Relative stabilities of carbocations

The rates of hydration of alkenes increase dramatically with increasing alkyl substitution (see table at left). This is usually attributed to the relative stabilities of carbocations formed as intermediates in the initial (and rate-hmiting) step of the reaction, e.g., for hydration of propene. 

Various quantitative methods have been developed to express the relative stabilities of carbocations. One of the most common of these, though useful only for relatively stable cations that are formed by ionization of alcohols in acidic solutions, is based on the equation ... 

The behavior of the isomeric dihydronaphthalenes emphasizes the importance of the relative stabilities of carbocation intermediates in ionic hydrogenations. Treatment of 1,2-dihydronaphthalene with Et3SiH/TFA at 50-60° gives a 90% yield of tetralin after one hour. Under the same conditions, the 1,4-dihydronaphthalene isomer gives less than 5% of tetralin after 70 hours.224 This difference in reactivity is clearly related to the relatively accessible benzylic cation formed upon protonation of the 1,2-isomer compared to the less stable secondary cation formed from the 1,4-isomer.224... 

From a number of experiments it has been concluded that with short life carbocations (obtained from secondary and tertiary alkyl deviatives) inversion is generally observed. But in long life carbocation where there is spreading of the charge, the product is a racemic one, as in diphenyl methyl carbocation Ph2CHX. Therefore it also affords a means of estimating the relative stability of carbocations. 

Usually, mechanistic proposals based on product studies are supplemented by independent evidence from similar reactions (i.e. arguments based on analogies). Such proposals might also be supported by a knowledge of characteristic reactions of functional groups and properties of reactive intermediates (e.g. acidities of C—H groups, or relative stabilities of carbocations), and the proposals are a basis for the more detailed investigations by methods discussed in later chapters. 

Referring to the discussions presented in Chapter 5 regarding the relative stabilities of carbocations (and hyperconjugation), we are reminded that tertiary carbocations are more stable than secondary carbocations, which, in turn, are more stable than primary carbocations. Since, as shown in Scheme 7.9, protonation of propene results in cationic character at both a secondary carbon and a primary carbon, a greater presence of cationic character on the secondary site is expected compared to the primary. This allows... 

The relative stabilities of carbocations can be estimated from the free energy changes of the ion-molecule proton transfer equilibria of the corresponding alkenes (26). 

Traditionally, relative stabilities of carbocations have been derived from the comparison of the rates of solvolysis reactions following the SN1 mechanism, for which the designation Dm + An has recently been proposed [36], The comparison of solvolytic rate constants for substrates of a large structural variety is hampered by the fact that the published solvolysis rates refer to different solvents, different temperatures, and precursors with different leaving groups. Dau-Schmidt has, therefore, converted solvolysis rates of a manifold of alkyl chlorides and bromides to standard conditions, i.e., soiv of RC1 in 100% EtOH at 25° C (Scheme 6) [37]. Although from a theoretical point of view, ethanol is not an ideal solvent for observing unassisted SN 1-type reactions (nucleophilic solvent participation), it has been selected as the reference solvent because most available experimental data have been collected in solvents of comparable nucleophilicity, a fact which made conversions to 100% ethanol relatively unproblematic [38],... 

A key point is that in the first step the proton is bonded to the less alkylated carbon. This is because the carbocation 45 is tertiary and stable. The alternative possibility is to produce a primary carbocation Me2CHCH2+ that is significantly less stable. The order of relative stabilities of carbocations is tertiary > secondary > primary, and it is this relative sequence that dictates the site of proton attachment. Accordingly, one can state Markovnikov s rule, proposed in 1869 in the addition of H-X to a double bond, H becomes bonded to the carbon with fewer alkyl groups and X becomes attached to the more alkylated carbon. 

Data on the relative stabilities of carbocations and the chemical shifts in the NMR spectra of the same cations do not at all always correlate with each other (for details see 253-255) 256.257) Qj other hand, the chemical shifts of... 

The greater the number of alkyl substituents bonded to the positively charged carbon, the more stable the carbocation will be. The order of relative stability of carbocations is tertiary benzyhc > allylic secondary > primary vinyl> phenyl. The nature of electron release by alkyl groups is not very clear. It may be an inductive effect, a resonance effect (hyperconjugation), or a combination of the two. When we refer to the inductive effect of the alkyl groups, it should be clear that this might well include a contribution from hyperconjugation. 

Figure 2.6 Relative stabilities of carbocations in quantitative terms.

These trends agree well with the trends in relative stabilization of carbocations from gas phase heterolytic C-Br bond dissociation energies in alkyl bromides CHj (O.Okcal/mol) < (36kcal/mol) < (CH3)2CH ... 

The relative stabilities of carbocations are related to the number of alkyl groups attached to the positively charged trivalent carbon. 

This behavior, as we shall see in Section 7.7B, is related to the relative stabilities of carbocations. 

Relative Stabilities of Carbocations Regioselectivity and Markovnikov s Rule... 

Although the concept of the relative stabilities of carbocations had not been developed in Markovnikov s time, their relative stabilities are the underlying basis for his rule that is, the proton of H—X adds to the less substituted carbon of a double bond because this mode of addition produces the more stable carbocation intermediate. 

The foundation for Markovnikov s rule is the relative stability of carbocation intermediates. (5.3)... 

Relative stabilities of carbocations. As we learned in 8ection 5.3A, 3° carbocations are the most stable carbocations, requiring the lowest activation energy for their formation,... 

The structure of the haloalkane. S g1 reactions are governed by electronic factors, namely, the relative stabilities of carbocation intermediates. 8 2 reactions are governed by steric factors, namely, the degree of crowding around the site of substitution. 

In Section 11.4.1, ionization of the C-Br bond in tertiary halide 64 gives carbocation 66. Carbocations were discussed in Chapter 10, Section 10.2, in connection with the acid-base reaction of an alkene with acids such as H-X (HCl, HBr, etc.). To understand formation of a carbocation in a substitution reaction, remember that the stability of a carbocation is related to the number of substituents attached to the positive carbon. The formation of carbocations from alkenes was described in Chapter 10, Section 10.2, as was the relative stability of carbocations. 

The rates of S l reactions are governed mainly by electronic factors, namely the relative stabilities of carbocation intermediates. The rates of Sj 2 reactions, on the other hand, are governed mainly by steric factors, and their transition states are particularly sensitive to bulky groups at the site of reaction. The ability of groups, because of their size, to hinder access to a reaction site within a molecule is called steric hindrance. 

Relative Stabilities of Carbocations Let US first consider the effect of the alkyl group of the haloalkane on S l reactions. As discussed, the rate-determining step of an Sj l mechanism is formation of a carbocation therefore, the stability of the resulting carbocation is a dominant consideration. 

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