碳正离子课程学习.pptx

会计学1碳正离子碳正离子第一页，共65页。CH3CCH3H+Isopropyl cation(a secondary carbocation)is more stable than CH3CH2+Carbocation Stability ContinuedCH3CCH3CH3+tert-Butyl cation(a tertiary carbocation)is more stable than(CH3)2CH+第2页/共65页第二页，共65页。Positively chargedcarbon pulls electrons in bonds closer to itself+Figure 4.15 Stabilization of Stabilization of carbocations via the inductive effectcarbocations via the inductive effect第3页/共65页第三页，共65页。Positive charge isdispersed,i.e.,sharedby carbon and thethree atoms attachedto it.Figure 4.15 Stabilization of carbocations via the inductive effect第4页/共65页第四页，共65页。Electrons in CCbonds are more polarizable than thosein CH bonds;therefore,alkyl groupsstabilize carbocationsbetter than H.n nElectronic effects transmitted through bonds are called inductive effects.Figure 4.15 Stabilization of carbocations via the inductive effect第5页/共65页第五页，共65页。Electrons in this bond can be sharedby positively chargedcarbon because the orbital can overlap with the empty 2porbital of positivelycharged carbonFigure 4.16 Stabilization of carbocations via hyperconjugation第6页/共65页第六页，共65页。Notice that an occupiedorbital of this type isavailable when sp3hybridized carbon is attached to C+,but is not available when His attached to C+.Therefore,alkyl groupsstabilize carbocationsbetter than H does.Figure 4.16 Stabilization of Figure 4.16 Stabilization of carbocations via hyperconjugationcarbocations via hyperconjugation第7页/共65页第七页，共65页。Carbocations from Alcohol Structures&Reaction Rates第8页/共65页第八页，共65页。n nThe more stable the carbocation,the fasterit is formed.n nTertiary carbocations are more stable thansecondary,which are more stable than primary,which are more stable than methyl.n nTertiary alcohols react faster than secondary,which react faster than primary,which react fasterthan methanol.Slow step is:ROHH+R+OHH第9页/共65页第九页，共65页。High activation energy for formation of methyl cation.CH3OHH+CH3+OHHEactCH3+OHH+第10页/共65页第十页，共65页。RCH2OHH+RCH2+OHHSmaller activation energy for formation ofprimary carbocation.RCH2+OHH+Eact第11页/共65页第十一页，共65页。R2CHOHH+R2CH+OHHActivation energy for formation of secondary carbocation is less than that for formation ofprimary carbocation.R2CH+OHH+Eact第12页/共65页第十二页，共65页。EactR3COHH+R3C+OHHActivation energy for formation of tertiary carbocation is less than that for formation ofsecondary carbocation.R3C+OHH+第13页/共65页第十三页，共65页。Carbocations第14页/共65页第十四页，共65页。Stability and Structureplanar4.1 4.1 碳正离子碳正离子 carbocation第15页/共65页第十五页，共65页。Carbocationic Reaction IntermediatesCarbocationic Reaction Intermediates Factors that influence carbocation formationFactors that influence carbocation formationNature of the leaving groupNature of the leaving group Structural factorsStructural factorsSolvent effectsSolvent effects Salt effectsSalt effectsIsotope effectsIsotope effectsFactors that Influence Carbocation FormationFactors that Influence Carbocation Formation Nature of the leaving groupNature of the leaving groupFactors that Influence Carbocation FormationFactors that Influence Carbocation Formation Anionic leaving groupsAnionic leaving groupsFactors that Influence Carbocation FormationFactors that Influence Carbocation Formation Structural factorsStructural factorsFactors that Influence Carbocation FormationFactors that Influence Carbocation Formation Solvent effectsSolvent effects:Any property of a solvent system that can lower the energy of activation for heterolytic bond cleavage will favor carbocation formation.Any property of a solvent system that can lower the energy of activation for heterolytic bond cleavage will favor carbocation formation.The Role of Solvent in Carbocation FormationThe Role of Solvent in Carbocation Formation Dielectric constant-a rough measure of the ability of the solvent to separate oppositely charged ionsDielectric constant-a rough measure of the ability of the solvent to separate oppositely charged ionsHydrogen-bonding abilityHydrogen-bonding abilityAcid-base propertiesAcid-base propertiesNucleophilicity:As the nucleophilicity of a solvent decreases,the likelihood of discrete carbocation formation increases.Nucleophilicity:As the nucleophilicity of a solvent decreases,the likelihood of discrete carbocation formation increases.Salt Effects:Ionic strengthSalt Effects:Ionic strength Common Ion(Mass Law)EffectCommon Ion(Mass Law)Effect Special Salt EffectSpecial Salt Effect Factors that Influence Carbocation FormationFactors that Influence Carbocation Formation Isotope effectsIsotope effectsNature of Some Carbocationic IntermediatesNature of Some Carbocationic Intermediates StabilityStability Stereochemistry of CarbocationsStereochemistry of Carbocations Stereochemistry of CarbocationsStereochemistry of Carbocations Kinetics of Carbocation FormationKinetics of Carbocation Formation 第16页/共65页第十六页，共65页。More stable第17页/共65页第十七页，共65页。Nature of the leaving groupNature of the leaving group第18页/共65页第十八页，共65页。Anionic leaving groupsAnionic leaving groups第19页/共65页第十九页，共65页。Structural factorsStructural factors第20页/共65页第二十页，共65页。Solvent effectsSolvent effects:Any property of a solvent system that can lower the energy of activation for heterolytic bond cleavage will favor carbocation formation.Any property of a solvent system that can lower the energy of activation for heterolytic bond cleavage will favor carbocation formation.The Role of Solvent in Carbocation FormationThe Role of Solvent in Carbocation Formation Dielectric constantDielectric constant-a rough measure of the ability of the solvent to separate oppositely charged ions-a rough measure of the ability of the solvent to separate oppositely charged ionsHydrogen-bonding abilityHydrogen-bonding abilityAcid-base propertiesAcid-base propertiesNucleophilicity:Nucleophilicity:As the nucleophilicity of a solvent decreases,the likelihood of discrete carbocation formation increases.As the nucleophilicity of a solvent decreases,the likelihood of discrete carbocation formation increases.第21页/共65页第二十一页，共65页。Salt Effects:Ionic strengthSalt Effects:Ionic strength 第22页/共65页第二十二页，共65页。Common Ion(Mass Law)EffectCommon Ion(Mass Law)Effect 第23页/共65页第二十三页，共65页。Special Salt EffectSpecial Salt Effect 第24页/共65页第二十四页，共65页。Isotope effectsIsotope effects第25页/共65页第二十五页，共65页。Nature of Some Carbocationic IntermediatesNature of Some Carbocationic Intermediates StabilityStability 第26页/共65页第二十六页，共65页。Stereochemistry of CarbocationsStereochemistry of Carbocations 第27页/共65页第二十七页，共65页。第28页/共65页第二十八页，共65页。Kinetics of Carbocation FormationKinetics of Carbocation Formation 第29页/共65页第二十九页，共65页。The instability of the bridgehead position is balanced by the extra stability gained from the conjugation with the three cyclopropyl groups.第30页/共65页第三十页，共65页。Electropositive and polarizable atoms have a remarkable ability to stabilize nonadjacent positive charge in carbocations.The best known manifestation of this phenomenon is the effect of silicon,whereby a silicon atom two bonds from the nominal center of positive charge stabilizes the system through hyperconjugation,exemplified by structures 1 and 2.The electropositivity and polarizability of silicon enhance its ability to delocalize positive charge through conjugation(often called vertical participation,although there is some nuclear movement).J.Org.Chem.,64(8),2729-2736,1999.第31页/共65页第三十一页，共65页。This arenium ion which decomposed with a rate constant of 104-105 s-1,clearly short lived.It is stable at room temperature.Its charge is present to a large extent on silicon.J.Org.Chem.,64(8),2729-2736,1999.第32页/共65页第三十二页，共65页。sp2第33页/共65页第三十三页，共65页。(CH3)3C(CH3)2CHCH3CH2CH3 It is a general rule that the more concentrated any charge is,the less stable the species bearing it will be.第34页/共65页第三十四页，共65页。Since simple alkyl cations are not stable in ordinary strong-acid solutions,the study of this species was greatly facilitated by the discovery that many of them could be kept indefinitely in stable solutions in mixtures of fluorosulfuric acid and antimony pentafluoride.Such mixture,usually dissolved in SO2 or SO2ClF,are among the strongest solutions known and are often called superacids.第35页/共65页第三十五页，共65页。第36页/共65页第三十六页，共65页。第37页/共65页第三十七页，共65页。第38页/共65页第三十八页，共65页。第39页/共65页第三十九页，共65页。第40页/共65页第四十页，共65页。第41页/共65页第四十一页，共65页。They are especially stable and are easily formed in 96%H2SO4.第42页/共65页第四十二页，共65页。第43页/共65页第四十三页，共65页。Simple conjugationHyperconjugation/conjugationHomoconjugationDelocalization第44页/共65页第四十四页，共65页。HyperconjugationDouble hyperconjugationHomo-hyperconjugationDelocalization第45页/共65页第四十五页，共65页。第46页/共65页第四十六页，共65页。第47页/共65页第四十七页，共65页。第48页/共65页第四十八页，共65页。第49页/共65页第四十九页，共65页。第50页/共65页第五十页，共65页。第51页/共65页第五十一页，共65页。第52页/共65页第五十二页，共65页。第53页/共65页第五十三页，共65页。第54页/共65页第五十四页，共65页。第55页/共65页第五十五页，共65页。第56页/共65页第五十六页，共65页。第57页/共65页第五十七页，共65页。第58页/共65页第五十八页，共65页。第59页/共65页第五十九页，共65页。第60页/共65页第六十页，共65页。第61页/共65页第六十一页，共65页。第62页/共65页第六十二页，共65页。第63页/共65页第六十三页，共65页。第64页/共65页第六十四页，共65页。感谢您的观看感谢您的观看(gunkn)！第65页/共65页第六十五页，共65页。