第2章：碳氢化合物.ppt

2.2 碳氢化合物的稳定性 2.2.5 Aromatic hydrocarbons Benzene, with relatively few hydrogens comparing with aliphatic compound, is a particularly stable compound because it has an unusually large resonance energy 151kJ/mole) Questions: Why is a conjugated diene more stable than an isolated diene? Why is benzene a particularly stable compound? 2.3 共轭烯烃、芳香性与共振论和分子轨道理论 Conjugated Dienes 1. Single bond between C2-C3 formed by sp2-sp2 overlap, the length of the bond is shorter, therefore the bond is stronger. CH3--CH3 Csp3-Csp3 154pm CH2=CH--CH3 Csp2-Csp3 150pm CH2=CH--CH=CH2 Csp2-Csp2 146pm 2.3 共轭烯烃、芳香性与共振论和分子轨道理论 2. Structure of Conjugated Dienes: 3. The conjugated diene had delocalized electrons. We can draw the resonance structure: resonance hybrid: 2.3 共轭烯烃、芳香性与共振论和分子轨道理论 分子轨道理论阐述1，3-丁二烯的结构 2.3 共轭烯烃、芳香性与共振论和分子轨道理论 Structure of Benzene 1. All the carbon are sp2 hybridized 2. The p orbital on each carbon can overlap with two adjacent p orbitals. 3. p-electrons in benzene are delocalized, we call such compound to have resonance. 2.3 共轭烯烃、芳香性与共振论和分子轨道理论 电子离域与共振 上述结构较好地表示了电子的离域，但是却无法告诉我们该结构中含有多少双键。因此化学家常用共振结构来表示。 2.3 共轭烯烃、芳香性与共振论和分子轨道理论 Some basic concepts 1. A compound with delocalized electrons is said to have resonance 2. The approximate structure using localized electrons is called a resonance contributor,a resonance structure, or a contributing resonance structure 3. The catual structure,drawn using delocalized eletrons, is called a resonance hybrid. 2.3 共轭烯烃、芳香性与共振论和分子轨道理论 Rules for Drawing Resonance Contributors 1. Only electrons move. The nuclei of the atoms never move 2. The only electrons that can move are π electrons and nonbonding electrons 3. The total number of electrons in the molecule does not change, neither do the numbers of paired and unpaired electrons 2.3 共轭烯烃、芳香性与共振论和分子轨道理论 4. The electrons can be moved in one of the following ways 1) Move πelectrons toward