arene chemistry

• Aliphatic


  Straight or branched chain organic substances
• Aromatic/Arene


  Includes one or more ring of six carbon atoms with delocalised bonding
• All previous organic substances have been aliphatic
• Benzene


  The simplest arene, has the molecular formula C6H6
• Benzene's structure


  • Six C atoms in a hexagonal ring, with one H atom bonded to each C atom
  • Each C atom is bonded to two other C atoms and one H atom by single covalent σ-bonds
  • One unused electron on each C atom in a p orbital, perpendicular to the plane of the ring
  • The six p electrons are delocalised in a ring structure above and below the plane of carbon atoms
  • Benzene is a planar molecule
  • 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
• The H-C-C bond angle is 120o in benzene
• Delocalisation


  The six electrons in the pi bonds are delocalised and spread out over the whole ring, not attached to a particular atom
• In formulae draw a circle to show the delocalised system
• Theoretical cyclohexa-1,3,5-triene

  The amount of energy would be 3 times as much as cyclohexene
• Actual benzene

  The amount of energy is less than 3 times cyclohexene due to delocalisation
• Delocalisation energy


  The increase in stability connected to delocalisation
• In cyclohexa-1,4-diene, there would not be delocalisation as the pi electrons are too far apart and so don't overlap
• In cyclohexa-1,3-diene, there would be some delocalisation and extra stability as the pi electrons are close together and so overlap
• Naming aromatic compounds


  • The simplest molecules are derivatives of benzene and have benzene at the root of the name
  • If two or more substituents are present, their positions must be indicated by the use of numbers to give the lowest possible numbers
  • When two or more different substituents are present, they are listed in alphabetical order and di, tri prefixes should be used
  • The C6H5- group is known as the phenyl group
• Benzene does not generally undergo addition reactions as this would involve breaking up the delocalised system
• Benzene's reactions are usually electrophilic substitutions as it has a high electron density and so attracts electrophiles
• Benzene is a carcinogen and is banned for use in schools
• Methylbenzene is less toxic and also reacts more readily than benzene as the methyl side group releases electrons into the delocalised system making it more attractive to electrophiles
• Nitration of benzene


  1. Reagents: concentrated nitric acid in the presence of concentrated sulfuric acid (catalyst)

  2. Mechanism: Electrophilic substitution

  3. Electrophile: +NO2

  4. Overall equation for formation of the electrophile: HNO3 + 2H2SO4 → +NO2 + 2HSO4- + H3O+
• This nitration reaction with benzene is done at 60oC. On using higher temperatures a second nitro group can be substituted
• Friedel Crafts acylation


  1. Reagents: acyl chloride in the presence of anhydrous aluminium chloride catalyst

  2. Conditions: heat under reflux (50OC)

  3. Mechanism: Electrophilic substitution

  4. Equation for formation of the electrophile: AlCl3 + CH3COCl → [CH3CO]+ + [AlCl4]-
• Reducing a nitroarene to aromatic amines

  1. Reagent: Sn and HCl or Fe and HCl

  2. Conditions: Heating

  3. Mechanism: reduction
• As the reduction reaction is carried out in HCl, the ionic salt C6H5NH3+Cl- will be formed, which is soluble in water. Reacting this salt with NaOH will give the insoluble phenylamine
• This reduction reaction can also be done with catalytic hydrogenation (H2 using a Ni catalyst)
• Effect of delocalisation on side groups with lone pairs


  If a –OH group, a Cl atom or an NH2 group is directly attached to a benzene ring the delocalisation in the benzene ring will extend to include the lone pairs on the N,O and Cl. This changes the properties and reactions of the side group.
• Chlorobenzene: The C-Cl bond is made stronger. Typical halogenoalkane substitution and elimination reactions do not occur. Also the electron rich benzene ring will repel nucleophiles.
• Phenol: Delocalisation makes the C-O bond stronger and the O-H bond weaker. Phenol does not act like an alcohol- it is more acidic and does not oxidise.
• Phenylamine: Less basic than aliphatic amines as lone pair is delocalised and less available for accepting a proton.
• Aromatic Synthetic Routes

  • Electrophilic substitution: Nitration, Friedel Crafts acylation
  • Nucleophilic addition-elimination: Reaction with acyl chlorides
  • Reduction: Reducing a nitroarene to aromatic amines
  • Other reactions: Esterification, Dehydration