Aromatic Chemistry

Aromatic Chemistry

1. Introduction to Arenes

   • Arenes are hydrocarbons based on benzene C6H6
     • Although benzene is an unsaturated molecule, it is very stable
   • Bonding and Structure
     • The C-C bond length in benzene are intermediate between those expected for a C-C single bond and a C-C double bond
     • Each carbon has three covalent bonds and one delocalised electron in a p-orbital
       • The delocalisation means that electrons are spread over the six carbon atoms forming a ring
         • They form a region of high electron density above and below the ring.
           • The delocalised system makes benzene and its derivatives unusually stable
     • The enthalpy change for the hydrogenation of the Kekule model of benzene with alternative double bonds would be expected to be three times the enthalpy change of the hydrogenation of cyclohexane.
       • However the enthalpy change of benzene is less than this showing that benzene is more stable than the unsaturated ring structure.

2. Physical Properties, Naming and Reactivity

   • Physical Properties of Arenes
     • Benzene is a colourless liquid at room temperature
     • It has a similar boiling point with that of hexane but a much higher melting point the hexane's.
       • This is because benzene's flat,hexagonal molecules pack together very well in a solid state so are therefore harder to seperate
     • Arenes are non-polar so do not mix with water, but mix with other hydrocarbons and other non-polar solvents
   • Naming
   • The Reactivity Of Aromatic Compounds
     • Most reactions are electrophilic substitution reactions because
       • The ring is an area of high electron density, because of the delocalised bonding and is therefore attacked by electrophiles
       • The aromatic ring is very stable. It needs energy to be put in to break the ring before the system is destroyed. It means that the ring almost always stays intact in the reaction of arenes.

3. Reactions of Arenes

   • Combustion
     • Arenes burn in air with flames that are noticebly smoky.
       • This is because they have a high carbon:hydrogen ratio
         • There is usually burned carbon remaining when they burn in air this produces soot
   • Electrophilic Substitution Reactions
     • During electrophilic substitution reactions the electrons are attracted towards the elecrophile and the electrophile attracted to the high electron density
     • The Mechanism of Electrophilic Substitutions
     • Nitration
       • Nitration is the substitution of a NO2 group for one of the hydrogen atoms on an arene ring
         • The electrohile NO2+ is generated in the reaction mixture of concentrated nitric acid and concentrated sulfuric acid.
       • Nitration is and important step in the production of explosives like TNT and making amines
     • Friedal-Crafts Acylation Reaction
       • These reactions use the aluminium chloride as a catalyst
         • The mechanism for acylation is a substitution, with RCO substituting for a hydrogen on the aromatic ring
         • The AlCl3 catalyst is reformed when AlCl4- reacts with H+
       • Acyl chlorides provide the RCO group. They react with AlCl3 to form AlCl4- + RCO+
         • This reaction takes place because the aluminium atom in aluminium chloride has only six electrons in its outer main level and readily accepts a lone pair from the chlorine atom of ROCl

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