Unit 4: Module 1 - Rings, Acids and Amines

Unit 4: Module 1 - Rings, Acids and Amines

1. BENZENE: Structure

   • Formula: C6H6
   • Cyclic
     • Two ways of representing it...
       • Kekule's
         • If it was correct..
           • 3 x length of C-C bonds (147pm)
           • 3 x length of C=C bonds (135pm)
           • HOWEVER
             • X-ray diffraction have shown all the bond lengths have the same value (140pm)
           • Hydrogenation Enthalpy Change of C=C bond is -120 Kj/mol
             • so with 3 C=C bonds, enthalpy change should be -360 Kj/mol
               • HOWEVER
                 • Experimental enthalpy change is -208 Kj/mol...... Far less exothermic than expected
                   • Indicates that Benzene is more stable than the Kekule structure would be. This is thought to be due to the delocalised electrons.
       • Delocalised Model
         • P-Orbitals of all 6 Carbon atoms overlap to create Pi Bonds
           • 3 Electrons in the top ring, 3 in the bottom ring (due to the repulsion of negative charges)

2. BENZENE: Aromatic Compounds

   • Compounds containing a benzene ring are called arenes or 'aromatic compounds'
     • Chlorobenzene
     • Nitrobenzene
     • 1,3-dimethylbenzene
   • Others are named as compounds with a phenyl group (C6H5) attached
     • Phenol
     • 2-methylphenol
     • phenylamine

3. Reactions with Benzene

   • Alkenes react easily with Bromine water (the famous experiment where Br water decolourises)
     • If the Kekule model was correct, Benzene would be expected to react similarly (due to the C=C bonds)
       • Because of the electron rings above and below the carbon atoms, it makes the benzene ring VERY staable, and spreads out the negative charge.
         • This means Benzene is very unwilling to undergo addition reactions which would destroy the stable ring
           • Instead, benzene prefers to react by electrophilic substitution
             • Electrophile: Positively charged ions or polar molecules that are attracted to ares of high electron density
             • Arenes undergo Electrophilic Substitution with....
               • With Nitric Acid and a Catalyst (this catalyst is Sulfuric acid)
                 • Sulfuric acid (how to make the NO2+)
               • HALOGENS using a HALOGEN CARRIER
                 • PICTURE OF BROMINE OR CHLORINE REACTING WITH BENZENE IS TOO BLOODY HARD TO FIND - JUST LEARN IT

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