benzene

benzene

1. structure

   • C6H6 =mr
   • kekulé structure
     • low reactivity
       • -didn't react with Br water
       • -not contain C=C bonds
       • -not take part in other electrophilic additions
     • C-C bond lengths
       • C-C & C=C different lengths
       • benzene- C-C bonds only as all same lengths
       • kekulé structure wrong
     • hydrogenation
       • cyclohexene (1 C=C) -120 KJ mol-1
       • cyclo-1,3,5-triene (3 C=C) -360 KJ mol-1
       • benzene -208 KJ mol-1
         • more stable then cyclo-1,3,5-triene

2. relative ease of bromination

   • easy- ethene
     • middle- phenol
       • hard- benzene
         • -π e' delocalised in benzene ring -e' density low -can't polarise Br2 -halogen carrier needed for reaction
       • -lone pair of OH partially delocalised in ring -e'density higher than benzene -can polarise Br2
     • -e' localised C=C -e' density is high -can polarise Br2
   • points for the answer
     • order of reactivity (hexene>phenol>benzene)
     • benzene- π e' are delocalised
     • phenol- lone pair is partially delocalised into ring
     • hexene- e' localised between 2 carbons
     • benzene lower e' density than phenol which is lower than hexene
     • benzene can't polarise Br2

3. properties

   • common start of aromatic compounds
   • carsinogen
   • colourless & flamable

4. reactions

   • nitration of benzene
     • conc HNO3 conc H2SO4 50'c
       • C6H6 + HNO3 = C6H5NO2 + H2O
   • halogenation
     • -requires halogen carrier -used as catalyst -carrier same as halogen
   • reducing reaction
     • propanal + reducing agent ( [H] ) = propan-1-ol
     • reducing agent (NaBH4)
       • generates hydride ions (H-)
   • nucleophilic addition