问题：关于SN2反应的离去基团???
类型：求助 (悬赏分:3分)
提问：乱劈要财
等级：▲
版块：大学化学(兔乖乖,化学王子,)
信誉：75%
回复：8
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时间：2006-10-30 02:01:52 编辑加入/取消收藏订制/取消短消息举报该贴

看胡宏纹版有机化学书的时候看到他上面说SN2反应中离去基团的碱性越强,其离去倾向越弱,(比如:RI>RBr>RCl>RF)这是为什么?麻烦哪位好汉帮忙解释一下,谢谢了!!

[该帖子已被沧海一声笑在2007-5-13 14:01:09编辑过]

回复人：外行,▲▲▲▲ (有机合成充满商机，是一个充满魅力的世界。。。。。。) 时间：2006-10-30 10:09:39 编辑	1楼
这和原子半径有关系。一般原子半径越大，越易极化，与原来母体分子的结合力越弱，因此越易离去。RI>RBr>RCl>RF就是这个道理。

回复人：sj,▲▲▲ (不管外部发生何等情况，我始终坚定自己是一名真正的化学工作者。) 时间：2006-10-30 11:30:48 编辑	2楼
楼主你的说法有误,应该是SN2反应中离去基团的碱性越强,其离去倾向越强.越易给电子的基团碱性越强,此种离子的变形性较大.具体内容请参阅南开大学张宝申,王积涛版的有机化学书.那里面对SN反应中的条件,机理等问题讲得很细.

回复人：乱劈要财,▲ (高分子合成－絮凝剂方向) 时间：2006-10-30 12:48:36 编辑

3楼

我比较同意一楼的朋友的解释。
二楼的朋友我用的是胡宏纹的版本的有机化学，我又看了一遍书，作者确实是说离去基团的碱性越强，其离去倾向就越小，而且作者还举例说像HO-、RO－、NH2－都是强碱，因此OH、OR、NH2在亲核取代反应中都很难被亲核取代。
对于二楼朋友的解释中说越易给电子的基团碱性越强，因此越易给电子的基团离去倾向就越强，但是我们知道离去基团是要带着一对电子离去，如果它给电子能力强，那么它如何带一对电子离去呢？

回复人：乱劈要财,▲ (高分子合成－絮凝剂方向) 时间：2006-10-30 12:49:43 编辑	4楼
感谢以上两位朋友的解释，谢谢了！

回复人：haopengnia,▲ (药学院的学生) 时间：2006-10-30 13:38:03 编辑	5楼
离去总得说有两个关键因素一是碱性强弱还有一个因素是键能大小酸性 HI>HBr>HCl>HF 固碱性F->Cl->Br->I- 并且由于键能大小是C-F>C-Cl>C-Br>C-I 所以离去能力是F-<Cl-<Br-<I- 看看中国科学技术大学出版的有机化学里面很好懂 [该帖子已被haopengnia在2006-10-30 18:06:20编辑过]

回复人：乱劈要财,▲ (高分子合成－絮凝剂方向) 时间：2006-10-30 18:02:35 编辑	6楼
感谢感谢！

回复人：haopengnia,▲ (药学院的学生) 时间：2006-10-30 18:07:49 编辑	7楼
不好意思上次说错了我修改了不好意思啊

回复人：admin,★ (论坛管理员-欢迎大家访问化学化工论坛) 时间：2007-05-13 11:29:48 编辑

8楼

A leaving group is an atom or molecule that detaches from an organic molecule, which, after detachment, is called the residual or main part. The ability for a functional group to leave is called lability. Leaving groups affect the intrinsic reactivity, not the nucleophilic discrimination factors.

The lower the pKa of the conjugate acid, the better the leaving group. This is because the lower the pKa of the conjugate acid of the leaving group, the more it stays in its anion (l.g.'s may be neutral), leaving group, form. This anion stability means that the leaving group is less likely to react with the cation formed by the leaving group's absence (this is the case with bad leaving groups). Without stabilization, a leaving group will become a nucleophile due to its negative charge, and if this happens, the reaction goes in circles. This is why a strong base is a poor leaving group. SN1 reactions prefer halide ions as leaving groups.

In room temperature water, the sequence of lability is:

Less lability
amine NH2-
methoxy CH3O-
hydroxyl HO-
carboxylate CH3COO-
F-
water
Cl-
Br-
I-
azide N3-
thiocyanate SCN-
nitro NO2
Greater Lability
NO3 is also weaker than F-.
A direct application of the difference in leaving group stability is in the carboxylic acid derivatives. The worse the leaving group, the more stable the chemical, which makes these form a hierarchy that can be separated in the lab. Amines, which are the worst leaving group, are the most stable compound, and attacking any carboxylic acid derivative with an amine produces an amide. Esters are the second most stable followed by anhydrides and finally halides.

During the SN2 nucleophilic attack, a partial negative charge forms on the leaving group. During SN1 reactions, the leaving group anionizes and leaves. In general, amine, methoxy and hydroxyl groups never act as leaving groups in a substitution reaction.

In the non-mechanistic transformations, leaving group is the actual substituent group present in the substrate and product.

摘自维基百科

得分人：外行：1,sj：1,haopengnia：1,

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