[理学]20120417有机中间体1定稿打印.ppt

Carbocations (碳阳离子) 中科院01 Alkyl groups and hydrogen can migrate in rearrangement reactions to give more stable intermediate carbocations. The type of rearrengement SN1 Reaction Unimolecular nucleophilic substitution. Two step reaction with carbocation intermediate. Rate is first order in the alkyl halide, zero order in the nucleophile. Racemization occurs.【r?simizei??n】 当芳环上取代基中的－X或－CO－处于适当的位置时,均可发生分子内反应。如： O Cl AlCl3 O O 当芳环上电子云密 度高的易发生亲电取代反应。 主要产物 科大 a. SN1： 烯丙醇、苄醇、叔醇、仲醇。 2、亲核取代反应机理：P 265 ??????nucleophilic? [,nju:kli?ufilik] ?-碳有支链的伯或仲醇和卤化氢反应时，发生分子重排。 科大 3. 醇脱水成烯 Zaitsev消除取向 H＋催化：有重排 Al2O3：不重排 醇的反应活性：3? 2? 1? 反应取向：查依采夫规则 强酸催化下的醇消除是碳正离子历程 Acid Catalyzed Dehydration of Secondary and tertiary alcohols Protonation, establishing of good leaving group. Elimination of water to yield carbocation in rate determining step. C C H O H H + C C H O H 2 B - H 2 O C C H B C C + H B Elimination of H+ from carbocation to yield alkene. 醇的b－消除 protonation?[,pr?ut?nei??n] elimination?[??l?m??ne???n] 主要得双键上烃基取代较多的烯,氢少消氢 Orientation of elimination E1 reactions faithfully follow Zaitsev’s rule! This means that the major product should be the product that is the most highly substituted. E1 reactions do not require an anti coplanar orientation of H and X. E1反应机理 SN1反应机理 慢 慢 快 快 进攻?-H 进攻C+ 碱性强，升温对E1有利。 中性极性溶剂对SN1有利。 E1反应机理和SN1反应机理的比较 4. Pinacol Rearrangement: Nucleophilic Rearrangement *** p116 an example of stabilization of a carbocation by an adjacent lone pair. -H2O H+ H+ 1. Protonation of a hydroxyl group Mechanism of the pinacol rearrangement -H2O 2.Elimination of water to yield tertiary carbocation. Resonance-stablized carbocation Deprotonation a.不对称的取代乙二醇中，质子化 电子云密度大的羟基优先 动力:氧正离子,更稳定,满足八隅体结构 b.迁移－亲核性强的基团优先，能提供电子的基团优先 规律： Phenyl group move first Phenyl with electron donor group move first How to decide which will be major? First decide which carbocation is more stable Then consider the relative migratory aptitude of the groups that will undergo 1,2-sh