线性自由能关系AOC-13

1第13章线性自由能关系（LinearFree-EnergyRelationships,LFERs)13.1Hammett方程---meta和para常数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logk或logK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23p-NO2m-NO2p-ClHp-Mep-OMem-NH2p-NH2－2.0－1.2－0.400.82.0－0.80.400.40.81.2lgk/k0从实验得出，mlgK/K0=lgk/k0根据2.303RTlgK=－△G2.303RTlgk=－△G≠m(lgK－lgK0)=lgk－lgk0m(－△G/2.303RT+△G0/2.303RT)=－△G≠/2.303RT+△G0≠/2.303RT则m(－△G+△G0)=－△G≠+△G≠0m△△G=△△G≠lgK/K0苯甲酸和取代苯甲酸的酸性离解常数（K）的对数比lgK/K0与它的乙酯水解速度常数k的对数比lgk/k0ArCO2HArCO2－+H+ArCO2Et+OH－ArCO2－+EtOH自由能差值之间的正比关系式:线性自由能关系4lgKK0=lgkk0=：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（ReactionConstant）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25C时于水中的离解为标准反应，取=1.000Hammett方程5HammettSubstituentConstantsSubstituentmetaσparaσSubstituentmetaσparaσSubstituentmetaσparaσH00F+0.34+0.06CF3+0.43+0.54CH3-0.07-0.17Cl+0.37+0.23CO2H+0.36+0.41C2H5-0.07-0.15Br+0.39+0.23CO2CH3+0.32+0.39(CH3)3C-0.10-0.20I+0.35+0.28COCH3+0.38+0.50C6H5+0.06-0.01OCOCH3+0.39+0.31SO2CH3+0.60+0.72(CH3)3Si-0.04-0.07NHCOCH3+0.210.00CN+0.56+0.66OCH3+0.12-0.27SH+0.25+0.15NO2+0.71+0.78OH+0.12-0.37SCH3+0.15+0.00(CH3)2S(+)+1.00+0.92NH2-0.16-0.66CO2(–)-0.100.00(CH3)3N(+)+0.88+0.82O(–)-0.71-1.00SO3(–)+0.05+0.09N2(+)+1.76+1.9167σmeta=lgKX-metaK0σpara=lgKX-paraK0mσ=－△G≠/2.303RT+△G≠0/2.303RTmσ=lgk－lgk0=lgk/k0,m=1(相当于直线斜率）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ValuesforacidDissociationsCO2HXacidSolventTemp.(C)。ρH2O251.00CO2HXEtOH251.957CO2HNO2XH2O250.905OHXH2O252.113ArCH2CO2HArCH2CO2－+H+(inH2O)=0.56ArCH2CH2CO2HArCH2CH2CO2－+H+(inH2O)=0.24请比较9ValuesDerivedfromRatesofHeterolyticReactions请比较：ArCH2CO2Et+OH－ArCH2CO2+EtOH=1.00ReactionSolventTemp.(°C)XCO2Me+OHXCO2XCO2Et+HXCO2HXCOCl+H2OXCO2HXCHO+HCNXCH(OH)(CN)ClHX+EtOHOEtHX+HCl60%acetone60%acetone50%acetone95%EtOHEtOH25100020252.2290.1060.7972.3295.09010若干芳香族亲电取代反应的值反应溴化(inHOAc)13.1氯化(inMeNO2)13.0(inHOAc-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)2.411(I)(II)(III)I00=0lgk/k0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12log(K/Ko)orlog(k/ko)=σρ13苯甲酸乙酯在99.9%的浓硫酸中的水解logk0.71.40.70.0－0.7－2.1对于供电子基或弱吸电子基，对应的机理为：ArOHOEt+ArOOEtH+CArO++EtOHHydrolysisofbenzoatesin99.9%H2SO4,kisinunitsofhr－1.H2OArCO2HForelectron-releasingsubstituents(lowvalues)theHammettplothasanegativeslope.Thissuggestsamechanisticpathwayinwhichpositivechargedevelopsinthetransitionstateofthereaction(or,moreprecisely,atransitionstatewherethechargechangeisinapositivedirection).Amechanismthatisconsistentwiththisobservationistheformationofasubstitutedbenzoylcation,asshownbelow,whichthenreactswithwatertogivethereactionproduct;inthismechanisticpathwaytherate-determiningheterolysisoftheacyl-oxygenbondisfacilitatedbyeletron-releasingsubstituents,sinceapositivelychargedcarbonatomisgeneratedadjacenttothering.14Forelectron-withdrawinggroups(largevalues)theHammettplothasapositiveslope.Thissuggeststhemechanisticpathway:Thepositiveslopesuggestseithernegativechargeformationinthetransitionstate,oralternatively,areductioninreactantpositivecharge.Achangeinthesubstituentwillchangethemechanismwhentherelativestabilityofthetwocompetingcationsinverts.ArOEtOHArOHEtO+H2OEtOH15Hammett方程改进：引入其它计算值的方法。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:XCClXC+X+XCOH标准反应：25C,90%丙酮水溶液。+: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16Hammett方程应用举例例1：对氯苯甲酸pKa=3.98,苯甲酸的pKa=4.19.计算p-Cl值lgKp-Cl/lgKH==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=0.30lgkp-NO2lgkH=1.02lgkH=lgkp-Br+0.30lgkH=lgkp-NO2+1.02lgkp-Brlgkp-NO2=0.72kp-Br/kp-NO2=5.2517TaftEquationWhiletheHammettequationaccountsforhowfield,inductive,andresonanceeffectsinfluencereactionrates,theTaftequationalsodescribesthestericeffectsofasubstituent.describesthestericeffectsofasubstituent.TheTaftequationiswrittenas:describesthestericeffectsofasubstituent.TheTaftequationiswrittenas:18σ*:PolarSubstituentConstantsEs:StericSubstituentConstantsSelectedEsandσ*values19Thebookoforganicchemistryisnotjusttoberead,butalsotobewritten!He(orshe)whositsatbottomofawelltocontemplatethesky,willfinditsmall.20Hardcover:1824pagesPublisher:Wiley-Interscience;5thedition(January15,2001)ISBN:0471585890ProductDimensions:9.4x6.4x2.6inchesEditorialReviews...thebookremainstremendousvalueformoney...morepagesperbuckthanmostothertexts...soitwillremainafirmfavouriteasageneralorganictextandaneasy-to-use1-volumereference...willundoubtedlyappearinallorganicchemistrylibrariesandprobablyonmanychemistspersonalbookshelvestoo.(OrganicProcessResearch&DevelopmentJournal,Vol.5,No.6,November2001)--ThistextreferstotheHardcoveredit