6.1 Aromatic Compounds

• There are two major classes of organic chemicals:

  • Aliphatic: straight or branched chain organic substances
  • Aromatic or arene: includes one or more ring of six carbon atoms with delocalised bonding
• All organic substances looked at so far have been aliphatic
• Benzene belongs to the aromatic class
• Benzene's structure:

  • Molecular formula: C6H6
  • Basic 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
  • Leaves 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
• In 1865, Kekule suggested a structure for Benzene consisting of alternate single and double covalent bonds between the carbon atoms, but this structure is not correct
• Benzene is a planar molecule with all C-C bonds being the same length and having a length and bond energy between a C-C single and C=C double bond
• The H-C-C bond angle in Benzene is 120°
• Enthalpies of Hydrogenation:

  • Cyclohexene: -120 kJ/mol
  • Cyclohexane: -360 kJ/mol
  • Theoretical value for benzene: -208 kJ/mol
  • Actual value for benzene: -152 kJ/mol
  • Delocalisation energy makes delocalised benzene more thermodynamically stable
• Summary of evidence for why benzene has a delocalised structure:

  • Bond length intermediate between short C=C and long C-C
  • ΔH hydrogenation less exothermic than expected compared to ΔH hydrogenation for Kekule structure
  • Only reacts with Br2 at high temp or in presence of a halogen carrier
• Naming aromatic molecules:

  • Derivatives of benzene have benzene at the root of the name
  • Examples: Methylbenzene, ethylbenzene, chlorobenzene, bromobenzene, nitrobenzene, benzenecarboxylic acid, benzaldehyde
• When two or more substituents are present on the benzene ring:

  • Positions must be indicated by numbers to give the lowest possible numbers
  • Different substituents are listed in alphabetical order with di, tri prefixes used
  • Examples: 1,3-dimethylbenzene, 1-chloro-4-methylbenzene, 4-hydroxybenzenecarboxylic acid
• The benzene ring can be regarded as a substituent side group on another molecule, known as the phenyl group
• Reactions of benzene:

  • Benzene does not generally undergo addition reactions
  • Most reactions involve substituting one H for another atom or group of atoms
  • Reactions are usually electrophilic substitutions
• Toxicity of benzene:

  • Benzene is a carcinogen and banned for use in schools
  • Methylbenzene is less toxic and reacts more readily than benzene due to the methyl side group releasing electrons into the delocalised system
• Nitration of benzene:

  • Important step in synthesising useful compounds like explosives and amines
  • Reagents: conc nitric acid in the presence of concentrated sulfuric acid (catalyst)
  • Mechanism: Electrophilic substitution
  • Electrophile: NO2+
• On using higher temperatures, a second nitro group can be substituted onto different positions on the ring
• Higher temperatures can lead to a second nitro group being substituted onto different positions on the benzene ring
• If the benzene ring already has a side group, such as methyl, the nitro group can also join on different positions
• Change in functional group: benzene → bromobenzene
• Reagents: bromine
• Conditions: iron(III) bromide catalyst (FeBr3)
• Mechanism: Electrophilic substitution
• This reaction can also be done with chlorine
• Halogenation of Benzene
• Reagents: Br2
• Overall Equation for reaction: Br2 + HBr → 2,4,6 – tribromophenol + 3 HBr
• Phenol does not need a FeBr3 catalyst like benzene and undergoes multiple substitutions, whereas benzene will only add one Br
• Phenols are used in the production of plastics, antiseptics, disinfectants, and resins for paints
• Reaction of Phenol with Nitric acid
• With 4M HNO3, single substitution occurs
• Reagent: 4M HNO3
• Conditions: room temperature
• Effect of side groups on substitution
• Electron-donating groups like OH, NH2 will force further substitutions to occur on the 2- and 4- positions of the ring
• Electron-withdrawing groups (such as NO2) will have a 3-directing effect in electrophilic substitution of aromatic compounds
• 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
• Aldehydes are organic compounds that contain the functional group -CHO.
• Aromatic compounds contain one or more benzene rings in their structure.