Unusual Stability Hydrogenation of just one double bond in benzene is endothermic (85.8 predicted) 36 kcal (57.2 predicted) resonance energy energy +1. 8 kcal resonance energy 574 kcal 55.4 kcal 49.8 kcal 28.6 energy
Chapter 16 6 Unusual Stability Hydrogenation of just one double bond in benzene is endothermic! =>
H Annulenes All cyclic conjugated hydrocarbons were proposed to be aromatic However, cyclobutadiene is so reactive that it dimerizes before it can be isolated And cyclooctatetraene adds br read lily Look at Mo s to explain aromaticity Chapter 16
Chapter 16 7 Annulenes • All cyclic conjugated hydrocarbons were proposed to be aromatic. • However, cyclobutadiene is so reactive that it dimerizes before it can be isolated. • And cyclooctatetraene adds Br2 readily. • Look at MO’s to explain aromaticity. =>
MO Rules for benzene Six overlapping p orbitals must form six molecular orbitals Three will be bonding three antibonding LoWest energy MO will have all bonding interactions, no nodes As energy of Mo increases, the number of nodes increases Chapter 16
Chapter 16 8 MO Rules for Benzene • Six overlapping p orbitals must form six molecular orbitals. • Three will be bonding, three antibonding. • Lowest energy MO will have all bonding interactions, no nodes. • As energy of MO increases, the number of nodes increases. =>
H MO’ s for benzene all antIbonding node 4 node antibonding no node bonding -node丌 all bonding
Chapter 16 9 MO’s for Benzene =>
Energy Diagram for Benzene The six electrons fill three bonding pi orbitals All bonding orbitals are filled (closed shell) an extremely stable arrangement energy T nonbonding li 10
Chapter 16 10 Energy Diagram for Benzene • The six electrons fill three bonding pi orbitals. • All bonding orbitals are filled (“closed shell”), an extremely stable arrangement. =>