aromatic chemistry

• sigma bond:

  • electrons in orbitals that are pointing towards each other
  • good overlap and electron density is in a small area
  • stronger bond
• Pi bond:

  • electrons in orbitals that are parallel to each other
  • less overlap so electron density is more spread out
  • weaker bond
• benzene is a cyclic hydrocarbon with the chemical formula C6H6
• kekule structure - cyclohexane-1,3,5-triene
• x-ray crystallography -

  • the bond lengthes measured did not match the structure
• thermochemical evidence -

  • energy released during hydrogenation was not what was expected
• chemical evidence -

  • benzene does not decolurise bromine water
• x-ray crystallography evidence -

  the bond length of all carbon to carbon bonds in benzene is an intermediate of the 2
• thermochemcial evidence -

  • the actual enthalp is lower than expected, it is energetically less favourable
• modern structure of benzene

  • all bonds the same length
  • delocalsied electrons
  • planar ring shape
  • all bonds anles are 120
• current ideas about bonding in benzene structure

  • each C atom uses 3 electrons to amke strong sigma bonds (2 to C atoms and 1 to H atoms)
  • 4th electron is in a p orbital perpendicular to the ring
  • overlap of these p orbitals forms a system pi bonds which spread the electron density of these electron across the whole structure
• benzene structure

  • delocalised electrons stabalise the strutcure
• reactions of benzene and its derivatives

  • the stability of the delocalised system in the benzene ring is highlighted by the comparison of thermochemcial data fromt he hydrogenation cyclohexene and benzene
• reactions of benzene

  • due to the stability of the delocalised ring structure benzene and its derivatives do not readily take part in typical reaction of alkenes such as addition reactions
  • therefore benzene does not decolourise bromine water
• electrophilic subsitiution mechanism

  • benzene and its derivative react with electrophiles via a subsitiution reaction - must be a powerful electrophile
  • this retains stability of the delocalised system
• nitration of benzene

  • refulx at 50C concentrated HNO3 and concentrated H2SO4
• nitration of benzene

  HNO3 + H2SO4 --> NO2+ + HSO4- + H2O
• uses of nitration

  • nitration produces compounds that are useful in the synthesis of other compounds
  • amines and dyes/ expolsives
• uses of nitration - amoines and dyes

  • nitrobenzene can be reduced using Sn and HCL under reflux to fomr phenylamine
  • phenyl amine and other aromatic amines are used to make dyes
• uses og nitration - explosives

  • nitro compiunds decompose violently when heated
  • TNT (1,4,6-trinitromethylbenzene) is made by adding multiplt nitro groups to methylbenzene
• dreidel-crafts acylation -

  • electrophile = acylium ion
  • reacting an acyl chloride with AlCl3 catalyst
• generating an electrophile

  • RCOCl + ALCl3 --> RCO+ + [AlCl4]-
• friedel crafts acylation -

  creates a phenylketone