Fate of Carbocations

As mentioned above, carbocations feature in many reactions, such as nucleophilic substitution (Sjjl) and elimination (El), additions of electrophiles to double and triple bonds, electrophilic aromatic substitution, and additions to carbonyl compounds and enolate chemistry (albeit in masked form). 

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In Chapter 2, the possible fates of carbocations were discussed. These include quenching with nucleophiles (especially water), loss of a proton,... 

Stabilization of a carbocation intermediate by benzylic conjugation, as in the 1-phenylethyl system shown in entry 8, leads to substitution with diminished stereosped-ficity. A thorough analysis of stereochemical, kinetic, and isotope effect data on solvolysis reactions of 1-phenylethyl chloride has been carried out. The system has been analyzed in terms of the fate of the intimate ion-pair and solvent-separated ion-pair intermediates. From this analysis, it has been estimated that for every 100 molecules of 1-phenylethyl chloride that undergo ionization to an intimate ion pair (in trifluoroethanol), 80 return to starting material of retained configuration, 7 return to inverted starting material, and 13 go on to the solvent-separated ion pair. 

An alternative view of these addition reactions is that the rate-determining step is halide-assisted proton transfer, followed by capture of the carbocation, with or without rearrangement Bromide ion accelerates addition of HBr to 1-, 2-, and 4-octene in 20% trifluoroacetic acid in CH2CI2. In the same system, 3,3-dimethyl-1-butene shows substantial rearrangement Even 1- and 2-octene show some evidence of rearrangement, as detected by hydride shifts. These results can all be accoimted for by a halide-assisted protonation. The key intermediate in this mechanism is an ion sandwich. An estimation of the fate of the 2-octyl cation under these conditions has been made ... 

Carboxylic acids are oxidized by lead tetraacetate. Decarboxylation occurs and the product may be an alkene, alkane or acetate ester, or under modified conditions a halide. A free radical mechanism operates and the product composition depends on the fate of the radical intermediate.267 The reaction is catalyzed by cupric salts, which function by oxidizing the intermediate radical to a carbocation (Step 3b in the mechanism). Cu(II) is more reactive than Pb(OAc)4 in this step. 

We studied both carbanions and carbocations using the stereochemical fate of stereogenic centers as a probe of reaction mechanism and solvation phenomena. These studies involved substitution, elimination, and rearrangement reactions, most of which involved carbanions and carbocations as short-lived reaction intermediates. The chiral systems were designed and synthesized, the kinetics of the reactions were examined, and the... 

The regioselectivity of Friedel-Crafts acylations of unsymmetrical alkenes can often be predicted simply by consideration of the alternative carbenium ions formed in an initial electrophilic attack. Pathways via tertiary carbocations are generally preferred over those involving secondary ions. It is the subsequent fate of the initially generated ion that determines the products formed. As has been indicated already, elimination of a proton completes a substitution, although there is a predominance of nonconjugated unsaturated ketone formed, and treatment with base is required to form the conjugated product."... 

In pursuing the fate of diol epoxides in spontaneous and acid catalysed solvolysis reactions (in dioxane-water, 0.1 M NaC104). Islam et al [156] have conclusively proved that the reactive intermediate in the rate-limiting step of spontaneous hydrolysis is a triol carbocation (TC). Trapping by the strong nucleophiles, aside and N-acetylcysteine anions rules out the possibility of a zwitterionic intermediate. While this is a certainty for syn-... 

The positively charged carbon atom in a carbocation is an extremely electron-dehcieni (electrophilic) carbon. As such, its behavior is dominated by a need to obtain an electron pair from any available source. The Sn I reaction illustrates the most obvir>"s fate of a < nr .- -t on c a.bination with an external Lewis base, forming a new bond to carbon. However, the electron deficiency of cationic carbon is so great that even under typical SnI solvolysis conditions, surrounded by nucleophilic solvent molecules, some of the cations won t wait to combine with external electron-pair sources. Instead, they will seek available electron pairs within their own molecular structures. The most available of the.se are electrons in carbon-hydrogen bonds one carbon removed from the cationic center (at llic so-called carbon) ... 

Experiments have established that the reaction of cyclopentene and HCl produces intermediate carbocation 2, written in a bracket to show that it is a reactive (transient) product. The intermediate is the product of the initial reaction, but it is unstable and reacts further to give the product that is isolated in a second chemical step. What is the fate of the carbocation intermediate The answer is that a second chemical reaction between the carbocation and the counter-ion occurs to form a new a-bond. In the case of 2, the chloride counter-ion donates two electrons to the positive carbon to form a new a-bond (C-Cl) in chlorocyclopen-tane 3, as shown. The chloride ion is a nucleophile (see Chapter 6, Section 6.7). If the overall sequence is examined, the jr-bond breaks, the H-Cl bond breaks, and new C-H and C-Cl bonds are formed in two separate chemical steps. 

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