Ionization potentials carbocations

Although alkyl groups in general increase the rates of electrophilic addition, we have already mentioned (p. 974) that there is a different pattern depending on whether the intermediate is a bridged ion or an open carbocation. For brominations and other electrophilic additions in which the first step of the mechanism is rate determining, the rates for substituted alkenes correlate well with the ionization potentials of the alkenes, which means that steric effects are not important. Where the second step is rate determining [e.g., oxymercuration (15-3), hydroboration (15-17)], steric effects are important. ... 

Heats of formation of carbocations, which are the basis data for these scales have been derived from the heats of formation of the neutral precursors (e.g., radicals) and the adiabatic ionization energies [ = ionization potentials, Eq. (2)] [20]. 

The proximity of the diffusion limit also inhibits a detailed discussion of the data in Table 7, but a significant difference to the substituent effects discussed in Section III.D.4 is obvious. Whereas the reactivities of terminal alkenes, dienes, and styrenes toward AnPhCH correlate with the stabilities of the new carbenium ions and not with the ionization potentials of the 7r-nucleophiles [69], the situation is different for the reactions of enol ethers with (p-ClC6H4)2CH+ [136]. In this reaction series, methyl groups at the position of electrophilic attack activate the enol ether double bonds more than methyl groups at the new carbocationic center, i.e., the relative activation free enthalpies are not controlled any longer by the stabilities of the intermediate carbocations but by the ionization potentials of the enol ethers (Fig. 20). An interpretation of the correlation in Fig. 20 has not yet been given, but one can alternatively discuss early transition states which are controlled by frontier orbital interactions or the involvement of outer sphere electron transfer processes [220]. 

The energy necessary to produce the carbocation R+ from a molecule RX exceeds the ionization potential of the radical R by the bond dissociation energy of RX. 

Several criteria, including hydride affinities of carbocations (R+—H heterolytic BDE) and adiabatic ionization potentials of the corresponding free radicals (Table 1.30), indicate the order of decreasing stability of fluoromethyl carbocations to be CHF2+ > CH2F+ > CF3+ > CH3+ (Scheme 1.54) [1],... 

The rate of epoxidation of alkenes is increased by alkyl groups and other ERG substituents, and the reactivity of the peroxy acids is increased by EWG substituents." These structure-reactivity relationships demonstrate that the peroxy acid acts as an electrophile in the reaction. Low reactivity is exhibited by double bonds that are conjugated with strongly EWG substituents, and very reactive peroxy acids, such as trifluoroperoxyacetic acid, are required for oxidation of such compounds. " Strain increases the reactivity of alkenes toward epoxidation. Norbornene is about twice as reactive as cyclopentene toward peroxyacetic acid." trani-Cyclooctene is 90 times more reactive than cyclohexene." Shea and Kim found a good correlation between relief of strain, as determined by MM calculations, and the epoxidation rate. ° There is also a correlation with ionization potentials of the alkenes. Alkenes with aryl substituents are less reactive than unconjugated alkenes because of ground state stabilization and this is consistent with a lack of carbocation character in the TS. 

As the carbocation becomes more stable, the transition structure would be expected to become somewhat more product-like (Chapter 6), and some selectivity of the cation for the more nucleophilic species might be expected. For a discussion, see (a) Sneen, R. A. Carter, J. V. Kay, P. S. J. Am. Chem. Soc. 1966,88,2594 (b) Raber, D. J. Harris, J. M. Hall, R. E. Schleyer, P. v. R. /. Am. Chem. Soc. 1971,93,4821. Ritchie (reference 33) discussed the possible origins of such correlations. Shaik, S. S. /. Org. Chem. 1987,52,1563 found that nucleophilicities toward one cation in water correlated with the vertical ionization potential of the nucleophile. A discussion of the barrier for carbocation-nucleophile combinaHons was given by Richard, J. P. Tetrahedron 1995,51,1535. 

The extent of participation of the carbon-carbon double bond in the ionization of anti-7-norbomenyl systems is a function of the substitution at C-7. The placement of an aryl substituent at C-7 diminishes the relative rate acceleration due to participation by the double bond. Evidently, the extent of participation is a function of the stability of the potential carbocation. When an aryl group is present at C-7, the resulting benzyl-type stabilization decreases the importance of participation by the double bond. The degree of stabilization is sensitive to substituents on the phenyl ring. For p-methoxyphenyl, phenyl, and p-trifluoromethylphenyl, the rate factor for the unsaturated relative to the saturated system is 3,40, and 3.5 x 10, respectively. The double bond clearly has a much larger effect on the poorly stabilized p-trifluoromethyl-substituted system. This dependence of the extent of participation on other stabilizing features is a general trend and has been observed with other types of carbocations. ... 

The ionization mechanism for nucleophilic substitution proceeds by rate-determining heterolytic dissociation of the reactant to a tricoordinate carbocation (also sometimes referred to as a carbonium ion or carbenium ion f and the leaving group. This dissociation is followed by rapid combination of the highly electrophilic carbocation with a Lewis base (nucleophile) present in the medium. A two-dimensional potential energy diagram representing this process for a neutral reactant and anionic nucleophile is shown in Fig. 

The proton affinity of a carbene can be derived by means of Eq. 9 (M = RR C ) if AHf of the carbene and of the corresponding carbocation have been estimated independently (Table 7). Appearance potentials (AP) are convenient, although sometimes inaccurate, sources of AHf (RR CH+).129 Adiabatic ionization energies (IEa) of free radicals, in combination with dissociation enthalpies... 

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