Unit 4

• Structural isomers


  Molecules with the same molecular formula but a different arrangement of atoms.
• Stereoisomers


  Molecules with the same structural formula but a different arrangement of atoms in 3D space.
• Geometric isomers


  Molecules with two different groups attached to each carbon atom either side of the double bond.
• Optical isomers

  Molecules with four different groups attached to a central carbon atom known as a chiral centre.
• Enantiomers


  Molecules that are non-superimposable mirror images of each other. They will rotate plane polarised light in opposite directions.
• Racemic mixture


  An equimolar mixture of two enantiomers. It has no effect with plane polarised light.
• Optical activity


  The ability of a substance to rotate plane polarised light.
• Aliphatic


  Straight-chained
• Aromatic


  Containing a benzene ring
• Bond lengths of Benzene

  Double bonds are shorter than single bonds. However, electron density diagrams show that Benzene has a perfectly hexagonal structure. This means that all Benzene's bonds must be the same length, somewhere between that of a single bond and a double bond.
• Reaction of Benzene with Bromine water

  Benzene will not react with Bromine water unless it is put under harsh conditions. This suggests it does not contain single or double bonds but something in between.
• Thermodynamic data on Benzene

  The enthalpy change when Benzene is hydrogenated is 152kj/mol more than what would be expected from Kekulé's structure, meaning it is more stable.
• Structure of Benzene

  Each carbon atom has a spare electron in a p-orbital. These p-orbitals overlap above and below the ring structure, known as a π-ring. Electrons are delocalised, meaning that Benzene is more stable than Kekulé's structure as it is harder for an electrophile to attack it.
• Reactions of Benzene

  Benzene molecules tend to undergo electrophilic substitution reactions, as these allow the π-ring to remain intact, maintaining the compound's stability.
• Conditions for the chlorination of Benzene

  • occurs at room temperature
  • occurs in the absence of light to prevent radicals from forming and free radical substitution from occurring
  • requires a halogen carrier to produce the attacking species
• Distinguishing chlorobenzene from (chloromethyl)benzene

  A silver nitrate test for chlorine ions will show that chlorobenzene will not form chlorine ions. This is because chlorine's lone pair of electrons become incorporated into the π-ring system. This shortens and strengthens the C-Cl bond.