1
00:00:00,000 --> 00:00:02,010
今天我们要讲的是

2
00:00:02,030 --> 00:00:09,680
区分开 亲核性

3
00:00:09,700 --> 00:00:14,280
即亲核试剂的强弱

4
00:00:14,300 --> 00:00:19,290
与碱性

5
00:00:19,300 --> 00:00:24,030
这个差别看似很微小

6
00:00:24,050 --> 00:00:25,850
实际上却很大

7
00:00:25,870 --> 00:00:27,310
一会儿我就会告诉大家为什么

8
00:00:27,330 --> 00:00:28,540
刚学的时候容易混淆

9
00:00:28,550 --> 00:00:30,600
学SN2反应的时候

10
00:00:30,620 --> 00:00:32,410
反应中的亲核试剂

11
00:00:32,430 --> 00:00:35,030
有一个多余的电子

12
00:00:35,050 --> 00:00:36,860
带一个负电荷

13
00:00:36,880 --> 00:00:42,520
就以甲基碳为例吧

14
00:00:42,540 --> 00:00:43,430
我画出来

15
00:00:43,440 --> 00:00:46,470
其中一个氢指向纸面外

16
00:00:46,490 --> 00:00:48,890
还有一个氢指向纸面内

17
00:00:48,900 --> 00:00:50,480
还有一个在上面

18
00:00:50,500 --> 00:00:56,460
然后这里有一个离去基团

19
00:00:56,480 --> 00:00:58,350
在SN2反应中

20
00:00:58,360 --> 00:01:02,390
亲核试剂把这个电子给这个碳

21
00:01:02,410 --> 00:01:04,100
这样碳就带有部分正电荷

22
00:01:04,130 --> 00:01:05,000
我画一下

23
00:01:05,020 --> 00:01:09,050
离去基团的电负性更强

24
00:01:09,070 --> 00:01:11,890
所以它带部分负电荷

25
00:01:11,910 --> 00:01:15,680
电子传给碳

26
00:01:15,690 --> 00:01:17,150
碳得到电子的一瞬间

27
00:01:17,170 --> 00:01:18,470
或者说同时

28
00:01:18,480 --> 00:01:22,410
电负性强的离去基团就可以

29
00:01:22,420 --> 00:01:27,400
完全夺走碳上的这个电子

30
00:01:27,420 --> 00:01:31,510
之后就变成这样啦

31
00:01:31,520 --> 00:01:33,140
甲基碳还在这儿

32
00:01:33,160 --> 00:01:35,110
一个氢指在后

33
00:01:35,120 --> 00:01:38,980
一个指在前 一个在上面

34
00:01:39,000 --> 00:01:42,780
离去基已离去

35
00:01:42,790 --> 00:01:44,910
它不但带着原来的电子

36
00:01:44,920 --> 00:01:50,000
还夺走了这个玫红色电子

37
00:01:50,020 --> 00:01:52,420
所以它现在带一个负电荷

38
00:01:52,430 --> 00:02:00,180
这个负电原本在亲核试剂上

39
00:02:00,200 --> 00:02:03,700
现在亲核试剂连到了碳上

40
00:02:03,710 --> 00:02:05,810
我举这个例子是为了说明

41
00:02:05,830 --> 00:02:07,100
它的角色是亲核试剂

42
00:02:07,120 --> 00:02:08,100
它大爱原子核

43
00:02:08,120 --> 00:02:10,200
它贡献了多余的电子

44
00:02:10,220 --> 00:02:17,490
同时它也是路易斯碱

45
00:02:17,500 --> 00:02:18,740
下面我们来复习一下

46
00:02:18,750 --> 00:02:21,340
路易斯碱 最广泛的定义

47
00:02:21,360 --> 00:02:24,280
我觉得它已经涵盖了

48
00:02:24,290 --> 00:02:25,650
所有可以称之为碱的东西

49
00:02:25,660 --> 00:02:31,880
路易斯碱就是 电子供体

50
00:02:31,900 --> 00:02:33,840
这个反应中就是这样

51
00:02:33,860 --> 00:02:37,130
亲核试剂将一个电子供给碳原子

52
00:02:37,150 --> 00:02:38,810
所以它的角色又是路易斯碱

53
00:02:38,830 --> 00:02:40,560
第一次看到这个的你可能会问

54
00:02:40,570 --> 00:02:44,080
为什么化学家们要这样折磨自己

55
00:02:44,090 --> 00:02:46,070
定义什么亲核试剂呢？

56
00:02:46,100 --> 00:02:47,900
干嘛不直接叫它们“碱”？

57
00:02:47,920 --> 00:02:49,710
为什么还要用

58
00:02:49,730 --> 00:02:52,740
亲核性和碱性两个概念？

59
00:02:52,760 --> 00:02:54,910
它们的区别在于

60
00:02:54,920 --> 00:02:59,870
亲核性是一个动力学概念

61
00:02:59,890 --> 00:03:02,450
即它反应速率如何？

62
00:03:02,470 --> 00:03:03,990
反应有多快？

63
00:03:04,000 --> 00:03:06,700
反应需要的能量少到什么程度？

64
00:03:06,720 --> 00:03:08,480
如果试剂的亲核性很强

65
00:03:08,490 --> 00:03:13,420
它会反应得很顺利

66
00:03:13,430 --> 00:03:15,890
但它不能表示

67
00:03:15,910 --> 00:03:19,990
反应物或产物有多稳定

68
00:03:20,000 --> 00:03:22,620
只能说明它们有多容易反应

69
00:03:22,640 --> 00:03:29,740
而碱性 是一个热力学的概念

70
00:03:29,750 --> 00:03:35,780
它表示反应物和产物有多稳定性

71
00:03:35,800 --> 00:03:52,510
它说明 反应的可能性有多大

72
00:03:52,530 --> 00:03:53,220
举个例子

73
00:03:53,240 --> 00:03:56,230
我们见过氟的反应

74
00:03:56,250 --> 00:03:58,470
思考一下

75
00:03:58,490 --> 00:04:00,390
在那个反应中

76
00:04:00,410 --> 00:04:01,620
其实 应该叫氟离子

77
00:04:01,630 --> 00:04:03,270
氟离子是这样的

78
00:04:03,290 --> 00:04:05,970
原子有7个价电子

79
00:04:05,990 --> 00:04:08,680
然后它夺取了一个电子

80
00:04:08,700 --> 00:04:10,540
变成了氟离子

81
00:04:10,560 --> 00:04:14,530
氟离子也是一种碱

82
00:04:14,550 --> 00:04:28,730
它的碱性甚至比碘离子还要强

83
00:04:28,740 --> 00:04:31,950
但在质子溶剂中

84
00:04:31,970 --> 00:04:33,090
我写这儿

85
00:04:33,090 --> 00:04:46,310
质子溶液中 亲核性减弱

86
00:04:46,320 --> 00:04:47,560
再次强调 质子溶液

87
00:04:47,580 --> 00:04:50,200
有游离的质子

88
00:04:50,210 --> 00:04:51,550
而亲核性减弱的原因是

89
00:04:51,570 --> 00:04:54,770
是氟离子 它想更容易

90
00:04:54,780 --> 00:04:58,170
和碳什么的成键

91
00:04:58,190 --> 00:04:59,990
或和质子

92
00:05:00,010 --> 00:05:02,370
它比碘离子更想要

93
00:05:02,390 --> 00:05:04,280
和质子成键

94
00:05:04,300 --> 00:05:06,660
一旦如此 氟离子成的键

95
00:05:06,680 --> 00:05:09,400
能比碘离子的强

96
00:05:09,420 --> 00:05:11,300
其实离子状态下

97
00:05:11,320 --> 00:05:14,240
氟没有碘稳定

98
00:05:14,260 --> 00:05:16,820
如果氯离子能得到一个质子

99
00:05:16,840 --> 00:05:18,400
或者失去一个电子

100
00:05:18,420 --> 00:05:19,790
它会更满足

101
00:05:19,810 --> 00:05:21,670
但它的亲核性较弱

102
00:05:21,690 --> 00:05:24,760
不擅长在质子溶剂中反应

103
00:05:24,770 --> 00:05:27,500
而亲核性较弱的主要原因在于

104
00:05:27,520 --> 00:05:28,970
有东西

105
00:05:28,990 --> 00:05:30,310
阻止它反应

106
00:05:30,320 --> 00:05:33,050
我们已经讲过亲核性强的条件啦

107
00:05:33,060 --> 00:05:34,510
对于氟离子来说

108
00:05:34,530 --> 00:05:37,890
主要原因就是离子体积小

109
00:05:37,910 --> 00:05:40,000
它体积如此之小

110
00:05:40,020 --> 00:05:43,130
使得电子十分紧密

111
00:05:43,150 --> 00:05:45,970
它的电子云很密

112
00:05:45,990 --> 00:05:49,050
这使得它能让水中的氢

113
00:05:49,070 --> 00:05:54,280
能围绕它形成紧致的“水壳”

114
00:05:54,290 --> 00:05:56,110
这些氢都带部分正电荷

115
00:05:56,130 --> 00:05:59,190
它们被吸引到阴离子周围

116
00:05:59,210 --> 00:06:02,040
它们在氟离子周围

117
00:06:02,060 --> 00:06:05,300
形成了一个致密的外壳

118
00:06:05,310 --> 00:06:08,780
所以它在质子溶液中

119
00:06:08,790 --> 00:06:09,870
难以反应

120
00:06:09,890 --> 00:06:16,660
因此 它不好反应

121
00:06:16,680 --> 00:06:18,570
如果它能反应的话

122
00:06:18,590 --> 00:06:21,940
它能形成比碘离子更强的键

123
00:06:21,960 --> 00:06:24,410
它能形成比碘离子更强的键

124
00:06:24,430 --> 00:06:25,630
这是主要区别

125
00:06:25,640 --> 00:06:27,980
下面我们变化趋势的不同

126
00:06:28,000 --> 00:06:29,110
碱性

127
00:06:29,120 --> 00:06:32,080
它的强度和溶剂无关

128
00:06:32,090 --> 00:06:33,940
碱性是分子、原子或离子的

129
00:06:33,960 --> 00:06:36,720
热力学性质

130
00:06:36,730 --> 00:06:41,870
如果只看碱性

131
00:06:41,890 --> 00:06:44,280
最强的碱就是…

132
00:06:44,300 --> 00:06:46,160
我直接写出氢氧根吧

133
00:06:46,170 --> 00:06:47,730
它通常是

134
00:06:47,750 --> 00:06:50,600
氢氧化钠或者氢氧化钾

135
00:06:50,620 --> 00:06:53,660
但是如果溶解在溶剂中 比如水

136
00:06:53,680 --> 00:06:57,140
钠离子和氢氧根就分离了

137
00:06:57,150 --> 00:06:59,470
充当碱的其实是氢氧根

138
00:06:59,480 --> 00:07:01,360
是它想贡献电子

139
00:07:01,370 --> 00:07:06,220
氢氧根的碱性比氟离子强很多

140
00:07:06,240 --> 00:07:08,620
而氟离子又比氯离子更碱

141
00:07:08,640 --> 00:07:10,750
氯离子又比溴离子更碱

142
00:07:10,770 --> 00:07:15,240
溴离子的碱性又强于碘离子

143
00:07:15,260 --> 00:07:20,280
现在 你回看亲核性

144
00:07:20,300 --> 00:07:25,930
有什么区别

145
00:07:25,950 --> 00:07:27,780
我们发现 亲核性和溶剂息息相关

146
00:07:27,800 --> 00:07:29,710
因为溶剂会影响

147
00:07:29,730 --> 00:07:31,640
它们的反应速率

148
00:07:31,650 --> 00:07:33,310
那么亲核性呢

149
00:07:33,330 --> 00:07:35,950
质子溶剂和非质子溶剂

150
00:07:35,960 --> 00:07:40,030
就不一样了

151
00:07:40,050 --> 00:07:41,110
如果是质子溶剂

152
00:07:41,120 --> 00:07:44,820
亲核性最强的

153
00:07:44,840 --> 00:07:45,870
是碘离子

154
00:07:45,890 --> 00:07:48,240
因为氢键

155
00:07:48,250 --> 00:07:49,890
阻止不了什么

156
00:07:49,910 --> 00:07:51,060
不会形成致密“水壳”

157
00:07:51,070 --> 00:07:53,060
碘离子的电子云太大

158
00:07:53,080 --> 00:07:55,010
有人觉得电子云还很松软

159
00:07:55,030 --> 00:07:56,990
很容易被极化

160
00:07:57,000 --> 00:07:59,090
它的电子云被吸向碳的一侧

161
00:07:59,110 --> 00:08:00,330
再该干嘛干嘛

162
00:08:00,350 --> 00:08:04,780
所以这时 碘离子的亲核性较强

163
00:08:04,800 --> 00:08:06,840
可以说 比氢氧根强

164
00:08:06,850 --> 00:08:11,010
氢氧根离子又强于氟离子

165
00:08:11,030 --> 00:08:13,170
然而 在非质子溶剂中

166
00:08:13,190 --> 00:08:19,730
突然之间 溶剂的影响

167
00:08:19,740 --> 00:08:22,110
就不那么明显了

168
00:08:22,130 --> 00:08:23,780
于是就反过来

169
00:08:23,790 --> 00:08:26,230
这时 碱性就成了主导

170
00:08:26,240 --> 00:08:29,520
所以在非质子溶剂中

171
00:08:29,530 --> 00:08:41,290
碱性和亲核性就基本一致了

172
00:08:41,310 --> 00:08:47,000
这里我标个星号

173
00:08:47,010 --> 00:08:49,840
因为亲核性还有一个因素

174
00:08:49,860 --> 00:08:51,090
我没有讲到

175
00:08:51,110 --> 00:08:52,470
不过我马上就要讲了

176
00:08:52,480 --> 00:08:54,100
在这种情况下

177
00:08:54,110 --> 00:09:00,970
氢氧根比氟离子更易反应

178
00:09:00,990 --> 00:09:04,560
而氟离子又比碘离子易反应

179
00:09:04,580 --> 00:09:05,420
之所以

180
00:09:05,440 --> 00:09:10,800
氢氧根两次都…

181
00:09:10,810 --> 00:09:13,730
我的意思是 即便它也会与溶剂作用

182
00:09:13,750 --> 00:09:16,070
但它还是很强的亲核试剂

183
00:09:16,090 --> 00:09:19,890
因为分析氢氧根的时候

184
00:09:19,910 --> 00:09:21,270
一定要想到

185
00:09:21,290 --> 00:09:22,690
它有一个多余电子

186
00:09:22,710 --> 00:09:24,520
分析它的话

187
00:09:24,540 --> 00:09:26,610
可以想象成水没了一个H

188
00:09:26,630 --> 00:09:28,430
我画一下

189
00:09:28,450 --> 00:09:32,220
大家可以想象成水少了一个质子

190
00:09:32,240 --> 00:09:34,190
或者失去了一个电子剩质子

191
00:09:34,200 --> 00:09:36,880
所以原来 有2对孤对电子

192
00:09:36,890 --> 00:09:38,700
然后现在变成了3对

193
00:09:38,720 --> 00:09:42,680
这个氧有1 2 3 4 5 6

194
00:09:42,700 --> 00:09:44,170
7个价电子

195
00:09:44,180 --> 00:09:45,610
比中性原子多1个

196
00:09:45,630 --> 00:09:46,950
所以它带一个负电荷

197
00:09:46,970 --> 00:09:51,010
它已经多了一个电子

198
00:09:51,030 --> 00:09:51,950
带一个负电荷了

199
00:09:51,970 --> 00:09:54,870
但因为氧比氢的电负性强

200
00:09:54,890 --> 00:09:56,030
它还能稍微

201
00:09:56,050 --> 00:10:01,150
将氢上的电子拉过来一点

202
00:10:01,170 --> 00:10:06,960
所以它的碱性非常非常强

203
00:10:06,970 --> 00:10:09,340
即使水之类里的质子

204
00:10:09,350 --> 00:10:11,380
会影响一点

205
00:10:11,390 --> 00:10:15,070
但是它的亲核性

206
00:10:15,080 --> 00:10:16,530
仍比氟离子强

207
00:10:16,550 --> 00:10:18,740
如果不考虑溶剂

208
00:10:18,760 --> 00:10:20,920
氢氧根是个超强碱

209
00:10:20,940 --> 00:10:24,950
它也是极强的亲核试剂

210
00:10:24,970 --> 00:10:28,050
下面我们来讲亲核性的最后一点

211
00:10:28,070 --> 00:10:30,560
记得吗？亲核性和反应难易程度有关

212
00:10:30,580 --> 00:10:34,180
假设这里有…

213
00:10:34,190 --> 00:10:42,270
假设是两种氢氧化物 好吧？

214
00:10:42,290 --> 00:10:47,980
假如这是个孤立的氢氧根

215
00:10:48,000 --> 00:10:48,970
而这边呢

216
00:10:48,990 --> 00:10:51,130
它还有其他基团

217
00:10:51,140 --> 00:10:55,770
它还连着一长串

218
00:10:55,790 --> 00:10:56,690
管它连着啥呢

219
00:10:56,710 --> 00:10:59,000
我们来看看这两个离子

220
00:10:59,020 --> 00:11:00,940
如果想判断

221
00:11:00,960 --> 00:11:02,870
哪个亲核性更强

222
00:11:02,890 --> 00:11:04,810
你只需记住

223
00:11:04,830 --> 00:11:07,380
亲核性和反应的难易有关

224
00:11:07,400 --> 00:11:08,850
它有多容易靠近其它分子

225
00:11:08,870 --> 00:11:11,410
并与之发生反应

226
00:11:11,420 --> 00:11:14,920
这个周围有一堆原子

227
00:11:14,940 --> 00:11:16,860
反应可能很难发生

228
00:11:16,880 --> 00:11:19,450
再回来看看这个图

229
00:11:19,470 --> 00:11:21,780
它很难接近这个原子

230
00:11:21,800 --> 00:11:23,140
我们以前从碳的角度

231
00:11:23,160 --> 00:11:24,930
讲过过空间位阻效应

232
00:11:24,950 --> 00:11:26,230
但还没想过亲核试剂的

233
00:11:26,240 --> 00:11:27,430
空间位阻呢

234
00:11:27,450 --> 00:11:29,460
先看这个亲核试剂

235
00:11:29,470 --> 00:11:37,890
这个额外的电子大概很难

236
00:11:37,900 --> 00:11:40,390
接近目标原子

237
00:11:40,400 --> 00:11:41,490
它受到了阻碍

238
00:11:41,500 --> 00:11:44,390
但是这边呢 就很灵活了

239
00:11:44,410 --> 00:11:47,000
虽然参与反应的这个基团

240
00:11:47,020 --> 00:11:49,050
氧有一个负电荷

241
00:11:49,060 --> 00:11:49,980
有一个额外的电子

242
00:11:50,000 --> 00:11:52,660
在某种程度上这两个是一样的

243
00:11:52,680 --> 00:11:56,190
但是这边的离子比较小

244
00:11:56,210 --> 00:11:58,110
受到的阻碍少 更容易接近目标

245
00:11:58,130 --> 00:12:03,380
所以这是更好的亲核试剂

246
00:12:03,400 --> 00:12:05,780
所以我不能绝对地说

247
00:12:05,800 --> 00:12:09,390
在非质子溶剂中

248
00:12:09,400 --> 00:12:12,140
碱性和亲核性是完全一致的

249
00:12:12,160 --> 00:12:14,000
因为影响亲核性的因素

250
00:12:14,010 --> 00:12:17,060
还有空间位阻效应

251
00:12:17,080 --> 00:12:19,530
它会不会被环境或本身的空间位阻

252
00:12:19,550 --> 00:12:21,290
妨碍反应

253
00:12:21,310 --> 00:12:23,500
尽管它是个很强的碱？

254
00:12:23,520 --> 00:12:25,630
如果它能成键 那么化学键一定很强

255
00:12:25,650 --> 00:12:28,300
本集的中心就是

256
00:12:28,320 --> 00:12:30,280
碱性和亲核性是完全不同的概念

257
00:12:30,290 --> 00:12:32,490
所以才会有两个不同的名称

258
00:12:32,510 --> 00:12:35,180
亲核性是指 反应进行的难易程度

259
00:12:35,190 --> 00:12:37,900
和新化学键的强弱无关

260
00:12:37,920 --> 00:12:40,850
而碱性表示新键的强度

261
00:12:40,870 --> 00:12:42,740
反应的可能性有多大

262
00:12:42,760 --> 00:12:46,300
而和反应的难易程度无关