Stability of carbocations

As alluded to at the beginning of this section, carbocations generated during SNI mechanisms are subject to side reactions that include eliminations and rearrangements. Considering the possibility of these side reactions, one must question the stability of carbo-cationic species. To clarify, if carbocations were inherently stable, they would not be readily subject to additional transformations. Having already addressed the structure of carbocations, attention can now be focused on the factors influencing stability. 

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The pATr+ values allow for a comparison of the stability of carbocations. The carbocations that can be studied in this way are all relatively stable carbocations. The data in Table 5.1 reveal that electron-releasing substituents on the aryl rings stabilize the carbocation (more positive pA r+) whereas electron-withdrawing groups such as nitro are destabilizing. This is what would be expected from the electron-deficient nature of the carbocation. 

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. 

The stability of carbocations depends on the nature of alkyl groups attached to the positive charge. The relative stability of carbenium ions is as follows [2] with tertiary ions being the most stable ... 

Draw the structures of the two carbocation intermediates that might form during the reaction of 2-methylpropene with HBr (Problem 5.40). We ll see in the next chapter that the stability of carbocations depends on the number of alkyl substituents attached to the positively charged carbon—the more alkyl substituents there are, the more stable the cation. Which of the two carbocation intermediates you drew is more stable ... 

To understand why Markovnikov s rule works, we need to learn more about the structure and stability of carbocations and about the general nature of reactions and transition states. The first point to explore involves structure. 

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. 

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 delocalization of charge and the order of stability of carbocations parallel the number of attached methyl groups. 

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... 

The physical organic chemistry of very high-spin polyradicals, 40, 153 Thermodynamic stabilities of carbocations, 37, 57 Topochemical phenomena in solid-slate chemistry, 15, 63 Transition state analysis using multiple kinetic isotope effects, 37, 239 Transition state structure, crystallographic approaches to, 29, 87 Transition state structure, in solution, effective charge and 27, 1... 

In general, the greater the resonance and hyper-conjugation the greater is the stability of carbocation. The stability also depends on the field strengths. The following examples illustrate this point. 

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. 

Br 80% + Br 20% kinetic control product ratio determined by stability of carbocation... 

The operation of the anomeric effect and the stabilization of carbocations are beautifully illustrated in a conformational study of 2-oxanol (2-oxacyclohexanol) (Smith, B. J., /. Am. Chem. Soc., 1997, 119, 2699-2706). 2-Oxanol prefers the OH axial form by 12 kJ/mol and, upon protonation of the OH group, spontaneously loses water to form the oxonium ion. Use principles of orbital interaction theory to explain ... 

The simple hyperconjugation approach also predicts the stabilization of carbocations R3SiCH2CH2+, and of carbanions R3SiCII2, as shown in equations 5 and 6, respectively. 

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