WEBVTT

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今天我們要講的是

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區分開 親核性

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即親核試劑的強弱

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與堿性

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這個差別看似很微小

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實際上卻很大

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一會兒我就會告訴大家爲什麽

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剛學的時候容易混淆

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學SN2反應的時候

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反應中的親核試劑

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有一個多余的電子

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帶一個負電荷

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就以甲基碳爲例吧

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我畫出來

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其中一個氫指向紙面外

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還有一個氫指向紙面內

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還有一個在上面

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然後這裡有一個離去基團

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在SN2反應中

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親核試劑把這個電子給這個碳

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這樣碳就帶有部分正電荷

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我畫一下

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離去基團的電負度更強

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所以它帶部分負電荷

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電子傳給碳

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碳得到電子的一瞬間

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或者說同時

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電負度強的離去基團就可以

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完全奪走碳上的這個電子

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之後就變成這樣啦

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甲基碳還在這兒

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一個氫指在後

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一個指在前 一個在上面

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離去基已離去

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它不但帶著原來的電子

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還奪走了這個玫紅色電子

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所以它現在帶一個負電荷

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這個負電原本在親核試劑上

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現在親核試劑連到了碳上

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我舉這個例子是爲了說明

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它的角色是親核試劑

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它大愛原子核

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它貢獻了多余的電子

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同時它也是路易斯堿

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下面我們來複習一下

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路易斯堿 最廣泛的定義

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我覺得它已經涵蓋了

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所有可以稱之爲堿的東西

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路易斯堿就是 電子供體

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這個反應中就是這樣

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親核試劑將一個電子供給碳原子

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所以它的角色又是路易斯堿

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第一次看到這個的你可能會問

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爲什麽化學家們要這樣折磨自己

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定義什麽親核試劑呢？

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幹嘛不直接叫它們“堿”？

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爲什麽還要用

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親核性和堿性兩個概念？

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它們的區別在於

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親核性是一個動力學概念

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即它反應速率如何？

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反應有多快？

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反應需要的能量少到什麽程度？

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如果試劑的親核性很強

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它會反應得很順利

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但它不能表示

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反應物或產物有多穩定

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只能說明它們有多容易反應

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而堿性 是一個熱力學的概念

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它表示反應物和產物有多水力安定性

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它說明 反應的可能性有多大

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舉個例子

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我們見過氟的反應

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思考一下

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在那個反應中

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其實 應該叫氟離子

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氟離子是這樣的

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原子有7個價電子

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然後它奪取了一個電子

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變成了氟離子

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氟離子也是一種堿

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它的堿性甚至比碘離子還要強

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但在質子溶劑中

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我寫這兒

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質子溶液中 親核性減弱

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再次強調 質子溶液

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有遊離的質子

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而親核性減弱的原因是

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是氟離子 它想更容易

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和碳什麽的成鍵

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或和質子

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它比碘離子更想要

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和質子成鍵

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一旦如此 氟離子成的鍵

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能比碘離子的強

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其實離子狀態下

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氟沒有碘穩定

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如果氯離子能得到一個質子

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或者失去一個電子

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它會更滿足

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但它的親核性較弱

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不擅長在質子溶劑中反應

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而親核性較弱的主要原因在於

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有東西

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阻止它反應

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我們已經講過親核性強的條件啦

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對於氟離子來說

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主要原因就是離子體積小

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它體積如此之小

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使得電子十分緊密

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它的電子雲很密

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這使得它能讓水中的氫

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能圍繞它形成緊致的“水殼”

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這些氫都帶部分正電荷

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它們被吸引到陰離子周圍

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它們在氟離子周圍

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形成了一個致密的封閉

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所以它在質子溶液中

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難以反應

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因此 它不好反應

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如果它能反應的話

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它能形成比碘離子更強的鍵

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它能形成比碘離子更強的鍵

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這是主要區別

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下面我們變化趨勢的不同

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堿性

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它的強度和溶劑無關

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堿性是分子、原子或離子的

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熱力學性質

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如果只看堿性

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最強的堿就是…

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我直接寫出氫氧根吧

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它通常是

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氫氧化鈉或者氫氧化鉀

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但是如果溶解在溶劑中 比如水

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鈉離子和氫氧根就隔離了

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充當堿的其實是氫氧根

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是它想貢獻電子

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氫氧根的堿性比氟離子強很多

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而氟離子又比氯離子更堿

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氯離子又比溴離子更堿

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溴離子的堿性又強於碘離子

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現在 你回看親核性

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有什麽區別

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我們發現 親核性和溶劑息息相關

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因爲溶劑會影響

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它們的反應速率

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那麽親核性呢

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質子溶劑和非質子溶劑

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就不一樣了

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如果是質子溶劑

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親核性最強的

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是碘離子

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因爲氫鍵

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阻止不了什麽

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不會形成致密“水殼”

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碘離子的電子雲太大

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有人覺得電子雲還很松軟

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很容易被極化

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它的電子雲被吸向碳的一側

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再該幹嘛幹嘛

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所以這時 碘離子的親核性較強

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可以說 比氫氧根強

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氫氧根離子又強於氟離子

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然而 在非質子溶劑中

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突然之間 溶劑的影響

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就不那麽明顯了

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於是就反過來

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這時 堿性就成了主導

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所以在非質子溶劑中

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堿性和親核性就基本一致了

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這裡我標個星號

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因爲親核性還有一個因素

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我沒有講到

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不過我馬上就要講了

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在這種情況下

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氫氧根比氟離子更易反應

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而氟離子又比碘離子易反應

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之所以

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氫氧根兩次都…

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我的意思是 即便它也會與溶劑作用

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但它還是很強的親核試劑

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因爲分析氫氧根的時候

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一定要想到

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它有一個多余電子

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分析它的話

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可以想象成水沒了一個H

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我畫一下

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大家可以想象成水少了一個質子

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或者失去了一個電子剩質子

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所以原來 有2對孤對電子

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然後現在變成了3對

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這個氧有1 2 3 4 5 6

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7個價電子

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比中性原子多1個

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所以它帶一個負電荷

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它已經多了一個電子

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帶一個負電荷了

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但因爲氧比氫的電負度強

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它還能稍微

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將氫上的電子拉過來一點

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所以它的堿性非常非常強

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即使水之類裏的質子

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會影響一點

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但是它的親核性

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仍比氟離子強

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如果不考慮溶劑

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氫氧根是個超強堿

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它也是極強的親核試劑

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下面我們來講親核性的最後一點

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記得嗎？親核性和反應難易程度有關

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假設這裡有…

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假設是兩種氫氧化物 好吧？

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假如這是個孤立的氫氧根

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而這邊呢

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它還有其他基團

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它還連著一長串

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管它連著啥呢

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我們來看看這兩個離子

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如果想判斷

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哪個親核性更強

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你只需記住

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親核性和反應的難易有關

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它有多容易靠近其它分子

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並與之發生反應

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這個周圍有一堆原子

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反應可能很難發生

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再回來看看這個圖

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它很難接近這個原子

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我們以前從碳的角度

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講過過空間位阻效應

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但還沒想過親核試劑的

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空間位阻呢

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先看這個親核試劑

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這個額外的電子大概很難

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接近目標原子

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它受到了障礙

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但是這邊呢 就很靈活了

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雖然參與反應的這個基團

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氧有一個負電荷

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有一個額外的電子

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在某種程度上這兩個是一樣的

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但是這邊的離子比較小

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受到的障礙少 更容易接近目標

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所以這是更好的親核試劑

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所以我不能絕對地說

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在非質子溶劑中

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堿性和親核性是完全一致的

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因爲影響親核性的因素

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還有空間位阻效應

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它會不會被環境或本身的空間位阻

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妨礙反應

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盡管它是個很強的堿？

00:12:23.520 --> 00:12:25.630
如果它能成鍵 那麽化學鍵一定很強

00:12:25.650 --> 00:12:28.300
本集的中心就是

00:12:28.320 --> 00:12:30.280
堿性和親核性是完全不同的概念

00:12:30.290 --> 00:12:32.490
所以才會有兩個不同的名稱

00:12:32.510 --> 00:12:35.180
親核性是指 反應進行的難易程度

00:12:35.190 --> 00:12:37.900
和新化學鍵的強弱無關

00:12:37.920 --> 00:12:40.850
而堿性表示新鍵的強度

00:12:40.870 --> 00:12:42.740
反應的可能性有多大

00:12:42.760 --> 00:12:46.300
而和反應的難易程度無關