Aromatic Compounds

• Aliphatic


  Straight or branched chain organic substances
• Aromatic or arene


  Includes one or more ring of six carbon atoms with delocalised bonding

  
• Benzene belongs to the aromatic class

  
• 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
  • Six p electrons are delocalised in a ring structure above and below the plane of carbon atoms

  
• Kekule's suggested structure for Benzene consisting of alternate single and double covalent bonds between the carbon atoms is not correct

  
• Evidence suggests all the C-C bonds in Benzene are the same length and have a length and bond energy between a C-C single and C=C double bond

  
• Benzene is a planar molecule

  
• Delocalised


  Not attached to a particular atom
• The H-C-C bond angle is 120o in Benzene

  
• The enthalpy of hydrogenation for Benzene (-208kJ/mol) is less exothermic than the theoretical value (-360kJ/mol) due to delocalisation energy
• Delocalisation energy


  The increase in stability connected to delocalisation
• Evidence for Benzene's delocalised structure: bond length intermediate between C=C and C-C, less exothermic hydrogenation, only reacts with Br2 at high temp or in presence of a halogen carrier

  
• Phenyl group


  The C6H5- group
• Benzene does not generally undergo addition reactions because these would involve breaking up the delocalised system
• Benzene's reactions are usually electrophilic substitutions

  
• 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: conc nitric acid in the presence of concentrated sulfuric acid (catalyst)

  2. Mechanism: Electrophilic substitution

  3. Electrophile: NO2+

  4. Equation for Formation of electrophile: HNO3 + 2H2SO4 → NO2+ + 2HSO4- + H3O+
• Nitration of benzene and other arenes is an important step in synthesising useful compounds like explosives and dyes
• Halogenation of benzene


  1. Reagents: bromine, iron(III) bromide catalyst FeBr3

  2. Mechanism: Electrophilic substitution

  3. Equation for Formation of electrophiles: AlCl3 + Cl2 → AlCl4– + Cl+, FeBr3 + Br2 → FeBr4– + Br+
• Friedel Crafts Alkylation


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

  2. Conditions: heat under reflux

  3. Mechanism: Electrophilic substitution

  4. Formation of the electrophile: AlCl3 + CH3CH2Cl → CH3CH2+ AlCl4-
• 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. Formation of the electrophile: AlCl3 + CH3COCl → CH3CO+ AlCl4-
• Phenol


  Compound where the OH group is directly attached to the benzene ring

  
• Phenols are very weakly acidic, weaker acids than carboxylic acids
• Reaction of phenol with bromine

  1. Reagents: Br2

  2. Conditions: room temp

  3. Product: 2,4,6-tribromophenol
• Phenols are used in the production of plastics, antiseptics, disinfectants and resins for paints
• Reaction of phenol with nitric acid

  1. Reagent: 4M HNO3

  2. Conditions: room temp

  3. Products: 2-nitrophenol, 4-nitrophenol
• Electron-donating groups like OH, NH2 will force further substitutions to occur on the 2- and 4- positions of the benzene ring
• Electron-withdrawing groups like NO2 will have a 3-directing effect in electrophilic substitution of aromatic compounds
• Delocalisation in side groups with lone pairs


  Extends to include the lone pairs on N, O and Cl, changing the properties and reactions of the side group