introducing benzene & electrophilic substitution reactions

• Kekule model - represents the structure with alternating single and double bonds 


  delocalised model - shows a ring of electrons that are delocalised

  
• how the delocalised model is formed

  • each carbon contributes one electron from its 2p orbital to a π bonding system.
• how the delocalised model is formed

  • the p-orbitals overlap side by side around the ring, forming a delocalised system of 6 π-electrons
  • this creates an electron density above and below the plane of carbon atoms
  • the electrons are not fixed between specific atom pairs, but delocalised over the whole ring
• how the delocalised model is formed

  this delocalisation leads to equal C-C bond lengths between the carbon atoms and enhanced stability of the aromatic ring

  
• equivalent carbon-carbon bond lengths

  • x ray diffraction techniques have revealed that all carbon-carbon bonds in benzene measure 140 pm in length
  • this measurement sits between the length of a typical C-C single bond (134 pm) and a C=C double bond (154 pm)
  • this contradicts the Kekule model, which would suggest alternating lengths for single and double bonds
• enthalpy of hydrogenation

  • hydrogenation of cyclohexene, which has one C=C bond, results in a change in enthalpy of -120 kJ mol -1
  • Kekulé model claimed to have 3 double bonds, which would have an enthalpy of -360 kJ mol -1 if it followed the same logic
  • however, the actual enthalpy for benzene's hydrogenation is only -208 kJ mol -1
• enthalpy of hydrogenation

  this indicates that breaking the bonds in benzene requires more energy, suggesting a stability greater than what the Kekule model predicts
• the stability seen in benzene is attributed to the delocalisation of electrons above and below the hexagonal ring
• • arenes are aromatic hydrocarbons that contain a benzene ring
  • benzene is the simplest arene and has a planar ring structure
  • benzene with the molecular formula C6H6, is composed of hexagonal ring that include 6 carbon atoms
• substituted benzene - the names of the substitutents precede the word 'benzene' e.g chlorobenzene, nitrobenzene and methylbenzene
• phenyl derivatives - compounds named as derivatives of the phenyl group (C6H5-) e.g phenol, phenylamine
• nitration of benzene

  1. nitric acid reacts w the sulfuric acid catalyst to generate the electrophilic nitronium (NO2+) ion: HNO3 + H2SO4 -> NO2+ + HSO4- + H2O
  2. benzene undergoes electrophilic substitution with the electrophile NO2+, displacing a proton and forming nitrobenzene: C6H6 + NO2+ -> C6H5NO2 + H+
  3. the displaced proton reacts with HSO4- to regenerate the H2SO4 catalyst: H+ + HSO4- -> H2SO4

  
• controlling nitration

  cooling the nitration reaction to less than 55 celsius ensures that only mononitration occurs, producing nitrobenzene as the major product
• nitro compounds like nitrobenzene can be catalytically reduced to form aromatic amines. these are key intermediates used to manufacture dyes, drugs and polymers 


  some nitrated compounds are useful explosives, such as 2,4,6-trinitrotoluene (TNT). the 3 nitro groups make TNT very unstable and easily detonated
• trinitrobenzene - 3 moles of nitric acid, 1 mole of benzene 


  C6H6 + 3HNO3 -> C6H3(NO2)3 + 3H2O
• friedel-crafts alkylation mechanism

  1. formation of the electrophilic methyl carbocation (CH3+) intermediate: CH3Cl + AlCl3 -> CH3+ + AlCl4-
  2. electrophilic substitution occurs on benzene with CH3+, displacing H+ and forming methylbenzene: C6H6 + CH3+ -> C6H5CH3 + H+
  3. the proton reacts with AlCl4-, regenerating the AlCl3 catalyst H+ + AlCl4- -> AlCl3 + HCl

  
• Friedel-Crafts acylation mechanism

  1. formation of the electrophilic acetyl cation (CH3CO+) intermediate: CH3COCl + AlCl3 -> CH3CO+ + AlCl4-
  2. electrophilic substitution occurs on benzene with CH3CO+, displacing H+ and forming phenylethanone: C6H6 + CH3CO+ -> C6H5COCH3 + H+
  3. the proton reacts with AlCl4-, regenerating the AlCl3 catalyst: H+ + AlCl4- -> AlCl3 + HCl

  
• to place a halogen on the benzene molecule, we need to show how the electrophile are made first 


  iron powder is reacted with Cl2 gas

  results in the formation of anhydrous FeCl3 (halogen-carrier catalyst)
• halogenation of benzene

  1. chlorine reacts with iron (III) chloride catalyst to form the electrophilic chloronium (Cl+) ion: Cl2 + FeCl3 -> Cl+ + FeCl4-
• halogenation of benzene

  2. benezene undergoes electrophilic substitution with the electrophile Cl+, displacing a proton and forming chlorobenzene: C6H6 + Cl+ -> C6H5Cl + H+
• halogenation of benzene

  1. chlorine reacts with iron (III) chloride catalyst to form the electrophilic chloronium (Cl+) ion: Cl2 + FeCl3 -> Cl+ + FeCl4-
  2. benezene undergoes electrophilic substitution with the electrophile Cl+, displacing a proton and forming chlorobenzene: C6H6 + Cl+ -> C6H5Cl + H+
  3. the displaced proton reacts with FeCl4- to regenerate the FeCl3 catalyst: H+ + FeCl4- -> FeCl3 + HCl

  
• upon addition of Br2 to alkenes, even at room temperature, a reaction occurs which results in decolourisation of bromine

  i.e C2H4 + Br2 (l) -> (r.t no catalyst) C2H4Br2
• under similar conditions (of alkenes), benzene will not react with Br2.

  this is due to a very stable delocalised ring , which will only donate electrons, with the aid of catalysts
• so C=C has more electron density than the C-C bonds in benzene

  the concentration of electrons per bond is 3 in benzenes and 4 in a C=C

  so alkenes can react much easier with bromine
• if benzene had C=C bonds (kekule model) it should decolourise bromine in an electrophilic addition reaction but:

  • benzene does not undergo electrophilic addition reactions
  • benzene does not decolourise bromine under normal conditions
  • therefore it cannot have C=C bonds in its structure