4.2

• Benzene has the molecular formula C6H6
• Kekule's original suggestion in 1865 was that benzene was a six-membered ring with alternating single and double bonds
• Reasons why Kekule's structure was proved incorrect:

  • Benzene does not undergo addition reactions like compounds with double bonds
  • Each C-C bond length in benzene is equal, shorter than a C-C single bond and longer than a C=C double bond
  • The enthalpy of hydrogenation for benzene is -208 kJ mol-1, less than the expected -360 kJ mol-1 for three double bonds. The difference of 152 kJ mol-1 is called resonance energy, indicating a more stable structure
• Benzene is a planar molecule with bond angles of 120° between three adjacent carbon atoms
• Each carbon atom in benzene is bonded to two other carbon atoms and a hydrogen atom by σ bonds, with the fourth outer shell electron in a 2p orbital above and below the carbon ring
• The p orbitals overlap in benzene, forming π bonds that give a delocalised electron structure above and below the plane of the ring
• Benzene's delocalised electron structure makes it more stable, known as 'delocalisation' or 'resonance' energy
• Benzene does not undergo addition reactions like alkenes because it would disrupt the stable delocalised π electron system
• Electrophile in nitration of benzene is the nitronium ion, NO2+
• Benzene reacts with bromine in the presence of an iron(III) bromide catalyst in a halogenation reaction
• Chloroalkanes react with aqueous sodium hydroxide in a nucleophilic substitution reaction to give alcohols
• Chlorobenzene does not react with aqueous sodium hydroxide due to the stable π system of electrons repelling nucleophiles
• Phenols are more acidic than alcohols because the phenoxide formed is stabilised by delocalisation of negative charge
• Phenylamine is less basic than alkyl amines due to interaction of the lone pair of electrons on nitrogen with the delocalised electrons in the benzene ring
• Friedel-Crafts alkylation involves the reaction of benzene with a halogenoalkane using AlCl3 as a catalyst