第6章：有机化学反应.ppt

Active Intermediates Structures and Stability（结构与稳定性） —— Carbocations and Radicals ——碳正离子 和 碳自由基 sp2-hybridized three sigma-bond radical ---- one electron in p orbital carbocation ---- a vacant p orbital Active Intermediates 烯丙型自由基和碳正离子结构 ---- 共轭效应 (Conjugation ) , 稳定 Active Intermediates For both carbocations and radicals, the observed order of stability is —— 30 20 10 +CH3 ( or .CH3 ) Hyperconjugation（超共轭作用） Active Intermediates 碳正离子的稳定性 P297 30苄基型30烯丙型 20苄基型20烯丙型 30 20 ~ 10苄基型~10烯丙型 10+CH3 碳自由基的稳定性 P297 30苄基型30烯丙型 20苄基型20烯丙型10苄基型~10烯丙型 30 20 10 .CH3 Active Intermediates Carbanions（碳负离子） ---- Like a carbocation or a radical, a carbanion has three sigma-bond, but it also bears unshared electron pair. So, a simple carbanion is pyramidal.( sp3-hybridized ) When the carbanionic carbon is adjecent to a pi-system, sp2-hybridization is preferred so that the negative charge can be distributed by resonance（共振） throughout the p orital array. Active Intermediates Active Intermediates Carbanions With three sigma-bond and a lone pair electrons, a carbanion is electronically similar to an amine, the carbanion and the amine are therefore said to be isoelectronic (等电子体) Active Intermediates Carbenes(卡宾) —— another class of neutral reactive intermediates —— highly electron-deficient ——Singlet carbene (单线态卡宾) ——Triplet carbene (三线态卡宾) Active Intermediates —— Singlet carbene (单线态卡宾) sp2-hybridized two sigma-bond, the electron pair in sp2-hybrid orbital, a vacant p-orbital —— reactive in an ambiphilic sense —— that is, toward both to electron-rich and eletron-poor reagents Active Intermediates ---- Triplet carbene (三线态卡宾) sp2-hybridized, or sp-hybridized two sigma-bond one electron in the p orbital one in the sp2-hybrid orbital or another p orbital ---- have radical-like reactivity Active Intermediates Classification of Reactions ---- Addition reactions A + B C e.g. Classification of Reactions ---- Eli