单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,2008/12/4,#,1,1,2,2,A pericyclic reaction is characterized as a change in bonding relationships,that takes,place as a,continuous,concerted reorganization of electrons.,The,term,concerted specifies that there is one single transition state,and,therefore,no intermediates are involved in the process.To maintain continuous electron flow,pericyclic reactions occur through cyclic transition states,.,More,precisely:The cyclic transition state must correspond to an,arrangementof,the,participating orbitals which has to maintain a bonding,interaction,between the,reaction components throughout the course of,the,reaction,3,A pericyclic reaction is chara,4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,12,12,13,13,14,14,15,15,16,16,17,17,18,18,19,19,20,20,21,21,22,22,23,23,24,24,25,25,26,26,27,27,28,28,The simplest approach(Supra/Supra)is forbidden under thermal activation.The less obvious approach(Antara/Supra)is allowed thermally but geometrically rather congested.It is believed to occur in some very specific cases(e.g.ketenes)where the steric congestion is reduced.,29,The simplest approach(Supra/S,30,30,31,31,32,32,33,33,34,34,35,35,36,36,37,37,38,38,39,39,40,40,41,41,42,42,43,43,44,44,45,45,