Aromatic Chemistry

• cyclohexa-1,3-diene is more stable than cyclohexa-1,4-diene because the c=c bonds are in closer proximity to each other, this means that there is an overlap of p orbitals, and therefore extra stability
• how do you use data to state and explain the stability in benzene compared with the hypothetical cyclohexatriene?

  • benzene is more stable than cyclohexatriene
  • the expected ΔH hydrogenation of benzene is 3 x (-120)= -360kJmol^-1
  • the actual ΔH hydrogenation is -208kJmol^-1, which is 152kJmol^-1 LESS exothermic
  • this is because of delocalisation
• compounds with a ring of delocalised electrons are weak bases, as they do not readily donate electrons
• the reagents for electrophilic substitution of benzene are conc HNO3 and conc H2SO4
• formation of the NO2+ ion?

  HNO + 2H2SO4 ----> NO2+ + 2HSO4- + H3O+
• which base is weaker, ammonia or phenylamine?

  phenylamine is a weaker base because the lone pair on the N atom is less available, as it is delocalised into the ring
• nucleophiles are repelled by the ring of delocalised electrons