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1、1,第13章 线性自由能关系 （Linear Free-Energy Relationships, LFERs ),30年代初：Hammett (Columbia大学)， Burkhardt (Manchester)在对取代苯衍生物的反应性研究中发现，间位或对位极性取代基对这些化合物侧链的影响与这些体系的log k或 log K之间存在线性关系，提出了著名的Hammett方程,线性自由能关系研究:预测某类反应的速度或一平衡的位置预测在何条件下可使机理发生改变,2,3,苯甲酸和取代苯甲酸的酸性离解常数（K）的对数比lg K/K0与它的乙酯水解速度常数k的对数比 lg k/k0,自由能差值之间的正
2、比关系式:线性自由能关系,4,：取代基常数（分为meta 和para，包括诱导电场效应和共轭效应）：反应特性常数（Reaction Constant）苯环上吸电子基有利于苯甲酸解离，取代基常数为正值；供电子基则为负值。如甲氧基p = 0.27; 硝基的p = 0.78:反应对极性效应敏感度大小的一个量度，由反应性本身（如溶剂、温度以及其它反应条件）决定。一般选取苯甲酸在25C时于水中的离解为标准反应，取 = 1.000,5,Hammett Substituent Constants,6,7,线性自由能关系的存在，需要满足以下条件的一个 焓变H保持恒定 熵变S保持恒定 H与S互成直线关系,苯甲酸
3、的离解和某些标准的芳香族化学反应符合这一情况,8,若干典型的反应常数, Values for acid Dissociations,ArCH2CO2H ArCH2CO2 + H+ (in H2O) = 0.56ArCH2CH2CO2H ArCH2CH2CO2 + H+ (in H2O) = 0.24,请比较,9, Values Derived from Rates of Heterolytic Reactions,请比较：ArCH2CO2Et + OH ArCH2CO2 + EtOH = 1.00,10,若干芳香族亲电取代反应的值,反 应溴化 (in HOAc)13.1氯化 (in MeNO2
4、)13.0 (in HOAc-H2O)8.8质子化 (H2SO4-CF3CO2H-H2O)8.6乙酰化 (MeCOCl, AlCl3, C2H4Cl2)8.6硝化 (H2SO4-HNO3)6.4氯化 (HOCl, H+)6.1烷基化 (EtBr, GaBr3)2.4,11, 1: 与标准反应（即苯甲酸系列在水中于25离解）相比离子化或反应的速度，对电子效应更敏感0 1: 吸电子基也提高离解常数或反应速度，但与标准反应相比对电子效应的敏感度较低若 0: 供电子基有利于反应速度的提高当考虑反应速度时，如果值很小，表明反应机理中涉及的中间体具有游离基特点或具有很小的电荷分离。如苯甲酸在乙醇中解离的值
5、要比在水中大许多。究其原因，乙醇是离子化较弱的溶剂，取代基对苯甲酸在乙醇中的酸强度比在水中的影响更大,12,log(K/Ko) or log(k/ko) = ,13,苯甲酸乙酯在99.9%的浓硫酸中的水解,For electron-releasing substituents (low values) the Hammett plot has a negative slope. This suggests a mechanistic pathway in which positive charge develops in the transition state of the reaction
6、 (or, more precisely, a transition state where the charge change is in a positive direction). A mechanism that is consistent with this observation is the formation of a substituted benzoyl cation, as shown below, which then reacts with water to give the reaction product; in this mechanistic pathway
7、the rate-determining heterolysis of the acyl-oxygen bond is facilitated by eletron-releasing substituents, since a positively charged carbon atom is generated adjacent to the ring.,14,For electron-withdrawing groups (large values) the Hammett plot has a positive slope. This suggests the mechanistic
8、pathway:,The positive slope suggests either negative charge formation in the transition state, or alternatively, a reduction in reactant positive charge.,A change in the substituent will change the mechanism when the relative stability of the two competing cations inverts.,15,Hammett方程改进：引入其它计算值的方法。
9、有的考虑共轭效应(R)和诱导效应（F）代替值，如+、 常数等直达共轭作用示例:,+: 给电子共轭作用较强的对位取代基，如-OMe等。: 吸电子共轭作用较强的对位取代基，如-NO2等（标准反应为对硝基,16,Hammett方程应用举例,lg Kp-Cl/lg KH = = 1 = lgKH (lgKp-Cl) = pKaH pKap-Cl = 4.19 3.98 = 0.21,例2：查表，计算对溴苄基溴与对硝基苄基溴在水中水解的 相对速度(已知 = -1.31),lgkp-Br/kH = (1.31)(0.23) lgkp- NO2/kH = (1.31)(0.78)lgkp-Br lgkH =
10、 0.30 lgkp- NO2 lgkH = 1.02lgkH = lgkp-Br + 0.30 lgkH = lgkp- NO2 + 1.02lgkp-Br lgkp- NO2 = 0.72 kp-Br/ kp- NO2 = 5.25,17,Taft Equation,While the Hammett equation accounts for how field, inductive, and resonance effects influence reaction rates, the Taft equation also describes the steric effects of
11、 a substituent.,describes the steric effects of a substituent. The Taft equation is written as:,describes the steric effects of a substituent. The Taft equation is written as:,18,Selected Es and * values,19,The book of organic chemistry is not just to be read, but also to be written! He (or she) who
12、 sits at bottom of a well to contemplate the sky, will find it small.,20,Hardcover: 1824 pages Publisher: Wiley-Interscience; 5th edition (January 15, 2001) ISBN: 0471585890 Product Dimensions: 9.4 x 6.4 x 2.6 inches Editorial Reviews .the book remains tremendous value for money.more pages per buck
13、than most other texts.so it will remain a firm favourite as a general organic text and an easy-to-use 1-volume reference.will undoubtedly appear in all organic chemistry libraries and probably on many chemists personal bookshelves too. (Organic Process Research & Development Journal, Vol. 5, No. 6, November 2001)-This text refers to the Hardcover edition.,Hardcover: 1495 pages Publisher: John Wiley 4th edition (July 1, 1992) ISBN: 0471601802 Product Dimensions: 3.0 x 6.5 x 9.8 inches Editorial Reviews,21,22,文 献 考 察,23,24,Happy Holiday,25,Good Luck!,
