天津大学,高等有机化学,第二章芳香性.ppt

1、,第二章 芳香性 (Aromaticity),Aromaticity is invented to account for the unusual stability of an important class of organic molecules with cyclic conjugation systems,Qualification: Cyclic conjugation system; 4n+2 electrons; Coplanar,新定义：纬向分子（diatropic)维持抗磁环电流,2.1 芳香化合物,（一）苯 （benzene),一、含苯芳香体系 (benzene arom

2、atic hydrocarbon system),稳定性：,（二）稠环芳烃,萘（naphthalene） 共振能：255KJ/mol,1.36,1.415,蒽（anthracene） 共振能 ：351KJ/mol,菲（phenanthrene） 共振能：385KJ/mol,二、杂芳香体系 （heterocyclic aromatic system）,121,杂芳香化合物中键长并未完全平均化,三、芳香离子 （aromatic ion）,Resonance energy: 100-113KJ/mol,mp=221,=0.8 D 6.3 D,四、 多电子芳香体系 4n+2电子,里氢 0.00ppm,

3、 -7.88ppm, 外氢 7.6ppm,Planarity Resonance energy: 155 KJ/mol The CC bond distances are not equal: 12 inner bonds of about 1.38 and 6 outer bonds of about 1.42,The 4n+2 systems are aromatic if they can be planar, aromaticity decreases with decreasing planarity.,五、其他芳香体系,富勒烯（Fullerene),Curl、 Kroto and

4、Smalley 1996 Nobel Prize,Conclusion,The presence of a diamagnetic ring current(diatropic) Equal or approximately equal bond distances Planarity Chemical stability The ability to undergo aromatic substitution,2.2 反芳香化合物 (Antiaromatic compounds),If compounds are much less stable, they are not only non

5、aromatic but actually antiaromatic！ The chief criterion for antiaromaticity is the prensence of a paramagnetic ring current, which causes protons on the outside of the ring to be shifted upfield while any inner protons are shifted downfield.(paratropic) When the closed loop contains 4n electrons, th

6、e molecule is destabilized by resonance. Antiaromaticity will be maximum when the molecule is planar and when bond distances are equal.,一、四电子体系,环丁二烯衍生物,Cyclobutadiene is rectangular,is not square,室温下能分离到的环丁二烯衍生物有：, 5.38,Push-pull effect,环辛四烯 (cyclooctatetraene),二、八电子体系,单键：1.46A 双键：1.33A,The reason for the lack of planarity is that a regular octagon has anlges of 135o , while sp2 angles are most stable at 120o. To avoid the strain, the molecule assumes a nonplanar shape, in which orbital overlap is grea