Aromatics

• Aliphatic are straight or branched organic chemicals
• Aromatic or arene includes one or more ring of six carbon atom with delocalised bonding
• Benzene is a planar molecule. The evidence suggests all the C-C bonds are the same and have a length and bond energy between a C-C single and C=C double bond.
• Molecular formula of benzene

  C6H6
• In benzene, each C atom is bonded to 2 other C atoms and one H atom by single covalent bonds. This leaves one unused electron on each C atom in the p orbital, perpendicular to the plane of the ring
• In benzene, 6 p electrons are delocalised in a ring structure above and below the plane of carbon atoms
• 6 electrons in the pi bonds are delocalised and spread out over the whole ring. Delocalised mean not attached to a particular atom
• H-C-C bond angle is 120 in benzene
• Name of organic molecule to show thermodynamic stability of benzene

  cyclohexa-1,3,5-triene
• Thermodynamic stability of benzene.

  Theoretically, because cyclohexa-135 triene have 3 double bonds, it is expected to have 3x the energy or cyclohexene.

  However, in actual benzene the amount of energy is less. The 6 pi electrons are delocalised and not arranged in 3 double bonds,
• why does benzene have less energy than expected?

  6 pi electrons are delocalised and are not arranged in 3 double bonds
• The increase in stability connected to delocalisation is called the delocalisation energy
• Cyclohexa-135triene vs cyclohexa-1,4-diene 


  in cyclohex-135 triene, there would be some delocalisation and extra stability as the pi electrons are close together and so overlap. Hydrogenation value would be less negative than 240.

  In cyclohexa 14 diene, there would not be delocalisation as the pi electrons are too far apart and so dont over lap the hydrogenation value would be -240
• Hydrogenation of cyclohexa 13 diene

  less negative than -240 (more stable)

  • because there are some delocalisation and extra stability because the pi electrons are close together and so overlaps
• Hydrogenation value of cyclohexa 14 diene

  -240 b

  • because there are no delocalisation as the pi electrons are too far apart and so dont overlap.
• Benzene does not generally undergo addition reaction because these would involve breaking up the delocalised system. Benzene has high electron density and attracts electrophiles. Reactions are usually electrophilic substitutions .
• Toxicity of benzene


  • carcinogen and is banned for use in schools
  • Methylbenzene is less toxic and also reacts more readily because methyl side groups are release electrons into delocalised system making it more attractive to electrophiles
• methyl benzene vs benzene

  • MB less toxic
  • MB reacts more readily, because methyl groups releases electrons into delocalised system making it more attractive to electrophiles
• Mechanism of nitration of benzene

  electrophilic substitution
• nitration of benezene

  • R- conc nitric acid, conc sulphuric acid
  • M- electrophilic substitution
  • NO2+
• Nitration of benzene done at 60' because any higher then a second nitro group will be substituted
• equation for formation of electrophile in nitration of benzene

  HNO3 + H2SO4 -> 2HSO4- +NO2+ +HNO3+
• Formation of NO2+ in nitration of benzene

  • 1st step HNO3 acts as a base and accepts a proton from sulphuric acid
  • The intermediate loses a water molecule leaving the electrophile NO2+
• Friedel crafts acylation 


  R - acyl chloride and anhydrous aluminium chloride

  C - heat under reflux

  M - electrophilic substitution
• Formation of electrophile for friedel crafts acylation

  AlCl3 + CH3COCl -> CH3CO+ + AlCl4-
• Reducing nitroarene to aromatic amines

  R - SN and HCl / Fe and HCl

  C - heating

  M - reduction
• Reagent for catalytic hydrogenation for reduction of nitroarene to aromatic amine

  H2 and Nickel catalyst
• Reduction of nitroarene, as the reaction is carried out in HCl, the ionic salt C6H5NH3+CL- will form, which is soluble in water. Reacting the salt with NaOH will give insoluble phenylamine.
• Chlorobenzene

  C-Cl bond is stronger, typical halogenoalkane substitutions cannot occur

  Electron rich benzene ring repel nucleophiles
• Phenol

  Delocalisation makes C-O bond stronger and O-H bond weaker. Doesn't act like an alcohol, more acidic and does not oxidise
• Phenylamine

  Less basic than aliphatic amines because lone pairs delocalised and less available for accepting protons