6.1.3

• Electrophilic Substitution

  benzene having delocalised electrons allows it to be very stable + unlikely to undergo addition reactions

  Steps :

  1. elections above + below the plane of atoms in the benzene ring attract an electrophile
  2. Electrophile accepts a pair of pi electrons from delocalised ring + makes a covalent bond

  → slowest step = rate - determining step

  1. A reactive intermediate is formed where the delocalised electrons have been disrupted
  2. unstable intermediate releases an H+ ion + stable product is formed

  summarised using a reaction mechanism
• Reaction of Benzene

  o nitration o halogenation
• Halogeration as aromatic ring is too stable it doesn’t directly react with halogens o halogen carrier such as iron (iron halide in situ), iron halides or aluminium halides are used

  → halogen carrier will generate a positive halogen ion

  Chlorination : AlF3 , FeCl3 or Fe

  Bromination : AlBr3, FeBr3, or Fe

  halogen carrier is a catalyst + regenerated at the end of halogenation as hydrogen ion is released from benzene ring forms HBr
• Nitration

  electrophilic substitution reaction for a - NO2 (nitro group) o the reagent is concentrated HNO 3 with concentrared H2 S04 as a catalyst CH+ HNO3 → C6H5N02 + H2O • HNO 3 + H2 SO4 are initially mixed together in an ice bath in a flask o benzene is added + a reflux is set up, keeping mixture at 50° to prevent further substitution reactions occurring

  • in reacture, H2 SO4 is needed to generate the NO 2+ electrophile from nitric acid
  • H2SO4 is regenerated after nitration + therefore a catalyst :

  HNO3 + H2SO4 → NO2+ + HSO4- + H2O
• Halogenation

  Although benzene and aromatic compounds are unsaturated they are not as reactive as alkenes.

  • pi bonds have a region of high electron density and will readily undergo addition reactions to become saturated.
  • Shaking ethene with bromine water will cause decolourisation as the coloured bromine is used to form the colourless 1,2-dibromoethane
• • If cyclohexene is mixed with bromine water, an addition reaction occurs

  → The first part of the mechanism is the bromine molecule having an induced dipole due to the interaction of the pi bond of the cyclohexene

  • As the bromine approaches the pi-bond the electrons in the bromine molecule are repelled, creating an induced dipole.
  • The electrons lie towards one end of the bond, creating a temporary dipole.
• • When benzene is shaken with bromine water, no reaction occurs. This surprising result occurs even density between the carbon atoms than an alkene.
  • though the electrons in benzene are delocalised in pi bonds. So benzene must have a lower electron density between carbon atoms than an alkene
• • When non-polar molecules like bromine approach the benzene ring, there is not enough electron density between the carbon atoms to induce a dipole and start the reaction
  • This is also the case with electrophile that can be generated and alkylation can occur when attempting to substitute alkyl halides like haloalkanes.

  → By using a halogen carrier, a stronger electrophile can be generated and alkylation can occur
• mechanism for nitration of benzene

  
• delocalised structure