Benzene
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give the formula for benzene
C6H6
why was the bonding and structure of benzene a mystery to chemists (2 marks)
• it is unsaturated but doesn’t undergo addition reactions • bond lengths are all equal due to delocalised system • benzene is more thermodynamically stable than expected (enthalpy of hydrogenation less than expected so compound is more thermodynamically stable)
how many delocalised pi orbitals are there in benzene
6
describe the shape and bonding in a benzene molecule
• planar • 120 degrees c-c bond angle • c-c bond lengths are all the same • single c-c (sigma) bonds • delocalised Pi electrons above and below due to sideways overall of P orbitals
Compare the stability of benzene with the hypothetical molecules hexa-1,3,5-triene. Use the data below in your answer (3 marks) benzene H = -120 hexa-1,3,5-triene H = -208
predict 3 x -120 = -360 for benzene benzene less exothermic because delocalised Pi electrons are stable and require more energy to break
how is the delocalised ring in benzene formed
each carbon donates an electron which forms a delocalised ring