6.1.1: BENZENE & ITS STRUCTURE

6.1.1: BENZENE & ITS STRUCTURE

1. Benzene

   • naturally occurring aromatic hydrocarbon
   • ring structure
   • simplest of arene homologous series
   • molecular formula: C6H6
   • empirical formula: CH
   • liquid @ room temp

2. Kekule's Model

   • suggested benzene was a 6 C ring with alternating single & double bonds
   • one group added= one isomer made
   • two groups added= 3 isomers made
   • proved incorrect
     • resistant to addition reactions (unlike alkenes)
       • due to double bonds would expect similar reactions to alkenes
       • benzene undergoes substitution of H atoms
     • enthalpy of hydrogenation means benzene more stable than predicted
       • addition of H to unsaturated chemical
       • enthalpy change for hydrogenation of benzene higher (by -208kj mol-1) than for cyclohexane
         • more energetically stable
     • all C bonds same length
       • x ray diffraction shows all bond lengths equal; kekule's model indicates single and double bonds would have difference lengths

3. Delocalised Structure

   • delocalised e- structure
   • explains all 3 experimental evidence
   • all bonds identical and same length
   • benzene very stable as a lot of energy reacted to sirupt delocalisation
   • each C donates e- from p orbital that combine to form a ring of e- above and below plane
     • e- move freely