高等有机化学chapter02课件

Chapter 2 Kinetics&ThermodynamicsThe objective of physical organic chemistry is an explanation and/or understanding of 1.)the motive forces that prompt spontaneous changes in all their variety,in terms of the fundamental physical properties of the compouds involved,and 2.)the factors that determine the rate at which a reaction proceeds.12.1 The tendency toward chemical reaction Driving Force AffinityCH4+3O2 CO2+2H2O+890kj(213kcal)2.1.1 the enthalpy of reactionThe First Law of thermaodynamicsdU=dq+dwWhere,dq heatdw workdw=dp dv p dvPconstantThermodynamics.2hence,dq=dU+pdvdH=dU+pdvD DH=D DU+pD Dventhalpy,Hintegrals The enthalpy of a given reaction:the changes in enthalpy during the process of a reaction.The standard state:1 atm,25oC.The enthalpy of formation of a given compound.The enthalpy of combustion of a given compound.3Estimating the enthalpy of a reaction From standard enthalpy of formation of compoundsD DHo=D DHofproducts-D DHof reactants From bond energies:(approximately)D DHo=(bond energies)formed-(bond energies)broken From the heat of combustionD DHo=D DHoc reactants-D DHoc productsThe feasibility of a given chemical process:there is a tendency for a spontaneous reaction to occur such that D DHo is negative,i.e.,an exothermic process.D DHo 0.4Endothermic raction:C(graphite)+H2O(gas)CO(gas)+H2(gas)D DHo:0 -242 -110 0D DH=-110 (-242)=+132 kj/mol (31.5kcal/mol)This reaction proceeds toward right while the temperature is high enough!Why?.52.1.2 the Entropy of ractionThe definition of entropy dS=dqrev/Tqrevreversible heat exchageFrom thermodynamic point of view S=k ln From statistical point of viewk Boltzmann constant.the number of microstates.The change of entropy with temperatureS(T2)=S(T1)+dH/T S(T2)=S(T1)+Cp/T Cp=(H/T)P.6The Second Law of Thermodynamics:The Third Law of Thermodynamics:Absolute entropy:S(T)=0TdH/TS(0K)=0The entropy change in chemical processes:Increasing:dissociative process Decreasing:associative process Solution of crystals;Mixing of reagents;Breaking of bonds;Crystallizations;Hydrogen bonding;Forming of bonds;in a closed system,spontaneous processes occur in the direction of increasing entropy.72.1.3 the Gibbs Free EnergyG=H-TSD DG=D DH T D DST=constantThe affinity of chemical reaction:the change in G A chemical reaction may occur spontaneously when D DG is negative,D DG 0.CH3+3O2 CO2 +2H2O(l)D DG=-197.7 kcal/mol.at T=300k,D DH=-212.7kcal/mol,D DS=-0.050 kcal/mol,.8C +H2O CO +H2D DH=31.5 kcal/mol,D DS=+0.0315 kcal/mol,at T=300k,D DG=31.5 3000.0315=+22 kcal/mol.at T=500k,D DG=31.5 5000.0315=+16 kcal/mol.at T=1000k,D DG=31.5 10000.0315=0 kcal/mol.A endothermic raction with a increase of entropy may proceed in either direction depending on the temperature.Chemical potential,m m,in solution.92.1.4 Chemical equilibrium v.s.D DGReversible process in thermodynamicsReversible chemical reactionIsomerization:500oCor H+cis-but-2-enetrans-but-2-eneEquilibrium Constant K:K=-cistransIn general,K=P PproductsainP PreactantsajmP PproductscinP Preactantscjm D DG=-RTlnKR,gas constant.10The relationship between D DG and K (A B at 298k)DG(kcal/mol)K Percentage(more stable)0 1.00 50 0.24 1.50 60 0.50 2.33 70 0.82 4.00 80 1.30 9.00 90 2.71 99.00 99 4.06 999.00 99.9 5.45 9999.00 99.99 .112.1.5 Brief Summary on Thermodynamics Fundamental application of D DG Estimation of a given chemical process occuring spontaneously under a given condition.Estimation of equilibrium composition for a reversible chemical process under a given condition.D DG 0D DG=RTlnK Measurement of thermodynamic data Experimental approach Molecular orbital calculations The limitations of the usefulness of thermodynamic data Complexity of solvation effects No information about the rates of chemical reactions.122.2 The rate of chemical reaction kineticsThe goal of a kinetic studyTo establish the quantitative relationship between the concentration of reactants,catalysts and the rate of the chemical raction process.The goal of a given reactionThe disapppearance of a reactant or the appearance of product.2.2.1 The kinetic expression and reaction mechanismThe empirical rate equation(ERE)or rate law:Rate=(products)=(reactants)=k P Pcindcdtdcdtc,concentration(activities);ci,the concentration of the ith reactant;k,rate constant(specific rate coefficient);n,the overal kinetic order of the reaction(n=ni).13A +B CRate=dAdt dBdt=dCdt=k ABNotes:k may include the term ci.n in normally but not necessarilly is integral.The importance of the ERE lies in its interpretation:those concentrations which it contains represent the species which together constitute the transition state(TS).14Some examples of reaction rate expressionR3N+MeI R3NMe+I+-Rate=k2 R3NMeIBimolecular raction,second order.heat+Rate=k1 cyclohexeneUnimolecular raction,first order.15CH3CH2CH2CH2Br+H2O CH3CH2CH2CH2OH+H+Br+-Rate=k2 1-BuBrH2OH2O=constant Rate=k1 1-BuBrpseudo-first orderCH3COCH3+Br2 CH3COCH2Br +HBraqueous bufferRate=k acetoneH+=k acetone=ksecond-order conditionspH=constant,first-order conditionspH=constant and a larger excess of acetone,zero-order conditions Bromine does not appear in the ERE.Single step reaction Multiple steps reaction.162.2.2 The rate-determining step in multiple steps reactiona).A B C Dk1k2k3k1b).A B CK-1k2ractive intermediatesThe overall rate of the reaction will depend on the rate of the slowest step of the sequence.The rate-determining step:the slowest step of the sequence.17a).A B C Dk1k2k3For a),If k1 k2 and k3Rate=k1 AIf k2 k1 and k3Rate=k2 BB=k1 A Rate=k1 k2 AIf k3 k-1,then k2D+k-1 k2DRate=k1AB D k1 k2 A B k2 D If k2D benzene.25The feature of the three provided kinetic expressions:Rate(A)Rate(B)Rate(C)=kobs HNO32benzeneNO3-Mechanism B has the distinctive feature that it is zero-order in the reactant benzene.Mechanism B and C differ only in the inclusion of water.If the concentration of water is several times larger than that of benzene,the term for the concentration of water would disappear so that the form of the kinetic expression alone would not distinguish between mechanisms A and C.262.The base-catalyzed reaction of benzaldehyde and acetophenonephCH=CHCOphphCHO +phCOCH3 phCHCH2COphOH(1)phCOCH3 +OEt phCOCH2 +EtOHk1k-1-(2)phCOCH2 +phCHO phCHCH2COphk2k-2-O(3)EtOH +phCHCH2COph phCHCH2COph+EtOk3k-3-OHO-(4)EtO +phCHCH2COph phCH=CH2COph+EtOH+OHk4OH-.27If step 1 is rate-controlling,Rate=k1phCOCH3EtO-No term ph-CHO.If step 2 is rate-controlling,woth step 1 as an rapid equilibrium,Rate=k2phCOCH2phCHO-=k2K1phCOCH3EtO phCHO-If step 4 is rate-controlling,Rate=k4EtO phCHCH2COph-OHK3=IEtO I-I=K3 I EtO-I =K2phCOCH2phCHO-phCOCH2=K1phCOCH3EtO-K2=I phCOCH2phCHO-K1=phCOCH2 phCOCH3EtO-.28The final rate expression:Rate=K1K2K3phCOCH3phCHOEtO-=KobsphCOCH3phCHOEtO-Experimental studies have revealed that it is third-order,indicating that either the second or fourth step must be rate-determining.Studies on the intermediate phCH(OH)CH2COph have shown that k4 is about four times as larger as k-3,so that about 80%of the intermediate goes on to product.These reactions are faster than th overall reaction under the same conditions.So the second step must be rate-controlling.Kinetic results can exclude from consideration all mechanisms that require a rate law from the observed one.Giving the composition of the activated complex for the rate-dermining step and preceding steps.292.2.5 the Limitations of kinetics In the cases of the mechanisms with identical predicted rate expression,“kinetically equivalent”,a choice between them is not possible on the basis of kinetic data.No information about the structure of the intermediate.No information about the value and nature of the rate constants kr.302.3 Reaction Pathway&Transition State Theory2.3.1 ActivationCH4 +O2 CO2 +H2ODH=-213 kcal/molr.t.,1atm.No raction at all.spark/flame CH2=CH2 +H2 CH3CH3DH=-32.7 kcal/molPt(surface of platinum)DHO-O=33.5 kcal/molphCOOCphO=O=phCO OCphO=O=productsDt 80oC.31the Maxwell distribution lawd nE=2pN()3/2E1/2 e-E/ktdE1pkTN:the total number of molecules.k:the Boltzman constant.A favourable DG is not of itself sufficient to promote reaction.Progress along the reaction coordination requires an initial increase in energyActivation Energy.Only those molecules possessing sufficient energy to overcome the activation barrier are capable of reaction.322.3.2 Reaction pathwayThere is one path between reactants and products that has a lower energy maximum than any other,this is the pathway that the reaction will follow.Involving the changes of structure and energy.Potential Energy Surface:The relationship between structure and potential energy,i.e.,describing energy as a function of the spatial arrangement of the atoms involved in the reaction.For example:AX +B A +XBThe two-dimensional energy profile for this exchange:AX +BA +XBA X B ababSaddle PointRP.332.3.3 the simplified model of raction coordinateHH H*H HH*The simplest possible reaction needs at least three parapmters to define the geometries changed fully during the procedure from ractant to product.Then,the change of geometrical coordinates during the course of a reaction is considered generally as“Reaction Coordinate”.D DGD DGoRPTSReaction CoordinateETS()The saddle point is then the point of highest energy on the reaction pathway and is of particular interest,know as the trasition state(TS)or activated complex().34D DGD DGoRPReaction CoordinateETS()The activation energy:D DE The difference in energy between the energy of TS and that of reactants.D DE=ETS Ereactants(Activation Barrier)The reaction heat:D DE The difference in energy between the energy of products and that of reactants.D DE=Eproducts Ereactants.35The energy profile of different types of reactions:Single-step reaction:Reaction CoordinateED DED DEoABTS()A BA BReaction CoordinateETS()ABD DED DEoD DE D DH,D DG DGor=DHorTDSor=-RT lnKDG rate constant,kr.36Multi-step reaction:Reaction CoordinateEA I BABID DE1D DEoTS1()TS2D DE2()A I BReaction CoordinateEABD DE1D DEoTS1()TS2D DE2()IIntermediates(reactive intermediates)Reactive species with high reactivities and short lifetimes,refering to the depth of the wells in which they reside.slowslowfastfast.372.3.4 the trasition state theory1.The trasition state and transition structureThe transition state corresponds to a structure metastable with respect to a change in its atomic coordinates along the reaction coordinate.The transition state is not a real molecule in that it may have partially broken and/or formed bonds and higher coordination numbers at reaction centers than that normally permitted by VB rules.TSReaction pathway Activated molecule Reactive intermediate Saddle point transition state activated complex transition structure.382.The transition state modelA X +B A +X BTSAssumption 1:TSA X Bpseudo-vibration,vE=hv kTv=kThk:Boltzmanns constant.h:Plancks constant.T:absolute temperature.at r.m.v 6 1012 s-1.39Assumption 2:A X +BA X BKReactants TSKK=TSAXBTS=K AXBAssumption 3:Rate of TS decomposition=v=kThkThRate of reaction=TS:the transimission coefficient,usually taken to be 1.kThRate of reaction=K A X BSo,=kr(obs)A X B.40SinceDG=-RT lnK K=e-D DG/RT Rate of reaction=e-D DG/RT A X BkTh=kr(obs)A X Bkr vs.T and DG This reveals that the magnitude of DG and the reaction temperature will be the factors that determine the magnitude of reaction constant,kr.For a multi-step reaction with several different TS at a given temperature,the higher activation energy(DG)of the TS implies that the step would be slower and therefor the rate-determining step would involve the TS with the highest DG.the activation energy,DG,is consequently a measure of the affinity of the reaction as a rate process.DG=DH-T DShere,DH 0;DS0,or,DS EAy EAxKinetically controlled product Thermodynamically controlled product the product ratio controlled kinetically:the product ratio controlled thermodynamically:Px Pykxky=EAxEAyshorter reaction time and low reaction temperature.Px PylnD DGro=D DGpxoD DGpyolonger reaction time and higher reaction temperature.50reaction coordinateERRRPAPBPBPBPAPATSATSATSATSBTSBTSBGAGBGAGBGAGAGBGBGBcase 1.case 2.case 3.case 1:Kinetic Control.case 2:Thermodynamic Control.case 3:Governed by either kinetic or thermodynamic factors.Example:CH3CCHC(CH3)2O=Thermodynamic enolateKinetic enolateweaker baseprotic solventstrong and sterically hinder baseaprotic solvent.517.The principle of least motionThose elementary reactions are favored for which involve least change in atomic positions and electronic configuations.Example:Cl(.528.Limitations of the transition state theory Equilibrium between reactants and TSPA +BTSK0 1 Activation energyPotential energy (E)Free energy (G)the transition state should be regarded as a region of reaction space or a point on potential surface.53