Organic Chemistry

• Thin Layer Chromatography (TLC)

  • Stationary Phase: Thin layer of solid (eg. silica gel or alumina) coated on an inert support
  • Mobile Phase: A liquid solvent (e.g. ethanol) which moves vertically up the TLC plate
• R, Value


  R value = distance travelled by spot / distance travelled by solvent
• Basis of Separation in TLC


  • The distance a component moves up the plate depends on how strongly it is adsorbed onto the stationary phase
  • The weaker the adsorption, the further it will move
• Gas Chromatography (GC)


  • Stationary Phase: A high boiling point liquid coated on the inside of a long column
  • Mobile Phase: An inert gas (e.g. He or N.)
• Retention Time


  The time for a component to pass from the column inlet to the detector. It is measured from zero to the centre of the peak.
• Basis of Separation in GC

  • The speed a component moves through the column depends on how soluble it is in the stationary phase
  • The more soluble, the longer the retention time
• Peak Area


  The area of each peak (not the peak height) is proportional to the amount of that substance in the sample
• Using an external calibration curve

  1. Create the calibration curve

  2. Use the calibration curve to determine the actual concentration of a particular substance
• Testing for Primary and Secondary Alcohols

  1. Add acidified potassium dichromate

  2. Warm gently

  3. Positive result: orange solution turns green
• Aldehydes will also give a positive result for the alcohol test, so you still need to test for an aldehyde
• Testing for Aldehydes


  1. Add ammoniacal silver nitrate (Tollens' Reagent)

  2. Heat

  3. Positive result: a silver mirror forms, as the silver(I) ions are reduced
• Testing for Carbonyls (Aldehydes and Ketones)

  1. Dissolve 2,4-dinitrophenylhydrazine (2,4-DNP) in methanol and concentrated sulfuric acid to make Brady's reagent

  2. Add the test compound and shake

  3. Positive result: a bright yellow/orange precipitate forms
• Identifying Carbonyl Compounds


  1. Recrystallise and dry the bright yellow/orange precipitate formed using Brady's reagent

  2. Measure its melting point

  3. Compare with known melting points of 2,4-DNP derivatives to determine the identity of the original carbonyl compound
• Optical Isomers/Enantiomers


  Stereoisomers that are non-superimposible mirror images of each other, caused by the presence of a chiral carbon atom in their structure
• Chiral Centre/Chiral Carbon Atom

  A carbon atom attached to 4 different atoms or groups of atoms
• Drawing Optical Isomers


  1. Draw the molecule, showing the tetrahedral structure around the chiral centre

  2. Draw a mirror image of the molecule
• Properties of Optical Isomers

  • A pair of optical isomers rotate plane-polarised light in opposite directions
• Racemic Mixture


  • A mixture containing equal amounts of each optical isomer
  • A racemic mixture does not rotate plane-polarised light because the effects of the two optical isomers cancel out
• Bonding in Benzene


  1. orbitals on carbon atoms overlap to form a delocalised π-bonding ring
• Bonding in Phenol


  • p-orbital on oxygen atom overlaps with delocalised π-bonding region in the benzene ring
  • Oxygen atom donates a lone pair to the benzene ring
  • Electron density in the ring increases (the ring is "activated")
• Bromination of Benzene


  • π-bonded electrons in benzene have lower electron density than σ-bonded electrons in alkenes
  • So benzene is unable to polarise Br₂
  • Therefore benzene will only react with Br₂ in the presence of a halogen carrier (whereas alkenes react spontaneously with Br₂)
• Bromination of Phenol


  • Phenol's ring has a higher electron density than benzene
  • So phenol can polarise Br₂
  • Therefore Br₂ molecules are attracted towards the ring and react spontaneously
• Electron Donating Groups (i.e. -OH and -NH₂)

  • Benzene rings with electron donating groups will have higher electron densities at the 2, 4 and 6 positions
  • Electrophiles are most likely to react at the 2, 4 and 6 positions
• Electron Withdrawing Groups (i.e. -NO₂)

  • Benzene rings with electron withdrawing groups will have lower electron densities at the 2, 4 and 6 positions
  • Electrophiles are most likely to react at the 3 and 5 positions
• Filtration under Reduced Pressure

  1. Pour the mixture of solid and liquid into a Buchner funnel containing filter paper

  2. Attach a vacuum pump to the side arm of the Buchner flask

  3. The reduced pressure in the flask forces the liquid through the funnel, leaving the solid on the filter paper
• Recrystallisation


  1. Dissolve the solid in the minimum amount of hot solvent

  2. Filter through fluted filter paper (removes insoluble impurities)

  3. Allow to cool slowly and form crystals (soluble impurities stay in solution)

  4. Filter under reduced pressure (removes soluble impurities)

  5. Wash crystals with ice cold solvent and allow to dry
• Melting Point Determination


  • Used to ascertain the purity of a sample (impurities lower the melting point and increase the temperature range over which a solid melts)
  • Used to determine the melting point for the purpose of identifying compounds (e.g. for identifying DNP derivatives of carbonyl compounds)
• Melting Point Determination


  1. Put a small sample of dry solid into a capillary tube

  2. Use a melting point apparatus (or oil bath) to raise the temperature very slowly

  3. Record the range of temperatures over which the solid melts
• Testing for Alkenes


  1. Shake with bromine water

  2. Positive result: bromine water turns colourless
• Testing for Haloalkanes


  1. Add ethanol and aqueous silver nitrate

  2. Warm gently

  3. Positive result: Chloroalkane - white precipitate, soluble in dilute ammonia

  4. Bromoalkane - cream precipitate, soluble in concentrated ammonia

  5. Iodoalkane - yellow precipitate, insoluble in ammonia
• Testing for Carboxylic Acids


  1. Add a carbonate (e.g. sodium carbonate)

  2. Positive result: effervescence (CO₂ produced)
• Testing for Phenols


  1. Test 1 - Test for acidity (e.g. add Universal Indicator or sodium hydroxide)

  2. Test 2 - Add a carbonate (e.g, sodium carbonate)

  3. Positive result: Test 1 - compound is acidic (e.g. Universal Indicator turns yellow/orange/red or compound reacts with sodium hydroxide)

  4. Test 2 - no effervescence (phenols do not react with carbonates)