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1、Structure, Bonding, and Stabilityof CarbocationsChapter 6 Most carbocations are too unstable to beisolated, but occur as reactive intermediates ina number of reactions.When R is an alkyl group, the carbocation isstabilized compared to R = H.CRRR+CarbocationsCHHH+Methyl cationleast stableCarbocation
2、StabilityCH3CHH+Ethyl cation(a primary carbocation) is more stable than CH3+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+Positively chargedcarbon pulls el
3、ectrons in bonds closer to itself +Figure 4.15 Stabilization of carbocations via the inductive effectPositive charge isdispersed , i.e., sharedby carbon and thethree atoms attachedto it.Figure 4.15 Stabilization of carbocations via the inductive effectElectrons in CCbonds are more polarizable than t
4、hosein CH bonds; therefore, alkyl groupsstabilize carbocationsbetter than H.Electronic effects transmitted through bonds are called inductive effects.Figure 4.15 Stabilization of carbocations via the inductive effectElectrons in this bond can be sharedby positively chargedcarbon because the orbital
5、can overlap with the empty 2porbital of positivelycharged carbonFigure 4.16 Stabilization of carbocations via hyperconjugationNotice 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 groupsstabi
6、lize carbocationsbetter than H does.Figure 4.16 Stabilization of carbocations via hyperconjugationCarbocations from Alcohol Structures & Reaction RatesThe more stable the carbocation, the fasterit is formed.Tertiary carbocations are more stable thansecondary, which are more stable than primary,which
7、 are more stable than methyl.Tertiary alcohols react faster than secondary, which react faster than primary, which react fasterthan methanol.Slow step is:ROHH+R+OHHHigh activation energy for formation of methyl cation.CH3OHH+CH3+OHHEactCH3+OHH+RCH2OHH+RCH2+OHHSmaller activation energy for formation
8、ofprimary carbocation.RCH2+OHH+EactR2CHOHH+R2CH+OHHActivation energy for formation of secondary carbocation is less than that for formation ofprimary carbocation.R2CH+OHH+EactEactR3COHH+R3C+OHHActivation energy for formation of tertiary carbocation is less than that for formation ofsecondary carboca
9、tion.R3C+OHH+CarbocationsStability and Structureplanar4.1 4.1 碳正离子碳正离子 carbocationCarbocationicCarbocationic Reaction Intermediates Reaction Intermediates Factors that influenceFactors that influence carbocation carbocation formation formationNature of the leaving groupNature of the leaving group St
10、ructural factorsStructural factorsSolvent effectsSolvent effects Salt effectsSalt effectsIsotope effectsIsotope effectsFactors that InfluenceFactors that Influence Carbocation Carbocation Formation Formation Nature of the leaving groupNature of the leaving groupFactors that InfluenceFactors that Inf
11、luence Carbocation Carbocation Formation Formation Anionic leaving groupsAnionic leaving groupsFactors that InfluenceFactors that Influence Carbocation Carbocation Formation Formation Structural factorsStructural factorsFactors that InfluenceFactors that Influence Carbocation Carbocation Formation F
12、ormation Solvent effectsSolvent effects: Any property of a solvent system Any property of a solvent system that can lower the energy of activation forthat can lower the energy of activation for heterolyticheterolytic bond cleavage will favor bond cleavage will favor carbocation carbocation formation
13、.formation.The Role of Solvent inThe Role of Solvent in Carbocation Carbocation Formation Formation Dielectric constant - a rough measure of the ability Dielectric constant - a rough measure of the ability of the solvent to separate oppositely charged ionsof the solvent to separate oppositely charge
14、d ionsHydrogen-bonding abilityHydrogen-bonding abilityAcid-base propertiesAcid-base propertiesNucleophilicityNucleophilicity: As the: As the nucleophilicity nucleophilicity of a solvent of a solvent decreases, the likelihood of discretedecreases, the likelihood of discrete carbocation carbocation fo
15、rmation increases. 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 InfluenceFactors that Influence Carbocation Carbocation Formation Formation Isotope effectsIso
16、tope effectsNature of SomeNature of Some Carbocationic Carbocationic Intermediates Intermediates StabilityStability Stereochemistry ofStereochemistry of Carbocations Carbocations Stereochemistry ofStereochemistry of Carbocations Carbocations Kinetics ofKinetics of Carbocation Carbocation Formation F
17、ormation More stableNature of the leaving groupNature of the leaving groupAnionic leaving groupsAnionic leaving groupsStructural factorsStructural factors Solvent effectsSolvent effects: Any property of a solvent system that can lower the Any property of a solvent system that can lower the energy of
18、 activation forenergy of activation for heterolytic heterolytic bond cleavage bond cleavage will favorwill favor carbocation carbocation formation. formation.The Role of Solvent inThe Role of Solvent in Carbocation Carbocation Formation Formation Dielectric constantDielectric constant - a rough meas
19、ure of the ability - a rough measure of the ability of the solvent to separate oppositely charged ionsof the solvent to separate oppositely charged ionsHydrogen-bonding abilityHydrogen-bonding abilityAcid-base propertiesAcid-base propertiesNucleophilicityNucleophilicity: : As the As the nucleophilic
20、ity nucleophilicity of a solvent of a solvent decreases, the likelihood of discretedecreases, the likelihood of discrete carbocation carbocation formation increases. formation increases. Salt Effects: Ionic strengthSalt Effects: Ionic strength Common Ion (Mass Law) EffectCommon Ion (Mass Law) Effect
21、 Special Salt EffectSpecial Salt Effect Isotope effectsIsotope effectsNature of SomeNature of Some Carbocationic Carbocationic Intermediates Intermediates StabilityStability Stereochemistry ofStereochemistry of Carbocations Carbocations Kinetics ofKinetics of Carbocation Carbocation Formation Format
22、ion The instability of the bridgehead position is balanced by the extra stability gained from the conjugation with the three cyclopropyl groups. Electropositive and polarizable atoms have a remarkable ability to stabilize nonadjacent positive charge in carbocations. The best known manifestation of t
23、his 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 charg
24、e through conjugation (often called vertical participation, although there is some nuclear movement). J. Org. Chem., 64 (8), 2729 -2736, 1999. 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 larg
25、e extent on silicon.J. Org. Chem., 64 (8), 2729 -2736, 1999. sp2(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. Since simple alkyl cations are not stable in ordinary strong-acid solutions, the study of this spec
26、ies 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.They are especially stable and are easily formed in 96% H2SO4.Simple conjugationHyperconjugation/conjugationHomoconjugationDelocalizationHyperconjugationDouble hyperconjugationHomo- hyperconjugationDelocalization
