Z: O,C,S,N
C C
HCl
H
C C
Cl
C O
C OH
C OH
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3) 由其它正离子转化而生成
NH 2
NaNO 3 HCl
N2
N2
H
H Ph3CSbF6 SbF6
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19
4) 在超酸中制备C正离子溶液 比100％的H2SO4的酸性更强的酸－超酸 (Super acid) 常见的超酸 与100％H2SO4的酸性比较 1000倍 103倍 1016倍
3
C
>
2 CH
>
CH2
>
CH2
环丙甲基正离子的结构：
C
CH2 CH2 CH2
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中心碳原子上的空的 p 轨道与环丙基中的弯曲 轨道进行侧面交盖，其 结果是使正电荷分散。
空的 p 轨道与弯曲轨道的交盖
随着环丙基的数目增多， 正碳离子稳定性提高。
10
NC + stable + CN 极性效应－场效应
金刚烷C+
Advanced Organic Chemistry Lecture 3 Xufeng Lin
Sept., 2013
91
1
Reactive Intermediates
Carbocations,Carbanions, Free radicals, Carbenes, and Nitrenes (Part I)
CH3 CH3-C-CH2Br CH3 _ Br CH3 + CH3-C-CH2 CH3 。+ 1 C CH3 CH3-C-CH2CH3
+
C2H5OH
CH3 CH3-C-CH2CH3 OC2H5
。 + 3 C
It is a general rule that the more concentrated any charge is, the less stable the species bearing it will be.
44
非经典正碳离子类型
1) π键参与的非经典正碳离子 反－7－原冰片烯基对甲苯磺酸酯,在乙酸中的溶剂解的速度， 构型保持,而且比相应的饱和化合物,速度快 1011倍。
TsO H
AcO
AcOH -TsO Ts = CH3 SO2
H
+
7
1 3
5 4 2
45
2电子3中心体系
91
2)σ键参与的非经典正碳离子
பைடு நூலகம்
91 J. Org. Chem. 2005, 70, 3263-3266
35
91
36
Delocalization Delocalization
+
+ + +
Simple conjugation
Homoconjugation
+
+
+
Hyperconjugation/ conjugation
37
91
Delocalization Delocalization
RF + SbF5
Me3CH FSO3H-SbF5
R SbF
+
-
Me3C+ SbF5FSO3- + H2
91 22
Cl The formation of carbocation is difficult
SbF6stable below-50oC
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23
离解
(CH3)3C-Br
slow
(CH3)3C
28
J. Org. Chem. 2001, 66, 1109-1114
91
29
Multicenter Bonding in Carbocations
?
91
30
J. Phys. Chem. A 2006, 110, 3785-3789
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J. Phys. Chem. A 2006, 110, 3785-3789
三苯甲基正离子利用核磁共振谱证明为螺旋浆结构
C
+
还有少数情况例外，如在炔基或苯基正离子， 正电荷不可能处于p 轨道
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5
溶剂效应
• 溶剂的极性－溶剂化效应使碳正离子稳定
need 82KJ/mol H2O (CH3)3C-Br need 820 KJ/mol gas phase
+
(CH3)3C
+ Br -
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Org. Lett., 2012, 14, 2674
40
Carbocation-Forming Reactions in Dimethyl Sulfoxide
91
41
J. Org. Chem., 2003, 68 (3), pp 1117
Nonclassical cation 非经典碳正离子
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6
Direct Observation of the Ionization Step in Solvolysis Reactions • J. Am. Chem. Soc., 2008, 130 (10), pp 3012–3022
91
7
诱导效应－共轭效应
烯丙型正碳离子：
CH2 CH CH2 CH2 CH CH2
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11
习题
• 二甲基环戊二烯在浓硫酸中形成的碳正离 子在1H NMR 上只见到三组峰，分别是6 个甲基氢、4个亚甲基氢和一个烯键氢， 试画出该碳正离子的共振结构
H3C
CH3
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12
H3C
+ CH3
H3C
+ CH3
H3C
+ CH3
烯丙基C＋
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13
芳构化效应
• 具有芳香性Hückel规则∏电子 4n＋2的环状C＋稳定性 大 Br H
基础有机是否碰到？
91 43
非经典正碳离子的结构
+ +
Br + C 溴鎓离子 C
由于Π、σ键邻基参与而形成非经典碳正离子 配位数可以为５个
H H
H *
H
H *
+
R
+
R
H
当有一个基团存在时倾向于以经典形式存在 若Ｒ为苯基时由于苄基正离子来自苯环的稳定化而形成经典正离子 91 若Ｒ为难离去基团时大多数情况下为经典的Ｃ正离子形式存在
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15
但对下列碳正离子的生成，由于几何形状的限 制，空间效应的影响，张力较大，稳定性较差。
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16
正碳离子的生成
1) 直接离子化
RX
Ph CH Cl Ph
H
R
通过化学键的异裂 X 而产生。
Ph2CH
Cl
R OH
ROH2
BF3
R
H2O
CH3COF
CH3CO
BF4
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17
2) 对不饱和键的加成
C Z H C ZH
91 32
R
C O
酰基阳离子
91
R
C O
33
CH3 H3C C O CH3
H3C H3C H3C
CH3 C O CH3
They are especially stable and are easily formed in 96% H2SO4.
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34
Isolation and X-ray Structures of Labile Benzoic- and Acetic-Acidium Carbocations
The structure of simple unsubstituted carbanions is not known with certainty since they have not been isolated, but it seems likely that the central carbon is sp3 hybridized, with the unshared pair occupying one apex of the tetrahedron.
连接于C＋的原子团对C＋稳定能力的大小
R2N>RO>Ar>RCH=CHR>R>H
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25
91
26
91
27
Intramolecular Hydride Migration in a Substituted 9-Fluorenyl Carbocation
J. Org. Chem. 2001, 66, 1109-1114 91
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21
Since simple alkyl cations are not stable in ordinary strongacid solutions, the study of this species was greatly facilitated by the discovery that many of them could be kept indefinitely in stable solutions in mixtures of fluorosulfuric acid and antimony pentafluoride. Such mixture, usually dissolved in SO2 or SO2ClF, are among the strongest solutions known and are often called superacids.
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5 进攻C1 5 6 1 + 4 3 HOAc 2 进攻C2 6 6