《不饱和碳氢化合物》PPT课件.ppt

第三章,不饱和碳氢化合物 Unsaturated Hydrocarbons,第三章重点内容,一、不饱和碳氢化合物的命名 二、烯烃的稳定性 三、双烯烃的结构与稳定性 四、苯的结构与稳定性 五、芳香性与Hckel规则 六、电子离域与共振 - 共振论 七、分子轨道理论及其应用,烯烃、炔烃和芳香烃,共轭二烯烃,7 6 5 4 3 2 1,2 4 6 8 10 12,1 3 5 7 9 11,1 2 3 4 5 6 7 8 9,9 8 7 6 5 4 3 2 1,2 3 4 5 6 7,1,7 6 5 4 3 2 1,3 4 5 6 7,2 1,1,2,4,1 3 5 7,2 4 6,氧化-还原反应,取代反应,3.3 烯烃的稳定性,化合物的稳定性 - 越稳定，越不容易发生化学反应；反过来，反应中更容易形成。 - 不稳定，表示化合物处在高能量状态，容易发生化学变化(并非绝对)。 燃烧热（Hc ） - CO2 + H2O + Hc - 燃烧热越高，化合物越不稳定,3.3 烯烃的稳定性,生成热（ Hf ） - nC + (n+1)H2 CnH2n+2 + Hf - 生成热越高，化合物越稳定,氢化热（ Hh ) - 双键 + nH2 饱和烃 Hh - 判断双键的稳定性 - 氢化热越高，化合物越不稳定,取代烯烃的稳定性 可由氢化热来判断。如： 单取代末端双键的稳定性 双取代末端双键 三取代双键 四取代双键,3.3 烯烃的稳定性,3.4 双烯烃的结构与稳定性,Class of dienes,The difference between the heats reflects the difference in stability between the nonconjugated and conjugated dienes conjugated isolated cumulated dienes,3.4 双烯烃的结构与稳定性,Stability of dienes,3.5 苯的结构与稳定性,An unusually large resonance energy (共振能): 151kJ/mol,Benzene - particularly stable because of the large resonance energy But cyclobutadiene and cyclooctatetraene contains cyclic two or four -bond, they are not stable.,3.6 Aromaticity and Hckels Rule,轮烯,3.6 Aromaticity and Hckels Rule,In 1938, a German chemist Erich Hckel recognized the difference between them and generalized this observation into Hckels Rule - any plannar, cyclic, conjugated system containing (4n+2) electrons (n=1, 2, 3, .) experiences unusual aromatic stabilization,It is also called “4n + 2 rule” 有4n + 2个p电子的环烯烃具有芳香性。 有4n 个p电子的环烯烃具有反芳香性和非芳香性。,3.6 Aromaticity and Hckels Rule,Questions,Why is a conjugated diene more stable than an isolated diene? Why is benzene a particularly stable compound? Why is cyclobutadiene and cyclooctatetraene not stable?,Explanation 1- 价键理论,Single bond between C-C formed by sp2-sp2 overlap, the length of the bond is shorter, therefore the bond is stronger. CH3CH3 Csp3-Csp3 154 pm CH2=CHCH3 Csp2-Csp3 150 pm CH2=CHCH=CH2 Csp2-Csp2 146 pm C6H6 Csp2-Csp2 139 pm,Explanation for Stability of Dienes,conjugated isolated dienes Bond length: Csp2 Csp2 Csp2 Csp3,isolated cumulated dienes sp hybridization, it is similar to alkyne (weaker p bond),3.5 苯的结构与稳定性,1. All the carbon are sp2 hybridized (ring plane) 2. The p orbital on each carbon can overlap with two adjacent p orbitals. 3. Particularly stable,丁二烯分子轨道能级图,Expanation 2 分子轨道理论,（2）苯分子轨道能级图,分子轨道理论阐述苯的结构,（2）苯分子轨道能级图 6个电子分布在成键的分子轨道上,Expanation 2 分子轨道理论,（3）cyclooctatetraene 两个在非键轨道的单电子，相当于双自由基，应具有反芳香性，但实际上不是平面分子，属于非芳香分子。,Expanation 2 分子轨道理论,（4）其他元环的分子轨道能级图,Expanation 2 分子轨道理论,（5）其他具芳香性的分子或离子,Expanation 2 分子轨道理论,3.6 Aromaticity and Hckels Rule,请思考： 为什么10-轮烯没有芳香性？ 而1,6-桥连10-轮烯具有芳香性？ 2. 14-轮烯，18-轮烯，22-轮烯又如何？,Explanation 3 - 共振理论,电子离域与共振 Electron Delocalization and Resonance,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或共振极限结构来表示。,p-electrons in conjugated dienes and benzene are delocalized 定域电子：被限制在两个原子核区域内运动的电子 离域电子：不局限于两个原子核区域内运动的电子,Explanation 3 - 共振理论,Not real structure,Real structure,Explanation 3 - 共振理论,Some basic concepts A compound with delocalized electrons is said to have resonance（共振） The approximate structure using localized electrons is called a resonance contributor, a resonance structure, or a contributing resonance structure（共振结构或共振式） The actual structure, drawn using delocalized electrons, is called a resonance hybrid（共振杂化体）,How to draw resonance contributors,Rules for Drawing Resonance Contributors Only electrons move. The nuclei of the atoms never move The only electrons that can move are electrons and nonbonding electrons The total number of electrons in the molecule does not change, neither do the numbers of paired and unpaired electrons,In order to draw contributors, the electrons in one resonance contributor are moved to generate the next contributor.,The electrons can be moved in one of the following ways,1. Move electrons toward a positive charge,The electrons can be moved in one of the following ways,2. Move electrons toward a bond,3. Move a nonbonding pair of electrons toward a bond,The electrons can be moved in one of the following ways,4. Move a single nonbonding electron toward a bond,The electrons can be moved in one of the following ways,共振杂化体比它的任何一个共振式都要稳定 共价键数目越多的共振式越稳定 共振式中的电荷越分散就越稳定 越稳定的共振式对共振杂化体