有机化学课件——芳烃.pdf

Chapter 8 Arenes Aromatic hydrocarbons . Often abbreviated ArH. Benzene Ar- : an aryl group. Aliphatic hydrocarbons: compounds that do not contain benzene or benzene-like rings as structural units. (Historically, aliphatic was used to describe compounds derived from fats and oils.) §8.1 Benzene and its homologs Kekulé and the structure of benzene In 1865, August Kekulé proposed structure of benzene based on three premises: 1. Benzene is C H . 6 6 2. All the hydrogens of benzene are equivalent. 3. The structural theory requires that there be four bonds to each carbon. Different!? Benzene is not cyclohexatriene, nor is it a pair of rapidly equilibrating cyclohexatriene isomers. What is it about benzene that makes it behave so much differently from other unsaturated compounds? The 140-pm bond distances in benzene are exactly midway between the typical sp2–sp2 single-bond distance of 146 pm and the sp2–sp2 double-bond distance of 134 pm. British chemist Sir Robert Robinson: benzene is regarded as a hybrid of the two Kekulé structures. Aromaticity: Special stability associated with aromatic compounds. (100%) (2–3 atm pressure) resonance energy ) l o m / J K ( An orbital hybridization view of bonding in benzene (a) The framework of bonds (a) shown in the tube model of benzene are σbonds. (b) Each