6.1 Aromatic compounds, carbonyls and acids

• In 1865 August Kekule suggested a six-membered Carbon ring of alternating double and single bonds.
• A problem of Kekule Benzene is that it has double bonds so you would assume that Benzene would behave like Alkenes but it doesn't
• Anything with C=C bonds should decolourise Bromine water
• Benzene does not decolourise bromine water because the delocalised electrons are shared between all the carbon atoms making them more stable than an alkene
• Bromine water is used to test alkenes
• Benzene doesn't do this and it doesn't undergo Addition reactions with HCl or HBr in the way that an Alkene would.
• Benzene nearly always undergoes Substitution reactions - a Hydrogen is replaced.
• If Kekule was correct then completely Hydrogenating Benzene - (adding 3 moles of Hydrogen) should release around three times as much energy - approximately 360 kJ/mol.
• Benzene releases 152 kJ/mol less energy than expected for Kekule Benzene when hydrogenated
• We can say that actual Benzene is more stable than Kekule's Cyclohexatriene would have been.
• we can interpret from the fact that benzene releases less energy than expected , that the bonding in actual Benzene being stronger than expected.
• If Benzene had alternating single and double bonds then the bond lengths would alternate between around 0.154 nanometres for C-C bonds and 0.134 nanometres for C=C bonds.
• X-ray crystallography demonstrates that all the bond lengths are equal at 139 nm. And all the bond angles are the same in a perfect hexagon.
• A C=C double bond consists of a sigma bond (overlapping s orbitals) and pi bond (overlapping p orbitals).
• If all the Carbons are equally spaced then all the p-orbitals overlap above and below the plain of the molecule.

  
• electrons are free to move around in a ring of de-localised pi-electrons
• When there is a Nitrile functional group

  We're not taught a way to name Nitriles as side-chains so we use the prefix Hydroxy to name the alcohol as a sidechain instead.
• If we add functional groups that are usually suffixes then they continue to do so and the new compound will end with Benzene.
• If a functional group has to be used as a suffix ( for example amine and alcohol) we can't use benzene as a suffix and we are left with phenyl as a prefix.
• The only significant exception is Benzoic Acid.

  which we might reasonably expect to start with the Phenyl prefix since the -oic acid part is a suffix.
• No numbering is required for methylbenzene since the Methyl group would be one C1 wherever we chose to draw it.
• Benzene rings are energetically stable due to the delocalisation energy, addition reactions would involve breaking the ring but not re-forming it.
• it is difficult to make aromatic compounds undergo addition.
• substitution can happen if the conditions for breaking the ring are met.
• Benzene rings don't polarise as much as Alkenes. This makes them less attractive to normal electrophiles than an Alkene would be.
• to make aromatic compounds undergo electrophilic substitution requires an electrophile with a positive charge.
• The broken ring must cover 4 of the C-C bonds
• The source of NO2 would be Nitric Acid (HNO3) but acids dissociate to make H+ ions and an anion.

  HNO3 --> H+ + NO3-
• NO3- ion would never be able to draw electrons out of the ring.
• we need a substance that can stop Nitric Acid behaving in a typically acidic way. (stronger acid)
• H2SO4 + HNO3 --> HSO4- + H2NO3+
• The unsightly looking protonated Nitric Acid then breaks up.

  H2NO3+--> H2O + NO2+
• A Halogen-carrier is required - a substance that will remove Br- (or Cl-) from Bromine (or Chlorine) and leave a Br+ or a Cl+ ion.