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《高等有機(jī)化學(xué)》PPT課件.ppt

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《高等有機(jī)化學(xué)》PPT課件.ppt

高等有機(jī)化學(xué)Advanced Organic Chemistry,Chapter 1. Introduction,1、About Contents of Advanced Organic Chemistry 2、The Relation between Advanced Organic Chemistry and Organic Syntheses 3、The method of Studying Advanced Organic Chemistry,參考書 (1),1)MARCHS ADVANCED ORGANIC CHEMISTRY; 6th Edition; Michael B. Smith, Jerry March (該書的第五版在2009年已由清華大學(xué) 李艷梅教授翻譯) 第一部分,分9章 基礎(chǔ)理論 第二部分,分10章 反應(yīng)與反應(yīng)機(jī)理 大量的原始文獻(xiàn) Organic Syntheses(OS)出處,參考書 (2),2)高等有機(jī)化學(xué) 王積濤編 3)物理有機(jī)化學(xué) 高振衡編 4)高等有機(jī)化學(xué) 美Carey and Sundberg, 夏熾(chi)中譯 分A、B兩卷: A卷:有機(jī)結(jié)構(gòu)理論, B卷:反應(yīng)與合成 5)高等有機(jī)化學(xué)基礎(chǔ)(第三版) 榮國斌編,Chapter 2. Covalent Bonding,Section 1. Wave Equation Section 2. Localized Chemical Bonding Section 3. Delocalized Chemical Bonding,Section 1. Wave Equation,宏觀物體的運(yùn)動(dòng)我們用牛頓三大定律來描述: F = ma F = -F 慣性定律 微觀粒子要用Schrdinger方程來描述: H(x,y,z) = E(x,y,z) 式中,E是體系的能量,h為Plunk 常數(shù),m為微觀粒子的質(zhì)量,V是位能,關(guān)于Schrdinger方程,Schrdinger是偏微分方程 不是推出來的,它是一種大膽的設(shè)想; 正確與否要靠事實(shí)來回答; 目前還沒有發(fā)現(xiàn)與該方程相沖突的事實(shí) 要 象承認(rèn)牛頓三大定律一樣,我們必須承認(rèn)它。 問題 既然這個(gè)方程是正確的,我們的化學(xué)家就應(yīng)該很容易掌握它; 根本不要作實(shí)驗(yàn), 只要按這個(gè)方程計(jì)算一下, 就可以得出分子的結(jié)構(gòu)、性質(zhì)、反應(yīng)條件、產(chǎn)率., 象我們計(jì)算太陽系 八大行星的運(yùn)行情況一樣。 可是實(shí)際上并非如此, 原因在于位能函數(shù)V。,在求解過程中要求對這四個(gè)量子數(shù)有下列限制:,當(dāng)出了問題時(shí)有提出一些修正: 諸如屏蔽效應(yīng)、鉆透效應(yīng).,我們能精確求解的原子:氫原子,關(guān)于Schrdinger方程,n = 1,2,3,; l = 0,1,2,3n-1 (s, p, d, f ) ; m = 0, 1, 2, l; ms = 1/2,通過求解得出n,l,m, ms等量子數(shù)(即軌道),然后把氫原子的結(jié)構(gòu)模型套到其他的原子上,根據(jù)近似方法不同,我們得到多種化學(xué)鍵理論:,關(guān)于Schrdinger方程,目前能精確求解的分子:氫分子離子 其他分子:只能近似求解,對于分子,近似方法,價(jià)鍵法 分子軌道法,About Valence-bond Method and Molecular Orbital Method,分子軌道法(Molecular Orbital Method),價(jià)鍵法(Valence-bond Method),包括 電子配對法 雜化軌道理論 這些都是求解的近似方法。,近似方法: 忽略了許多因素 得出的結(jié)論很粗糙 對于它的定量已無多少實(shí)用意義 有機(jī)化學(xué) 只對求解的一些定性結(jié)論感興趣 對它的求解過程及具體的解不感興趣 這些定性結(jié)論 即價(jià)鍵理論和分子軌道理論,問題:,The relation of Valence-bond Method and Molecular Orbital Method,平時(shí)我們對這些理論并不都用 究竟用什么理論處理,這要看具體問題 那一種來得方便簡單,且又準(zhǔn)確,我們就用那一種 我們在一些場合用價(jià)鍵理論,而在另一場合我們用分子軌道理論。 這兩種理論沒有說那個(gè)好的問題 因?yàn)閮烧叨际墙魄蠼?,都忽略了一些因素?對于鍵,我們用價(jià)鍵理論 對于共軛體系我們用分子軌道理論處理 在這種場合下如果我們再用價(jià)鍵理論,就會(huì)得出錯(cuò)誤的結(jié)論。 我們不能說誰比誰更準(zhǔn)確 兩種理論是互補(bǔ)的,我們大家一定要注意,Section 2. Localized Chemical Bonding,1、Definition of Localized Chemical Bonding. 2、Valence-bond (VB). 3、Hybridization.,1、Definition of Localized Chemical Bonding.,Localized Chemical Bonding maybe defined as bonding in which the electrons are shared by tow and only two nuclei. (Delocalized Chemical bonding: maybe defined as bonding in which the electrons are shared by more than tow nuclei.),所有的鍵和孤立的鍵都是定域鍵。 處理定域鍵時(shí)使用價(jià)鍵理論和分子軌道理論得到同樣的結(jié)論 價(jià)鍵理論比較直觀,形象、簡單,而分子軌道理論比較抽象復(fù)雜 所以我們處理定域鍵時(shí)使用價(jià)鍵理論。 處理離域鍵時(shí)使用分子軌道理論。,2、Valence-bond (VB),(略),3. Hybridization,(略),Section 3. Delocalized Chemical Bonding,1. Definition of Delocalized Chemical Bonding . 2.The Structure of Benzene . 3. Resonance Theory . 4. Aromaticity,Definition of Delocalized ChemicalBonding,Delocalized Chemical Bonding maybe defined as bonding in which the electrons are shared by more than tow nuclei.,2.The Structure of Benzene,1) Lewis 結(jié)構(gòu),即電子結(jié)構(gòu)。 2) 用Lewis 結(jié)構(gòu)表示苯的缺陷。 3)離域域處理苯。 4)其他離域鍵的分子舉例 。,1) Lewis Structure 即電子結(jié)構(gòu),2) 用Lewis 結(jié)構(gòu)表示苯的缺陷。,苯的Kekule結(jié)構(gòu)式:,很難想象,后來發(fā)展的共振論用兩個(gè)式子來表示:,6個(gè)C等同,6個(gè)C-H鍵等同!,C6H6,但這樣表示不能說明為什么鄰二取代物只有一種! 因?yàn)槿舭碖ekule結(jié)構(gòu)式鄰二氯苯應(yīng)該有下列兩種結(jié)構(gòu):,苯的化學(xué)性質(zhì)易取代,不易加成。 苯的氫化熱問題。 理論計(jì)算: 環(huán)己烯:28.6千卡/摩爾 環(huán)己三烯:28.63=85.8Kcal/mole (若三個(gè)雙鍵無相互作用),關(guān)于苯的穩(wěn)定性:離域能,實(shí)際氫化熱:49.8千卡/摩爾 差值:36千卡/摩爾 鍵長問題:CC: 1.48, (148pm, 0.148nm) C=C: 1.32 苯: 1.4 , 正六邊形,3)分子軌道(離域)處理苯。,分子軌道理論處理: 把三個(gè)雙鍵作為離域鍵來考慮 認(rèn)為6個(gè)電子是離域的,而不作為定域處理。 Hckel分子軌道理論(HMO): 骨架用價(jià)鍵理論處理 鍵用分子軌道處理。,苯的Hckel分子軌道法,每個(gè)C原子剩下1個(gè)p軌道,共剩下6個(gè)p軌道 這6個(gè)p軌道形成離域鍵,形成6個(gè)分子軌道 其中三個(gè)是成鍵軌道,6各C原子采取sp2雜化成鍵形成下列骨架 :,3個(gè)成鍵的軌道,1,2,3,整個(gè)軌道的電子云圖如下:,6個(gè)軌道能量: +2,+,+,-,-,-2 :庫侖積分,:交換積分(負(fù)值),又稱共軛積分。,苯的軌道能量和,-2 - - + , + +2,3個(gè)成鍵軌道能量和:6+8 環(huán)己烯的能量:2+2 環(huán)己三烯能量:6+6 差值:2/mol 即 36kcal/mol 稱為離域能、共軛能,HMO的不足,HMO解釋了苯的性質(zhì),穩(wěn)定性,鍵長平均化。 苯的另一種表示方式:,缺點(diǎn):忽略了電子間的排斥。 如果把鍵和鍵一起作為分子軌道進(jìn)行計(jì)算,結(jié)果會(huì)更精確。且會(huì)得出一些很有用的結(jié)果。現(xiàn)代計(jì)算機(jī)使這種計(jì)算成為可能,4)其他離域鍵的分子舉例,1,3-丁二烯 CH2=CH-CH=CH2,HMO法:,反鍵軌道,成鍵軌道, -1.618, -0.618,+1.618, + 0.618,成鍵電子能量: 4+4.472 孤立雙鍵能量: +, 2個(gè)孤立雙鍵能量:4+4, 離域能:0.472,1,3-丁二烯四個(gè)軌道的電子云圖,烯丙基,成鍵軌道,反鍵軌道,非鍵軌道,P 軌道,0個(gè)電子,1個(gè)電子,2個(gè)電子,軌道位相,電子云,氯乙烯,交橫共軛,3. Resonance Theory,分子的真實(shí)結(jié)構(gòu)不知道(即不能用一種Lewis 結(jié)構(gòu)表示),但可以用幾種Lewis結(jié)構(gòu)共同來描述,這幾種Lewis結(jié)構(gòu)稱做共振極限結(jié)構(gòu)。整個(gè)分子具有所有這些共振極限結(jié)構(gòu)的性質(zhì)。 共振極限結(jié)構(gòu)必須符合經(jīng)典概念,例如:碳不能為5價(jià)。 所有共振極限結(jié)構(gòu)中,原子核的位置必須是一樣的,只是電子的排列方式不同。 共振極限結(jié)構(gòu)之間用符號“ ”連接起來。 每個(gè)共振極限結(jié)構(gòu)對分子的貢獻(xiàn)大小不一樣,穩(wěn)定結(jié)構(gòu)的貢獻(xiàn)最大。 共振極限結(jié)構(gòu)越多越穩(wěn)定。,共振論實(shí)際上是價(jià)鍵理論的延伸。,要點(diǎn):,共振論舉例,如苯:,氯苯:,關(guān)于p-d成鍵,如:,注意: 碳硫鍵,不是由氧原子p軌道和硫的p軌道重疊形成的。,硫原子的電子結(jié)構(gòu):3S23P4,硫采取SP3雜化,該分子為四面體結(jié)構(gòu),孤對電子占住一個(gè)頂點(diǎn)。,那么硫原子用什么軌道與氧原子的p軌道形成鍵呢?,答案:d軌道。,這就是p-d成鍵!,其他的p-d成鍵舉例,氧化胺,氧化膦,亞砜,(沒有共振,缺乏穩(wěn)定性),ylide(內(nèi)鎓鹽),磷的依立德,硫的依立德,氮的依立德,只能寫出這種形式 又一次說明瞵的依立德比氮穩(wěn)定。,注意: 硫的依立德也不夠穩(wěn)定,盡管有共振。,4. Aromaticity,芳香性是指分子的特殊穩(wěn)定性。如苯環(huán)具有下列特性: (1)高度的不飽和,(2)特別穩(wěn)定。 性質(zhì)清楚,但難尋求到一種滿意的定義。 有人提出芳香性與電子環(huán)即芳香六隅體有關(guān),但測定這種電子環(huán)困難。 現(xiàn)在可以用NMR來測定了。通過NMR確定是否有反磁環(huán)流存在。,關(guān)于方向性的定義,反磁環(huán)流,若有反磁環(huán)流存在則:,所以環(huán)上氫的=78ppm,環(huán)外氫<2ppm。,環(huán)外氫在高場區(qū)的證實(shí),后來合成了 :,paracyclophane,CH2為=12ppm,的比平常的低得多。越到中間越低。,有人提出:是否有反磁環(huán)流存在作為芳香性的定義。 盡管有缺陷,仍為大多數(shù)人接受。,Hckel的 4n+2規(guī)則,含有4n+2個(gè)電子的平面環(huán)狀共軛多烯具有芳香性(n為整數(shù))。 例如下列的電子符合4n+2 規(guī)則,有芳香性:,下列的電子不符合4n+2 規(guī)則,沒有芳香性:,輪 烯,分析CmHm的 環(huán)狀烯烴。 當(dāng)m=2k+1(奇數(shù))時(shí),為自由基,不穩(wěn)定。 當(dāng)m=2k(偶數(shù))時(shí),是穩(wěn)定的環(huán)狀共軛多烯,具有C2kH2k分子式的環(huán)狀共軛多烯稱:輪烯。,m=4 的輪 烯,m=4, C4H4:平面環(huán)丁二烯。 為雙自由基,不穩(wěn)定,實(shí)際采取折疊式:,鍵長不平均化,1.506 (150.6pm), 1.376 分子折疊后,軌道不重疊,形成兩個(gè)孤立的軌道。,m=6 的輪 烯,C6H6, 苯. 滿足 4n+2 規(guī)則,有芳香性。,m=8 的輪 烯,C8H8, 環(huán)辛四烯,不滿足 4n+2 規(guī)則,沒有芳香性。 事實(shí)上,它不采取平面結(jié)構(gòu),而是采取下列結(jié)構(gòu):,平面結(jié)構(gòu)的環(huán)辛四烯是反芳香性的。 非平面結(jié)構(gòu)的環(huán)辛四烯是非芳香性的。,m=10 的輪 烯,m=10, 環(huán)癸五烯,按Hckel的 4n+2規(guī)則,有芳香性。但空間結(jié)構(gòu)不允許:,反-順-反-順-順式 全順式 反-順-順-順-順式,實(shí)際上都不具有芳香性。原因:角張力、空間張力之故。,1,6-亞甲基10輪烯,鍵長均等,有反磁環(huán)流。,m=12 的輪 烯,C12H12, 環(huán)十二碳烯,4n,平面,不穩(wěn)定。,m=14、18 的輪 烯,C14H14,十四碳環(huán)七烯(n=3),十八碳環(huán)八烯,(n=4),這兩個(gè)化合物環(huán)足夠大,無空間張力,分子是平面的。有芳香性,而且環(huán)內(nèi)質(zhì)子的化學(xué)位移負(fù)得比較厲害。 目前已合成了二十碳環(huán)十烯,二十二碳環(huán)十一烯,二十四碳環(huán)十二烯,只有二十二碳環(huán)十一烯具有芳香性。,雜原子芳香體系,符合4n+2規(guī)則,有芳香性。 吡啶與吡咯的堿性問題。 吡啶與吡咯的極性問題。,帶電荷的芳香體系,茂 zhuo 莘,C-H的酸性問題,pKa = 16.0,蘭烴,1.08D,互變異構(gòu)與共振的區(qū)別,共振式 互變異構(gòu)體,Hckel規(guī)則 的圖解法,原子軌道能級,n 3 4 5 6,n 7 8 9 10,畫一個(gè)正多邊形,使正多邊形的一個(gè)頂點(diǎn)朝下 分子軌道的能級在正多邊形的頂點(diǎn) 外接圓的圓心為原子軌道能級。,圖解法求分子軌道:,Chapter 3. Stereochemistry,Section 1. Optical Activity and Chirality,Optically active: Rotation of the plane of polarized light Chirality Chiral molecule: a molecule is not superimposable on its mirror image (the property of nonsuperimposability),Enantiomers,Racemates,Properties of enantiomers,Properties of racemates,1)組成:兩種異構(gòu)體等量混合。 2)氣態(tài)、液態(tài)和溶液的性質(zhì)與各別對映體的性質(zhì)一樣(理想混合)。 3)固體的某些性質(zhì)與各別對映體的性質(zhì)不一樣 如:M.P.、Solubility,1. Dependence of Rotation on Condition of Measurement,對溶液:,對純化合物:,(c: g/ml ) (l: dm =10cm ),( d: g/ml ),: Amount of rotation, (l, t, sol, c, p, ),引入比旋光度:,Dependence of Rotation on Temperature,溫度不僅改變的大小,而且改變方向。 如天冬酸: 20D = +4.36, 90D = -1.86, 75D = 0 理論上濃度不影響比旋光度 但考慮到分子的締合、離解、溶質(zhì)與溶劑的相互作用的因素,比旋光度可能因濃度而發(fā)生變化。,2. What kinds of molecule display optical activity,下列化合物沒有手性: 具有對稱面的化合物 具有對稱中心的化合物 具有n階更迭對稱軸的化合物,有一個(gè)對稱中心的分子,i,下面我們來看這個(gè)具有對稱中心的分子是否有手性。即其鏡像是否與分子本身重疊?,鏡像是否與分子本身完全重疊! 分子的鏡像與分子本身完全重疊 分子無手性。 結(jié)論:有對稱中心的分子無手性。,n階更迭對稱軸,下面是4階更迭對稱軸軸,記作S4。,n階更迭對稱軸:Cnv,即旋轉(zhuǎn)反映軸。,具有n階更迭對稱軸(即Sn軸)的分子無手性。 注意:只有S4軸是獨(dú)立存在的,其它的軸Sn 都不是獨(dú)立存在的。如:S2軸中有對稱中心i存在。,上面已述:分子存在對稱中心 i,不對稱分子的充分條件必要條件,不對稱碳的存在,對旋光性 既不是充分條件,也不是必要條件。,(1)有不對稱碳的化合物 (2)含其他4價(jià)不對稱原子的化合物。如磷、氮。 (3)含三價(jià)不對稱原子的化合物 (4)取代的金剛烷 (5)包含合適取代的八面體原子的化合物 (6)轉(zhuǎn)動(dòng)受限而使分子產(chǎn)生手性軸 (7)螺旋形產(chǎn)生手性(右手螺旋和左手螺旋) (8)其他類型,(1)有不對稱碳的化合物,只有1個(gè)不對稱碳的化合物必旋光 不管不對稱碳上的4個(gè)基團(tuán)差別如何小 只要有差別,就有旋光 哪怕一個(gè)是氫一個(gè)是氘。 旋光的大小依賴于四個(gè)基團(tuán)極性差的大 小。,(2)含其他4價(jià)不對稱原子的化合物,(3)含三價(jià)不對稱原子的化合物,A. 傘效應(yīng),一般無手性。 B. 在三元環(huán)上的N 原子且該原子與帶孤電子對的原子連接,在室溫下消除了傘效應(yīng),因此有旋光性。 C. 若N原子在橋頭,也沒有傘效應(yīng),例 :,(4)取代的金剛烷,(5)包含合適取代的八面體原子的化合物,(6)轉(zhuǎn)動(dòng)受限而使分子產(chǎn)生手性軸,如:,(7)螺旋形產(chǎn)生手性(右手螺旋和左手螺旋),如:,(8)其他類型,例如:二茂鐵的兩個(gè)茂環(huán)上,一個(gè)環(huán)有一甲基,一個(gè)環(huán)上有兩個(gè)不同的鄰位取代基。,3. The method of Expressing Configuration,Fischer投影式:,The Fischer projection and its properties 橫前豎后,不能離開平面翻轉(zhuǎn),可在紙平面上轉(zhuǎn)180度。,命名法,1. DL命名法 以D-(+)-甘油醛為標(biāo)準(zhǔn):,2. RS法,D-(+)-甘油醛,4. The method of determining Configuration,原則:想辦法把未知物構(gòu)型和已知物構(gòu)型聯(lián)系起來。 (1) 不干擾手性中心的反應(yīng),反應(yīng)前后的化合物構(gòu)型不變。利用這一原理可以確定一些化合物的構(gòu)型。 (2) 若知道反應(yīng)機(jī)理,則可以從反應(yīng)物的構(gòu)型推測生成物的構(gòu)型。如:SN2機(jī)理。 (3) 似外消旋化合物法。 (4) 生物化學(xué)法。 (5) 旋光比較 。 (6) Bijvoet的專門X-衍射給出直接結(jié)果。 (7) 其它方法:旋光色散和不對稱合成。,似外消旋化合物法,外消旋化合物的的熔點(diǎn)高于R、S兩種異構(gòu)體任何一種。 這時(shí)外消旋化合物的熔點(diǎn)曲線為下圖。,外消旋化合物:,有的外消旋混合物的R、S兩種異構(gòu)體之間有很強(qiáng)的分子間作用力,,能以1:1的比例成對地結(jié)晶出來,,這時(shí)的外消旋混合物就成了外消旋化合物。,返回,似外消旋化合物,有時(shí)我們發(fā)現(xiàn)有這么一種情況: 化合物A和化合物B很相相似(但不相同) 化合物A的R異構(gòu)體和化合物B的S異構(gòu)體也形成類似的外消旋化合物 我們把它稱做為似外消旋化合物 似外消旋化合物的熔點(diǎn)曲線和真正的外消旋化合物一樣。 利用似外消旋化合物這種現(xiàn)象我們可以確定未知物的構(gòu)型。 將未知的B與已知構(gòu)型的A混合,測定其熔點(diǎn)曲線。,似外消旋混合物舉例,例如下列兩個(gè)化合物形成似外消旋化合物 。,(4) 生物化學(xué)法,酶是一種手性分子,它通常只進(jìn)攻一種構(gòu)型的分子。,已知某種酶進(jìn)攻8種氨基酸的L型, 若對于第九種氨基酸也進(jìn)攻, 那么,第九種氨基酸也是L型。,(5) 旋光比較 。,在同系列里, 旋光通常沿著一個(gè)方向逐漸改變。 若該系列足夠多的成員的構(gòu)型已知的話, 那么遺漏成員的構(gòu)型用類推法可以確定。,(6) Bijvoet的專門X-衍射給出直接結(jié)果,培養(yǎng)單晶,做X射線單晶衍射。,(7)其它方法,旋光色散: 測定比旋光度與波長的關(guān)系曲線圖 不對稱合成: 用有機(jī)合成 的方法將化合物合成出來。,5. Molecules with more than one chiral center,其它內(nèi)容(略) 講一講赤式和蘇式的概念。 赤式和蘇式這兩個(gè)名詞來源于赤蘚糖和蘇阿糖, 它們都是含有兩個(gè)手性碳的丁醛糖。,赤蘚糖和蘇阿糖(Erythroses reactions catalyzed by very strong bases and proceeding through aryne intermediates; reactions initiated by electron donors; reactions in which the nitrogen of a diazonium salt is replaced by a nucleophile.,Reactions that are successful at an aromatic substrate are largely of four kinds:,It is noted that solvent effects can be important,MECHANISMS,The SNAr Mechanism The SN1 Mechanism The Benzyne Mechanism The SRN1 Mechanism,There are four principal mechanisms for aromatic nucleophilic substitution.,1. The SNAr Mechanism,Evidence for The SNAr Mechanism,There is a great deal of evidence for the mechanism, we shall discuss only some of it.,e.g.:,Intermediates of this type are stable salts, called Meisenheimer or MeisenheimerJackson salts and many more have been isolated. The structures of several of these intermediates have been proved by NMR and by X-ray crystallography.,have been isolated,The break of the Ar-X was not the rate-determining step,When X was Cl, Br, I, SOPh, SO2Ph, or p-nitrophenoxy, the rates differed only by a factor of 5. This behavior would not be expected in a reaction in which the ArX bond is broken in the rate-determining step. We do not expect the rates to be identical, because the nature of X affects the rate at which Y attacks.,The following reaction indicated that Ar-X bond is not broken until after the rate-determining step.,The very fact that fluoro is the best leaving group among the halogens in most aromatic nucleophilic substitutions is good evidence that the mechanism is different from the SN1.,When X=F, the relative rate was 3300 (compared with I=1).,Element effect of leaving group,An increase in the electronegativity of X causes a decrease in the electron density at the site of attack attack, resulting in a faster attack by a nucleophile.,The SN1 Mechanism,For aryl halides and sulfonates, even active ones, a unimolecular SN1 mechanism is very rare; It has only been observed for aryl triflates in which both ortho positions contain bulky groups (tert-butyl or SiR3). It is in reactions with diazonium salts that this mechanism is important.,The evidence for the SN1 mechanism,Among the evidence for the SN1 mechanism with aryl cations as intermediates, is the following:,1. The reaction rate is first order in diazonium salt and independent of the concentration of Y. 2. When high concentrations of halide salts are added, the product is an aryl halide but the rate is independent of the concentration of the added salts. 3. The effects of ring substituents on the rate are consistent with a unimolecular rate-determining cleavage.,4. When reactions were run with substrate deuterated in the ortho position, isotope effects of 1.22 were obtained.,It is difficult to account for such high secondary isotope effects in any other way except that an incipient phenyl cation is stabilized by hyperconjugation, which is reduced when hydrogen is replaced by deuterium.,5. That the first step is reversible cleavage was demonstrated by the observation that when Ar15NN was the reaction species, recorvered starting material contained not only Ar15N N, but also ArN 15N.,This could arise only If the nitrogen breaks away from the ring and then returns. Additional evidence was obtained by treating PhN 15N with unlabeled N2 at various pressures. At 300 atm, the recovered product had lost 3% of the labeled nitrogen, indicating that PhN2+ was exchanging with atmospheric N2.,There is kinetic and other evidence that step 1 is more complicated and involves two steps, both reversible:,Intermediate Ar+N2, which is probably some kind of a tight ionmolecule pair, has been trapped with carbon monoxide.,The Benzyne Mechanism,Some aromatic nucleophilic substitutions are clearly different in character from those that occur by the SNAr mechanism (or the SN1 mechanism). These substitutions occur on aryl halides that have no activating groups; Bases are required that are stronger than those normally used;,The incoming group does not always take the position vacated by the leaving group (most interesting of all). e.g.: the reaction of 1-14C-chlorobenzene with potassium amide:,50% 50%,Mechanism,Other evidence,1. If the aryl halide contains two ortho substituents, the reaction should not be able to occur. 2. It had been known many years earlier that aromatic nucleophilic substitution occasionally results in substitution at a different position.,This is called cine substitution and can be illustrated by the conversion of o-bromoanisole to m-aminoanisole. In this particular case, only the meta isomer is formed.,3. The fact that the order of halide reactivity is Br I Cl F (when the reaction is performed with KNH2 in liquid NH3) shows that the SNAr mechanism is not operating here.,The SRN1 Mechanism,Problem: If benzyne Mechanism take place, the ratio of A and B should be the same because the same aryne intermediate would be formed in both cases.,A B 0.63 : 1,A B 5.9 : 1,Fact:,Reaction condition: Strong base Result: 1. cine substitution along with normal substitution 2. The ratio of A and B was not the same 3. If Cl or Br replace I, the ratio of A and B was 1.46:1,Explaining,This mechanism is called the SRN1 mechanism. An electron donor is required to initiate the reaction. In the case above the electron donor is solvated electrons from KNH2 in NH3.,To explain the iodo result, it has been proposed that besides the benzyne mechanism,this free-radical mechanism is also operating here:,Evidence for SRN1 mechanism,1. The addition of potassium metal (a good producer of solvated electrons in ammonia) completely suppressed the cine substitution. 2. addition of radical scavengers (which would suppress a free radical mechanism) led to A:B ratios much closer to 1.46:1. 3. In the case above, some 1,2,4-trimethylbenzene was found among the products.,REACTIVITY,The Effect of Substrate Structure,In the discussion of electrophilic aromatic substitution, equal attention was paid to the effect of substrate structure on reactivity (activation or deactivation) and on orientation. The question of orientation was important because in a typical substitution there are 4 or 5 hydrogens that could serve as leaving groups. This type of question is much less important for aromatic nucleophilic substitution, since in most cases, there is only one potential leaving group in a molecule. Attention is largely focused on the reactivity of one molecule compared with another and not on the comparison of the reactivity of different positions within the same molecule.,SNAr Mechanism,The substitutions are accelerated by electron-withdrawing groups, especially in positions ortho and para to the leaving group. hindered by electron-donating groups.,Benzyne Mechanism,Factor A: The direction in which the aryne forms,In such cases, the more acidic hydrogen is removed.,Acidity is related to the field effect of Z. An electron-attracting Z favors removal of the ortho hydrogen An electron-donating Z favors removal of the para hydrogen.,But when a meta group is present, the aryne can form in two different ways:,Factor B: the aryne, once formed, can be ttacked at two positions.,The favored position for nucleophilic attack is the one that leads to the more stable carbanion intermediate, and this in turn also depends on the eld effect of Z. For -I groups, the more stable carbanion is the one in which the negative charge is closer to the substituent. These principles are illustrated by the reaction of the three dichlorobenzenes with alkali-metalamides to give the predicted products shown.,Main product,Main product,Main product,The Effect of the Leaving Group,The common leaving groups in aliphatic nucleophilic substitution (halide, sulfate, sulfonate, NR3+, etc.) are also common leaving groups in aromatic nucleophilic substitutions. But the groups NO2, OR, OAr, SO2R and SR, which are not generally lost in aliphatic systems, are leaving groups when attached to aromatic rings. Surprisingly, NO2 is a particularly good leaving group. An approximate order of leaving-group ability is F NO2 OTs SOPh Cl, Br, I N3 NR3+ OAr, OR, SR, NH2. This depends greatly on the nature of the nucleophile.e,.g. C6Cl5OCH3 treated with NH2- gives mostly C6Cl5NH2; The leaving group is methoxy rather than chlorines.,The Effect of the Attacking Nucleophile,It is not possible to construct an invariant nucleophilicity order because different substrates and different conditions lead to different orders of nucleophilicity. But an overall approximate order is NH2 Ph3C PhNH (aryne mechanism) ArS RO- R2NH ArO- OH- ArNH2 NH3 I- Br- Cl- H2O ROH. As with aliphatic nucleophilic substitution, nucleophilicity is generally dependent on base strength: Nucleophilicity increases as the attacking atom moves down a column of the periodic table. But there are some surprising exceptions, for example, OH-, a stronger base than ArO-, is a poorer nucleophile.,Examples of Reaction,1. Hydroxylation of Aromatic Compounds,Conditions: activating groups are present or exceedingly strenuous conditions are employed.,For the conversion of aryl halides to phenols, a good method is the use of aryl Grignard reagents:,2. Alkali Fusion of Sulfonate Salts,3. Replacement by SH or SR,4. Replacement by NH2, NHR, or NR2,5. Replacement of a Hydroxy Group by an Amino Group (Bucherer reaction),6. Homo-Coupling of Aryl Halides: The Ullmann Reaction,7. ArylAlkyne Coupling Reactions,8. Conversion of Aryl Substrates to Carboxylic Acids, Their Derivatives, Aldehydes, and Ketones,9. The von Richter Rearrangement,Chapter 11. Aliphatic Electrophilic Substitution,脂肪族親電取代反應(yīng)的一般通式:,反應(yīng)發(fā)生在脂肪碳上,底物上的離去基丟下成鍵電子對而離去,進(jìn)攻試劑帶著空軌道進(jìn)攻。(注意:與親核反應(yīng)不同的地方),親核取代反應(yīng)的一般通式:,Section 1. Mechanisms,關(guān)于對脂肪族親電取代反應(yīng)機(jī)理的研究遠(yuǎn)不如對脂肪族親核取代反應(yīng)的研究那么多和那么深刻,也不如對芳香族親電反應(yīng)那么多那么深刻。這里我們向大家介紹三種可能的機(jī)理:,1. Bimolecular Mechanism. SE2 and SEi SE: Substitution Electrophilic 2:雙分子。 i:internal,SE2:又可分為前面進(jìn)攻和后面進(jìn)攻。 與SN2比較: 對于親核的雙分子機(jī)理,根本不可能發(fā)生前面進(jìn)攻,因?yàn)檫M(jìn)攻試劑的電子對和離去基的電子對相斥。而SE2因整個(gè)體系是缺電子的,試劑可以前面進(jìn)攻而吸引,最后成鍵:,協(xié)同進(jìn)行,前面進(jìn)攻,構(gòu)型保留。 這三種機(jī)理中有兩種構(gòu)型保留,只有一種構(gòu)型翻轉(zhuǎn),就立體化學(xué)很難區(qū)分SE2(前面)和SEi,SEi:,保型(類似于SNi,但是雙分子機(jī)理),2. SE1 Mechanism 單分子機(jī)理,SE1類似于SN1,也分兩步,不同的是電子轉(zhuǎn)移情況不同:,立體化學(xué):中間體是碳負(fù)離子,若碳負(fù)離子由于共軛效應(yīng)的存在而使得它成平面結(jié)構(gòu),那么反應(yīng)產(chǎn)物是外消旋化的;若無共軛效應(yīng),碳負(fù)離子是錐形的有可能構(gòu)型保留,也有可能部分翻轉(zhuǎn)(傘效應(yīng)),這要視溶劑而定。,3. Electrophilic Substitution Accompanied by Double-Bond Shifts,(2) 當(dāng)然也可以先加上Y+ 然后X+ 離去:,(3) SE2,(4) SEi,因?yàn)镾E反應(yīng)機(jī)理研究很不完善,所以有很多反應(yīng)很難講是以那種機(jī)理發(fā)生的。,Section 2. Reactivity,對反應(yīng)性規(guī)律的分析,請大家按照對脂肪族親核取代反應(yīng)的同樣思維方法去分析問題。這里為大家分析問題提供一點(diǎn)線索,也就是請大家著重考慮以下性質(zhì):,1. Effect of Substrate,2. Effect of Leaving Group,離去基的酸性,酸性越弱越容易離去(穩(wěn)定)。,3. Effect of Solvent,Section 3. Some Examples of Reaction.,1. Hydrogen as Leaving Groups,A) Halogen electrophiles,1) :羰基的-H的鹵代:,a)鹵化劑 : 鹵化劑可以是Br2,Cl2,I2,但不能用氟。對于活潑的化合物可用XeF2氟化氙氟化。能用的氟化劑還有:,這里講的活潑的化合物是指-酮酯,-二酮。,b)底物 對于不對稱酮,較易鹵化的是C-H,其次是CH2,再次是CH3,通常是得到一種混合物,醛的鹵化有時(shí)發(fā)生在CHO氫上。 對于氯化,二取代發(fā)生在同一碳上,而對于溴化則在兩個(gè)碳上,形成, 二取代。 c) 反應(yīng)機(jī)理:,分酸催化和堿催化。 Acid catalysts:,Basis Catalysts:,形成 CHBr后,C-H的酸性更強(qiáng),更易在堿的作用下烯醇化。 Regioselectivity (位置選擇性): 用RC(OBR2)=CHR作底物與N-溴代琥珀酰亞胺(N-bromosuccinimide)反應(yīng),得區(qū)域選擇性反應(yīng)產(chǎn)物:,NBS是一種很好的溴化劑。,2) 羧酸和酰鹵的反應(yīng) Halogenation of Acids and Acyl Halides 此反應(yīng)與前面講的反應(yīng)相似:對于酸有:,在這里用PX3作催化劑,中間體是酰鹵。對酐和酰氯則無須催化劑。,B) Nitrogen Electrophiles,1) 氮烯和碳烯的插入反應(yīng) Insertion by Nitrenes and Carbenes,W=烷基、芳基 or OR基 生成產(chǎn)物:酰胺、酯 當(dāng)分子中有幾種C-H時(shí),C-H鍵的活性次序:tertiarysecondaryprimary 反應(yīng)一般以單線態(tài)反應(yīng),反應(yīng)過程中,手性原子的構(gòu)型不發(fā)生翻轉(zhuǎn)。反應(yīng)物一般是一個(gè)混合物,因此無合成價(jià)值。,上述反應(yīng)物一般是一個(gè)混合物,無合成價(jià)值。不過在成環(huán)時(shí),此反應(yīng)還時(shí)有用:,碳烯也有同樣的性質(zhì):,反應(yīng)特點(diǎn)與上面反應(yīng)的特點(diǎn)一樣: C-H活性順序 tertiarysecondaryprimary。在合成上也沒有多大的用途。 烷基卡賓一般發(fā)生重排,而難以發(fā)生分子內(nèi)插入:,當(dāng)重排不可能時(shí),則發(fā)生插入:,關(guān)于這些反應(yīng)的機(jī)理,認(rèn)為有兩種: 其一、協(xié)同機(jī)理(過度態(tài)三員環(huán)):,其二、自由基機(jī)理:,一般認(rèn)為單線態(tài)按其一反應(yīng),三線態(tài)按其二反應(yīng)。,Aliphatic Diazonium Coupling,如果CH2 的酸性足夠強(qiáng),在堿的催化下,它與重氮鹽發(fā)生偶合。該反應(yīng)經(jīng)常在醋酸

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