Purification

• Boiling point


  The temperature (at a given pressure) at which the liquid and gaseous are in equilibrium i.e. the temperature at which the free energy of the change liquid - gas is zero
• Distillation


  • Can be carried out at atmospheric pressure or reduced pressure
• Simple assembly for distillation at atmospheric pressure

  1. Boiling flask

  2. Still-head with thermometer and water-cooled condenser

  3. Receiver-adapter

  4. Distillate collection flask
• It is important that a few porous chips or boiling stones be added to the liquid before heating
• Distillation procedure


  1. Heating may be rapid at the beginning until boiling commences

  2. Temperature shown on thermometer rises rapidly until near boiling point

  3. Then rises slowly and finally remains constant at the boiling point of the liquid while distillation occurs
• Fractional distillation


  • A fractionating column is inserted vertically between boiling flask and condenser
  • A thermometer is inserted on top of the fractionating column
• Fractional distillation procedure


  1. Fractions of the distillate are collected at different temperature points

  2. Fraction at first steady temperature is the boiling point of the more volatile compound

  3. Fraction at second steady temperature is the boiling point of the less volatile compound
• Recrystallisation


  Purification of solid products based on differences in solubility at different temperatures
• Recrystallisation process


  1. Choosing a suitable solvent

  2. Making a saturated solution in hot solvent

  3. Allowing solution to cool to crystallize solid

  4. Filtering and washing the solid with cold solvent
• Qualities of a good solvent


  • Must not react with solid
  • Solid must be relatively insoluble in it at room temperature but soluble in hot solvent
  • Hot solution should yield well-defined pure crystals on cooling
  • Should dissolve impurities readily at room temperature
  • Should have low boiling point and viscosity
• Solubility rule: like dissolves like. More polar compounds dissolve better in polar solvents
• Removal of coloured impurities

  1. Boil solution with 1-2% animal charcoal

  2. Filter hot solution through celite to give clear colourless filtrate
• Melting point


  The temperature at which a solid changes into a liquid under 1 atm pressure
• A sharp melting point is indicative of high purity of a single compound
• Melting point determination


  1. Electrically: sample in sealed capillary tube, inserted into instrument with thermometer

  2. Manually: sample in capillary tube affixed to thermometer, heated in liquid paraffin
• Boiling point


  Characterization technique for liquids, obtained by distillation
• TLC


  Thin layer chromatography - simple technique for identification, reaction progress, side products, column chromatography
• TLC procedure


  1. Sample spotted on adsorbent plate, placed in solvent tank

  2. Solvent ascends by capillary action, separating components by polarity

  3. Compounds identified by Rf value (distance traveled/distance of solvent)

  
• Visualization of TLC


  • UV light for fluorescent compounds
  • Iodine vapour for colourless compounds
  • Spraying with reagents to colour spots
• Most organic compounds contain carbon, hydrogen and other elements like O, N, halogens, S, P, metals
• Detection of C and H

  They can be easily oxidized to CO2 and H2O respectively
• Procedure for detection of C and H

  1. Sample mixed with CuO, strongly heated in test tube

  2. CO2 produced bubbles through limewater, turning it milky
• Qualitative Determination of Elements


  The methods of estimation of the relative percentages of some of the elements are vital to organic chemists
• Detection Of Carbon and Hydrogen

  1. Chemical principle: Carbon and hydrogen can be easily oxidized to carbon (IV) oxide, CO2 and water, H2O, respectively

  2. Procedure: Sample of organic compound is mixed with freshly ignited copper (II) oxide and strongly heated in a test tube. Carbon is oxidized to CO2 which turns lime water milky

  3. Hydrogen is oxidized to water which condenses and turns anhydrous copper sulphate blue
• Detection Of Nitrogen, Sulphur and Halogens (Lassaigne's Sodium Fusion Test)

  1. Nitrogen is converted into cyanide ions, sulphur into sulphide ions and halogens into halide ions by fusion with sodium metal

  2. Test for Nitrogen: Prussian blue colour/precipitate with iron(II) sulphate and sulphuric acid

  3. Test for Sulphur: Black precipitate of lead sulphide with lead acetate, or purple colour with sodium nitroprusside

  4. Test for Halogens: White/pale yellow/yellow precipitate with silver nitrate, depending on halide ion
• Fluorine cannot be detected by Lassaigne's test as silver fluoride is soluble
• Test for Presence of Fluorine

  Acidify filtrate, boil to reduce volume, add calcium chloride. Gelatinous precipitate of calcium fluoride if fluoride is present
• Quantitative Determination of Elements

  Determines the percentage composition of elements in a compound, as a first step to determine the molecular formula
• Steps to determine percentage composition

  1. Determine molar mass of compound

  2. Calculate mass ratios/percentages
• Empirical Formula


  Simplest whole number ratio of atoms of elements in a compound
• Molecular Formula


  Exact formula of a compound, determined after finding the empirical formula and knowing the molar mass
• Calculating Empirical Formula from Percentage Composition

  • Example 1: Ascorbic acid (40.92% C, 4.58% H, 54.50% O) has empirical formula C3H4O3
  • Example 2: Compound with 24.75% K, 34.77% Mn, 40.51% O has empirical formula KMnO2
  • Example 3: Compound with 0.418 g Sb, 0.082 g O has empirical formula Sb2O3
• Calculating Empirical Formula from Combustion Analysis

  • Example 1: Isopropyl alcohol (0.561 g CO2, 0.306 g H2O from 0.255 g sample) has empirical formula C3H8O
  • Example 2: Hydrocarbon (2.613 g CO2, 0.778 g H2O from 0.800 g sample) has empirical formula CH2
• Determining Molecular Formula


  Use empirical formula and molar mass to calculate the molecular formula
• Determining Empirical and Molecular Formula

  • Example 1: Compound with 71.65% Cl, 24.27% C, 4.07% H, molar mass 98.96 g/mol has empirical formula CH2Cl and molecular formula C2H4Cl2
  • Example 2: Compound with 40.0% C, 6.71% H, 53.29% O, molar mass 60.05 g/mol has molecular formula C2H6O2
• A compound's melting point is the temperature at which it changes from solid to liquid.
• Boiling points increase as molecular mass increases, because there are more intermolecular forces between molecules with higher molar masses.