Aromatic compounds

• Benzene is the simplest aromatic compound
• Benzene was first discovered in 1825 by English scientist Michael Faraday, isolated and identified it in an oily residue from the gas used for street lighting
• Benzene is:

  1. a colourless, sweet smelling, highly flammable liquid
  2. found naturally in crude oil, is a component of petrol, and also found in cigarette smoke
  3. classified as a carcinogen, that is it can cause cancer
• A benzene molecule consists of a hexagonal ring of 6 carbon atoms, with each carbon atom joined to 2 other carbon atoms and to one hydrogen atom
• benzene is classed as an aromatic hydrocarbon or arene
• structure of benzene is shown in 2 ways
• derivatives of benzene:

  • historically aromatic was the term used to classify the derivatives of benzene as many pleasant-smelling compounds contained a benzene ring
  • many odourless compounds have been found to contain a benzene ring yet the term aromatic is still used to classify these compounds
  • many aromatic compounds can be synthesised from benzene
• the kekule and delocalised models of benzene:

  • for many yeras scientists attempted to establish a structure for benzene taking into account its molecular formula and the experimental evidence at the time
  • molecular formula C6H6 suggested a structure containing many double bonds or a structure containing double and triple bonds
  • compounds containing multiple bonds known to be very reactive - benzene appeared to be unreactive
• the Kekule model:

  • german chemist Friedrich august kekule suggested that the structure of benzene was based on a 6 membered ring or carbon atoms joined by alternate single and double bonds
  • claimed he had thought of the ring shape whilst daydreaming about a snake seizing its own tail
• evidence to disprove:

  • model unable to explain all of Benzene's chemical and physical properties
  • reactivity of benzene
  • lengths of the c-c bonds in benzene
  • hydrogenation enthalpies
• lack of reactivity of benzene:

  • if benzene contained the c=c bonds it should decolourise bromine in an electrophilic addition reaction however
  • benzene does not undergo electrophilic addition reactions
  • benzene does not decolourise bromine water under normal conditions
  • suggests benzene does not have any c=c bonds in structure
• lengths of c-c bonds in benzene:

  • using X-ray diffraction - possible to measure bond lengths in a molecule
  • Kathleen Lonsdale found in 1929 - all bonds in benzene were 0.139 nm in length
  • between the bond length of a double 0.134 nm and single bond 0.139
• hydrogenation enthalpies:

  • kekule structure containing alternate single and double bonds could be given the name cyclo-1,3,5-triene to indicate positioning of the double bonds
  • if had this structure - would expect to have an enthalpy change of hydrogenation 3x that of cyclohexane
  • when cyclohexane is hydrogenated one double bond reacts with hydrogen - enthalpy change hydrogenation is -120KJmol-1
• • kekule model predicted to contain 3 double bonds - expected enthalpy change for reacting 3 bonds with hydrogen would be 3x-120 = -360 KJ mol-1
  • actual enthalpy change hydrog is only -208KJmol-1 - 152 KJmol-1 less energy produced than expected
  • actual structure is therefore more stable than theoretical model
• the delocalised model of benzne:

  • developed by scientists who decided that the experimental evidence was sifficient to disprove the kek structure
• main features:

  • benzene is a planar, cyclic, hexaganol hydrocarbon containing 6 C and 6 H
  • each C used 3 of its available 4 electrons to bond to 2 other C and one H
  • each C has one electron in a p-orbital at right engles to plane of bonded C and H atoms
  • adjacent p-orbitals overlap sideways in both directions above and below plane of C atoms - ring of electron density
  • overlapping of the p-orbitals creates system of pi-bonds which spread over all 6 carbon atoms in ring structure
  • 6 electrons occupying system of pi-bonds said to be delocalised
• some groups shown as prefixes to benzene - short alkyl chains, halogens and nitro groups
• aromatic componds with 1 substituent group are monosubstituted.

  in aromatic compounds the benzene ring is often considered to be parent chain

  alkyl, halogens and nitro groups all considered to be prefixes
• when a benzene ring is attached to an alkyl chain with a functional group or to an alkyl group with 7 or more carbon atoms - benzene is considered a substituent

  prefix phenyl is used
• as with all systems there are some noticeable exceptions:

  • benzoic acid (benzenecarboxylc acid)
  • phenylamine
  • benzaldehyde (benzenecarbaldehyde)
• some compounds may contain more than one substituent group on the benzene ring e.g. disubstitued groups have 2 substituent groups

  ring is now numbered starting with one of the substituent groups

  alphabetical order using smallest numbers possible