3.3.10 Aromatic chemistry

• Cyclohexane-1,3-diene is more stable due to the C=C bond being closer together and so the p orbitals overlap creating some delocalisation of electrons so cyclohexane-1,3-diene is less exothermic than the double of cyclohexene
• Methylbenzene has a faster rate of reaction than benzene due to the methyl pushing electron density towards the ring via the inductive effect making the ring more electron rich so the electrophile is more strongly attracted.
• Benzene is a planar molecule and each carbon has a trigonal planar shape and a bond angle of 120. In benzene all six of the C-C bonds are the same length.
• Nitronium ion:

  Step 1: Sulfuric acid gives nitric acid H+

  HNO3 + H2SO4 -> H2NO3* + HSO4-

  Step 2: The protonated nitric acid breaks down

  H2NO3* + H2SO4 -> +NO2 + HSO4- + H3O+

  Overall equation:

  HNO3 + 2H2SO4 -> +NO2 + 2HSO4- + H3O+
• Nitration of benzene:

  Type of Mechanism: Electrophilic Substitution

  Reagents/Conditions: Conc HNO3 and Conc H2SO4
• Aromatic nitro compounds are used to make explosives

  Phenylamine is used to make dyes
• Reduction of Aromatic nitro compounds:

  Reagent: Sn/conc HCl

  C6H5NO2 + 6[H] -> C6H5NH2 + 2 H2O
• Acylation:

  Making the nucleophile:

  acyl chloride + AlCl3 -> R-C*=O + [AlCl4]-

  Forming catalyst AlCl3:

  +H + -[AlCl4] -> HCl + AlCl3
• Compare the shape and bond angles in benzene and cyclohexane.

  Benzene

  > Planar molecule

  > trigonal planar shape

  > bond angle : 120

  Cyclohexane

  > Boat shape

  > Tetrahedral

  > bond angle : 109.5
• Predict the values for the enthalpy of hydrogenation for cyclohexa-1,3-diene and cyclohexa-1,4-diene and explain your answer.

  Cyclohexa-1,3-diene is more stable than cyclohexane-1,4-diene because the C=C bonds are closer together so the P orbitals overlap creating some delocalisation of electrons. So cyclohexa-1,3-diene is less exothermic than cyclohexa-1,4-diene
• Why does the electrophilic substitution reaction happen faster with methylbenzene and slower with nitrobenzene compared to benzene?

  
  

  Methyl benzene has a faster rate of reaction than benzene with an electrophile because the methyl group pushes electron density towards the ring via the inductive effect makins the ring more electron rich and so the electrophile is more strongly attracted

  In nitrobenzene, the nitro group withdraws electron density making it less electron rich and the electrophile is less strongly attracted
• Why does benzene undergo electrophilic substitution reactions rather than electrophilic addition reactions?

  Electrophilic addition reaction breaks up the delocalised system in benzene and benzene is stable so therefore undergoes substitution rather than addition reaction
• Explain bonding and shape of benzene and stability and compare it to cyclohexane-1,3,5-triene

  Each C has three (covalent) bonds, Spare electrons (in a p orbital) overlap (to form an electron cloud) leading delocalisation = stable

  Stage 2 Shape: Planar , Hexagon/6 carbon ring/120° bond angle, C–C bonds equal in length / C–C bond lengths between single and double bond

  Stage 3 Stability: Expected enthalpy of hydration of cyclohexatriene = –360 kJ mol–1, enthalpy of hydration of benzene (is less

  exothermic) by 152 kJ mol–1, Benzene lower in energy than

  cyclohexatriene / Benzene is more stable
• Electrophilic substitution-( Benzene-> Nitrobenzene )


  Conditions: r.t.p

  Reagents: Conc H2SO4 + Conc HNO3 -> make electrophile ( +NO2 )

  Equation: 2H2SO4 + HNO3 -> +NO2 + 2(HSO4)- + H3O+

  Forms: Nitrobenzene
• Reduction of Nitrobenzene -> Phenylamine

  Condition: Conc HCl

  Reagent: Tin ( Sn ) catalyst / 6[H]

  Forms: Phenylamine and 2 waters
• Speed of nitration:

  As more Nitronium ions join onto the benzene molecule. The N pulls Electron density from the benzene ring therefore it is less strongly attracted to the electrophile
• Electrophilic Substitution/ Freidel-Craft's Acylation - ( Benzene-> benzaldehyde / phenylketone )


  Conditions: Aluminium chloride catalyst ( AlCl3 )

  Reagent: Acyl Chloride + catalyst form electrophile (ROC+)

  Equation: RCOCl + AlCl3 -> [AlCl4]- + ROC+

  Reformation of catalyst: [AlCl4]- + H+ -> AlCl3 + HCl

  Forms: benzaldehyde / phenylketone