution (Na2[Fe(CN)5NO]).
  • Observation: Appearance of a deep violet/purple color indicates sulfur.
    • Na2S+Na2[Fe(CN)5NO]→Na4[Fe(CN)5NOS] (sodium thionitroprusside, violet)
  • Test 2 (Lead Acetate Test): Acidify a portion of L.E. with acetic acid and add lead acetate solution.
  • Observation: Black precipitate indicates sulfur.
    • Na2S+(CH3COO)2Pbacetic acid​PbS(s,black)+2CH3COONa
• c. Detection of Halogens:
  • Test (Silver Nitrate Test): Acidify a portion of L.E. with dilute HNO3 (to remove any NaCN or Na2S, which interfere) and then add silver nitrate solution (AgNO3).
  • Observation: A precipitate indicates a halogen.
    • NaX+AgNO3→AgX(s)+NaNO3
    • AgCl: White precipitate, soluble in excess dilute ammonia solution.
    • AgBr: Pale yellow precipitate, sparingly soluble in dilute ammonia, soluble in concentrated ammonia.
    • AgI: Yellow precipitate, insoluble in dilute and concentrated ammonia.
  • Interference (if N or S are present): If the compound contains both N and S, NaSCN is formed during fusion (Na+C+N+S→NaSCN). Acidify L.E. with conc. HNO3 and boil to decompose NaCN and Na2S to prevent interference with Ag+ test.

4. Quantitative Analysis of Organic Compounds (Estimation of Elements)

This involves determining the precise percentage composition of different elements in an organic compound.

4.1. 1. Estimation of Carbon and Hydrogen (Liebig’s Method)

• Principle: A known mass of the organic compound is heated strongly in a current of dry oxygen with copper(II) oxide (CuO). Carbon is quantitatively oxidized to CO2, and hydrogen to H2O.
• Procedure:
  • The evolved CO2 is absorbed in a weighed U-tube containing concentrated KOH solution.
  • The evolved H2O is absorbed in a weighed U-tube containing anhydrous CaCl2 (or anhydrous Mg(ClO4)2).
• Calculations:
  • Mass of organic compound = w g
  • Increase in mass of CaCl2 tube = mass of H2O formed = x g
  • Increase in mass of KOH tube = mass of CO2 formed = y g
  • Percentage of Carbon:
    • Molecular mass of CO2 = 44 g (contains 12 g C)
    • Mass of C in y g CO2 = (12/44)×y g
    • % C=((12/44)×y/w)×100
  • Percentage of Hydrogen:
    • Molecular mass of H2O = 18 g (contains 2 g H)
    • Mass of H in x g H2O = (2/18)×x g
    • % H=((2/18)×x/w)×100

4.2. 2. Estimation of Nitrogen

• a. Dumas Method: (Suitable for all nitrogenous organic compounds)
  • Principle: A known mass of the organic compound is heated with copper(II) oxide in an atmosphere of CO2. Nitrogen is quantitatively converted to N2 gas. Any oxides of nitrogen formed are reduced to N2 by passing over hot copper gauze.
  • Procedure: The nitrogen gas collected over KOH solution (which absorbs CO2 produced in combustion).
  • Calculations:
    • Mass of organic compound = w g
    • Volume of N2 gas collected = V1 mL at T1 K and P1 pressure.
    • Convert V1 to STP conditions (V2 at 273 K and 1 atm).
    • Molar volume of N2 at STP = 22400 mL/mol.
    • Mass of N2 = (V2/22400)×28 g (where 28 is molar mass of N2)
    • % N=((28×V2)/(22400×w))×100
• b. Kjeldahl’s Method: (Not suitable for compounds containing N in nitro and azo groups, or N in rings like pyridine, as they don’t convert to ammonium sulfate)
  • Principle: A known mass of the organic compound is heated strongly with concentrated H2SO4 (digestion) in the presence of K2SO4 (raises boiling point) and a catalyst (e.g., CuSO4 or mercury). Nitrogen in the organic compound is quantitatively converted to ammonium sulfate. The ammonium sulfate is then treated with excess NaOH, and the liberated ammonia is absorbed in a known volume of standard acid. The unreacted acid is then back-titrated.
  • Reactions:
    • Organic compound (containing N) conc.H2SO4,K2SO4,catalyst​ (NH4)2SO4
    • (NH4)2SO4 + 2NaOH → Na2SO4 + 2NH3 + 2H2O
    • 2NH3 + H2SO4 (known volume and conc.) → (NH4)2SO4 (Ammonia absorbed)
    • Excess H2SO4 titrated with standard NaOH solution.
  • Calculations:
    • From the titration data, find moles of NH3 produced.
    • 1 mole of NH3 contains 14 g of Nitrogen.
    • Calculate percentage of Nitrogen.

4.3. 3. Estimation of Halogens (Carius Method)

• Principle: A known mass of the organic compound is heated strongly with fuming nitric acid in the presence of silver nitrate (AgNO3) in a sealed hard glass tube (Carius tube). Carbon and hydrogen are oxidized to CO2 and H2O, and the halogen is quantitatively converted to silver halide (AgX).
• Procedure: The precipitate of AgX is filtered, washed, dried, and weighed.
• Calculations:
  • Mass of organic compound = w g
  • Mass of AgX formed = x g
  • Percentage of Halogen:
    • For Chlorine: AgCl (143.5 g) contains 35.5 g Cl. % Cl=((35.5/143.5)×x/w)×100
    • For Bromine: AgBr (188 g) contains 80 g Br. % Br=((80/188)×x/w)×100
    • For Iodine: AgI (235 g) contains 127 g I. % I=((127/235)×x/w)×100

4.4. 4. Estimation of Sulfur (Carius Method)

• Principle: A known mass of the organic compound is heated strongly with fuming nitric acid in a Carius tube. Sulfur is quantitatively oxidized to sulfuric acid (H2SO4). The H2SO4 is then precipitated as barium sulfate (BaSO4) by adding barium chloride (BaCl2) solution.
• Procedure: The precipitate of BaSO4 is filtered, washed, dried, and weighed.
• Calculations:
  • Mass of organic compound = w g
  • Mass of BaSO4 formed = x g
  • Percentage of Sulfur:
    • Molecular mass of BaSO4 = 233 g (contains 32 g S)
    • % S=((32/233)×x/w)×100

4.5. 5. Estimation of Phosphorus

• Principle: A known mass of the organic compound is heated with fuming nitric acid in a Carius tube. Phosphorus is oxidized to phosphoric acid (H3PO4). This is then precipitated as ammonium phosphomolybdate or magnesium ammonium phosphate, which on ignition gives magnesium pyrophosphate (Mg2P2O7).
• Calculations:
  • Mass of organic compound = w g
  • Mass of Mg2P2O7 formed = x g
  • Percentage of Phosphorus:
    • Molecular mass of Mg2P2O7 = 222 g (contains 2 x 31 = 62 g P)
    • % P=((62/222)×x/w)×100

4.6. 6. Estimation of Oxygen

• Principle: Oxygen cannot be directly estimated. Its percentage is usually calculated by difference: % Oxygen=100−(% C+% H+% N+% S+% Halogens)

This detailed guide on Purification and Characterization of Organic Compounds covers all essential methods and calculations for NEET and JEE Main. Focus on the principles behind each technique, the specific reagents used, and the stoichiometric calculations for quantitative analysis. Practice numerical problems, especially for Liebig’s, Dumas’, Kjeldahl’s, and Carius’ methods.