Primary, secondary, tertiary carbocation stabilization

Rotational barriers in the substituted cations have been the subject of a number of classic studies at the relatively low (by modem standards) levels of theory. Although these smdies have not been tested with the more accurate recent methods, many of the fundamental insights are stiU very interesting. The 1972 work of 

For clarity, H-atoms are not shown in the substituted ethyl and methyl cations given below 

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It is important to point out that thermodynamic equilibria of hydrocarbons and those of derived carbocations are substantially different. Under appropriate conditions (traditional acid catalysts, longer contact time), the thermodynamic equilibrium mixture of hydrocarbons can be reached. In contrast, when a reaction mixture in contact with excess of strong (super) acid is quenched, a product distribution approaching the thermodynamic equilibrium of the corresponding carbocations may be obtained. The two equilibria can be very different. Since a large energy difference in the stability of primary < secondary < tertiary carbocations exists, in excess of superacid solution, generally the most stable tertiary cations predominate. This allows, for example, isomerization of n-butane to isobutane to proceed past the equilibrium concentrations of the neutral hydrocarbons, as the er -butyl cation is by far the most stable butyl cation. 

One important experimental fact is that the rate of reaction of alcohols with hydro gen halides increases m the order methyl < primary < secondary < tertiary This reac tivity order parallels the carbocation stability order and is readily accommodated by the mechanism we have outlined... 

The order of carbocation stability is methyl < primary < secondary < tertiary. Alkyl groups that are directly attached to the positively charged carbon stabilize carbocations. 

In El reactions, the rate of the reaction depends mainly on carbocation stability, just as we saw for S).jl reactions. (Remember primary > secondary > tertiary for carbocations, conjugation either with multiple bonds or lone pairs of electrons stabilizes carbocations, and nonplanar carbocations cannot readily be formed). Most of the quantitative data (Table 10.2) for carbocation stability, however, derive from substitution rather than elimination reactions, and even some of these are a little misleading. Rates of substitution of bromomethane and bromoethane tell us little about carbocations these are purely 5 2 reactions in which no carbocation participates. While good gas phase and superacid data do exist, what they can tell us about reactions in solution is limited. That said, the data do tell us clearly that tertiary cations are the most stable and that conjugation stabilizes carbocations. 

The second point to explore involves carbocation stability. 2-Methyl-propene might react with H+ to form a carbocation having three alkyl substituents (a tertiary ion, 3°), or it might react to form a carbocation having one alkyl substituent (a primary ion, 1°). Since the tertiary alkyl chloride, 2-chloro-2-methylpropane, is the only product observed, formation of the tertiary cation is evidently favored over formation of the primary cation. Thermodynamic measurements show that, indeed, the stability of carbocations increases with increasing substitution so that the stability order is tertiary > secondary > primary > methyl. 

One way of determining carbocation stabilities is to measure the amount of energy required to form the carbocation by dissociation of the corresponding alkyl halide, R-X - R+ + X . As shown in Figure 6.10, tertiary alkyl halides dissociate to give carbocations more easily than secondary or primary ones. As a result, trisubstituted carbocations are more stable than disubstituted ones, which are more stable than monosubstituted ones. The data in Figure 6.10 are taken from measurements made in the gas phase, but a similar stability order is found for carbocations in solution. The dissociation enthalpies are much lower in solution because polar solvents can stabilize the ions, but the order of carbocation stability remains the same. 

Figure 6.11 A comparison of inductive stabilization for methyl, primary, secondary, and tertiary carbocations. The more alkyl groups there are bonded to the positively charged carbon, the more electron density shifts toward the charge, making the charged carbon less electron-poor (blue in electrostatic potential maps).

Because of resonance stabilization, a primary allylic or benzylic carbocation is about as stable as a secondary alkyl carbocation and a secondary allylic or benzylic carbocation is about as stable as a tertiary alkyl carbocation. This stability order of carbocations is the same as the order of S l reactivity for alkyl halides and tosylates. 

For now, let s consider the effect of the substrate on the rate of an El process. The rate is fonnd to be very sensitive to the nature of the starting aUcyl halide, with tertiary halides reacting more readily than secondary halides and primary halides generally do not nndergo El reactions. This trend is identical to the trend we saw for SnI reactions, and the reason for the trend is the same as well. Specihcally, the rate-determining step of the mechanism involves formation of a carbocation intermediate, so the rate of the reaction will be dependent on the stability of the carbocation (recall that tertiary carbocations are more stable than secondary carbocations). 

Primary carbocations Should you wish to use carbocations in a reaction mechanism, you must consider the relative stability of these entities. Tertiary carbocations are OK, and in many cases so are secondary carbocations. Primary carbocations are just not stable enough, unless there is the added effect of resonance, as in benzylic or ally lie systems. 

Cleavage is favored at alkyl-substituted carbon atoms the more substituted, the more likely is cleavage. This is a consequence of the increased stability of a tertiary carbocation over a secondary, which in turn is more stable than a primary. 

As carbocations go, CH3+ is particularly unstable, and its existence as an intermediate in chemical reactions has never been demonstrated. Primary carbocations, although more stable than CH3+, are still too unstable to be involved as intermediates in chemical reactions. The threshold of stability is reached with secondary carbocations. Many reactions, including the reaction of secondary alcohols with hydrogen halides, are believed to involve secondary carbocations. The evidence in support of tertiary carbocation intermediates is stronger yet. 

The transition state is closer in energy to the carbocation and more closely resembles it than the alkyloxonium ion. Thus, structural features that stabilize carbocations stabilize transition states leading to them. It follows, therefore, that alkyloxonium ions derived from tertiary alcohols have a lower energy of activation for dissociation and are converted to their corresponding carbocations faster than those derived from secondary and primary alcohols. Simply put more stable carbocations are formed faster than less stable ones. Figure 4.17 expresses this principle via a potential energy diagram. 

Also shown in Table 1 are differences in pAiR. These are multiplied by 1.364 to give free energies for easier comparison with HIAs. They correspond to intrinsic differences between tertiary, secondary, and primary carbocation centers (CH+, CH2+, and CH3+) and the corresponding values for the carbon bound to an OH functional group (C-OH, CH-OH, and CH2-OH). In principle, carbocation stabilities may be expressed relative to any functional group, but clearly the convenience and prevalence of measurements of pAiR give a special place to the OH group. 

Because of the high stability of the tertiary ions, these are preferentially formed in the superacid systems from both tertiary and secondary, and even primary, precursors.353 If, however, the tertiary carbocation is not benzylic, rearrangement to a... 

A stable tertiary carbocation is formed. The order of stability of saturated carbocations decreases in the order tertiary > secondary > primary > methyl. 

The order of carbocation stability is tertiary > secondary > primary. There is only one C5Hn+ car-bocation that is tertiary, and so that is the most stable one. 

Electrophilic additions occur by a two-step mechanism. In the first step, the electrophile adds in such a way as to form the most stable carbocation (the stability order is tertiary > secondary > primary). Then the carbocation combines with a nucleophile to give the product. 

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... 

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