Stability of Carbocation Intermediates

SnI reactivity follows the order 3° 2° 1°. Is this the ordering of stabilities of the carbocation intermediates  

Display and examine electrostatic potential maps for ethyl cation, 2-propyl cation and 2-methyl-2-propyl cation. Which cation shows the greatest localization of positive charge If you find that the methyl groups delocalize the positive charge, where does the charge go Write resonance contributors for the three cations to rationalize your conclusion. (Note You may need to draw resonance contributors that contain a CC double bond and are missing a CH bond see also Chapter 7, Problem 8.) 

Electrostatic potential map for 3-ethyl-3-pentyl cation shows most positively-charged regions (in blue) and less positively-charged regions (in red). 

Reactions between cations and anions in the gas phase generally proceed with no activation energy. The simplest example is heterolytic bond dissociation. 

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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... 

Thomson Click Organic Interactive to rank the stability of carbocation intermediates. 

Markovnikov s rule works because of the stability of carbocation intermediates. Experiments tend to reveal tliat carbocations are planar molecules, with a carbon that has three substituents at 120° to each other and a vacantp orbital that is perpendicular to it in the 3" plane. The p orbital extends above and below the trisubstituent plane. 

From these studies, it can be concluded that it is not a simple matter to define the general substituent migratory aptitudes in chloiin and bacteriochlorin systems, because distant conjugated peripheral substituents can dramatically affect the stability of carbocation intermediates and hence the products. The facility of the rearrangement of various substituents depends not only on the intrinsic nature of the migratory group but also on the electronic and steric factors present elsewhere on the tetrapyrrolic systems. 

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

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. 

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. 

The rates of 1 reactions decrease in the order tertiary > secondary > primary methyl. This trend is exactly the reverse of the trend observed in Sj 2 reactions. The relative reactivity of haloalkanes in Sj 2 reactions corresponds to the relative stability of carbocation intermediates that form during the reaction. We recall from Chapter 3 that the order of stability of carbocations is tertiary > secondary > primary methyl. A relatively stable tertiary carbocation forms faster than a less stable secondary carbocation, which in turn forms very much faster than a highly unstable primary carbocation. However, Sj l mechanisms are possible at primary centers that are resonance stabilized, such as allyl and benzyl. 

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