The benzene ring containssxequally overlapping p Byarogenssnecalwsable:heasod w ○ 6R。 《草心@ hstethamolecueont 15-3 Molecular Orbitals of Benzene 8ara8aastheenerovofbenaenea 1.3.5-hexatriene be compared to those 西 aromatic transitions 字望 eweg 3 3 The benzene ring contains six equally overlapping p orbitals. Experimentally, benzene can be shown to be a completely symmetrical hexagon; there are no alternating single (long) and double (short) bonds. The electronic structure of the benzene ring consists of 6 sp2 carbon atoms, with each p-orbital overlapping the p-orbitals of its nearest neighbors. Benzene is especially stable: heats of hydrogenation. It is useful to compare the heats of hydrogenation of benzene, 1,3-cyclohexadiene and cyclohexene. In each case, cyclohexane is the final product. The heat of hydrogenation of benzene can be experimentally measured using special catalysts and is measured to be 29.6 kcal mol-1 less than the calculated heat for the hypothetical molecule, 1,3,5-cyclohexatriene. Benzene is much more stable than a molecule containing alternating double and single bonds. The difference in stability is called the resonance energy of benzene, about 30 kcal mol-1. 15-3 π Molecular Orbitals of Benzene Cyclic overlap modifies the energy of benzene’s molecular orbitals. The molecular π orbitals of benzene can be compared to those of 1,3,5-hexatriene: The cyclic π system is stabilized relative to the acyclic one. Two of the orbitals have been lowered in energy in benzene (π1 and π3) while one has been raised (π2). The drop in energy of π1 and π3 more than offsets the rise in energy of π2. Some reactions have aromatic transitions states. The transition states in the Diels-Alder reaction, the addition of osmium tetroxide to alkenes and the first step in ozonolysis all exhibit a cyclic overlap of six electrons in π orbitals or orbitals having π character: These concerted aromatic transition states have lower energies than the alternative sequential bond-breaking and bond-making mechanism