第六章 碳正离子.ppt

1、Structure, Bonding, and Stabilityof Carbocations,Chapter 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.,Carbocations,Methyl cationleast stable,Carbocation Stabi

2、lity,C,H3C,H,H,+,Ethyl cation(a primary carbocation) is more stable than CH3+,Isopropyl cation(a secondary carbocation) is more stable than CH3CH2+,Carbocation Stability Continued,tert-Butyl cation(a tertiary carbocation) is more stable than (CH3)2CH+,Positively chargedcarbon pulls electrons in bond

3、s closer to itself,Figure 4.15 Stabilization of carbocations via the inductive effect,Positive charge isdispersed , i.e., sharedby carbon and thethree atoms attachedto it.,Figure 4.15 Stabilization of carbocations via the inductive effect,Electrons in CCbonds are more polarizable than thosein CH bon

4、ds; therefore, alkyl groupsstabilize carbocationsbetter than H.,Electronic effects transmitted through bonds are called inductive effects.,Figure 4.15 Stabilization of carbocations via the inductive effect,Electrons in this bond can be sharedby positively chargedcarbon because the orbital can overla

5、p with the empty 2porbital of positivelycharged carbon,Figure 4.16 Stabilization of carbocations via hyperconjugation,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 car

6、bocationsbetter than H does.,Figure 4.16 Stabilization of carbocations via hyperconjugation,Carbocations from Alcohol Structures & Reaction Rates,The more stable the carbocation, the fasterit is formed. Tertiary carbocations are more stable thansecondary, which are more stable than primary,which are

7、 more stable than methyl. Tertiary alcohols react faster than secondary, which react faster than primary, which react fasterthan methanol.,Slow step is:,R,+,+,High activation energy for formation of methyl cation.,Smaller activation energy for formation ofprimary carbocation.,Activation energy for f

8、ormation of secondary carbocation is less than that for formation ofprimary carbocation.,Eact,Activation energy for formation of tertiary carbocation is less than that for formation ofsecondary carbocation.,Carbocations,Stability and Structure,planar,4.1 碳正离子 carbocation,Carbocationic Reaction Inter

9、mediates Factors that influence carbocation formation Nature of the leaving group Structural factors Solvent effects Salt effects Isotope effects Factors that Influence Carbocation Formation Nature of the leaving group Factors that Influence Carbocation Formation Anionic leaving groups Factors that

10、Influence Carbocation Formation Structural factors Factors that Influence Carbocation Formation Solvent effects: 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 Formation Diele

11、ctric constant - a rough measure of the ability of the solvent to separate oppositely charged ions Hydrogen-bonding ability Acid-base properties Nucleophilicity: As the nucleophilicity of a solvent decreases, the likelihood of discrete carbocation formation increases. Salt Effects: Ionic strength Co

12、mmon Ion (Mass Law) Effect Special Salt Effect Factors that Influence Carbocation Formation Isotope effects Nature of Some Carbocationic Intermediates Stability Stereochemistry of Carbocations Stereochemistry of Carbocations Kinetics of Carbocation Formation,More stable,Nature of the leaving group,A

13、nionic leaving groups,Structural factors,Solvent effects: 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 Formation Dielectric constant - a rough measure of the ability of the

14、solvent to separate oppositely charged ions Hydrogen-bonding ability Acid-base properties Nucleophilicity: As the nucleophilicity of a solvent decreases, the likelihood of discrete carbocation formation increases.,Salt Effects: Ionic strength,Common Ion (Mass Law) Effect,Special Salt Effect,Isotope

15、effects,Nature of Some Carbocationic Intermediates Stability,Stereochemistry of Carbocations,Kinetics of Carbocation Formation,The instability of the bridgehead position is balanced by the extra stability gained from the conjugation with the three cyclopropyl groups.,Electropositive and polarizable

16、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

17、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.,This arenium ion which decomposed

18、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.,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