Carbocations chain extension

In contrast with these results, catalytic cracking yields a much higher percentage of branched hydrocarbons. For example, the catalytic cracking of cetane yields 50-60 mol of isobutane and isobutylene per 100 mol of paraffin cracked. Alkenes crack more easily in catalytic cracking than do saturated hydrocarbons. Saturated hydrocarbons tend to crack near the center of the chain. Rapid carbon-carbon double-bond migration, hydrogen transfer to trisubstituted olefinic bonds, and extensive isomerization are characteristic.52 These features are in accord with a carbo-cationic mechanism initiated by hydride abstraction.43,55-62 Hydride is abstracted by the acidic centers of the silica-alumina catalysts or by already formed carbocations ... 

The Y-T equation (2) has been used extensively in studies of electrophilic substitution in the aromatic ring, and aliphatic nucleophilic substitution and related reactions forming a carbocation or a carbocationic (electron-deficient) centre at the conjugative position (mostly benzylic position) in the side chain. 

The reactions of thiiranes with electrophiles has been extensively studied and well reviewed <83HC(42/l)333,84CHEC-I(7)i3i>. Generally, electrophilic additions to thiiranes may be stereoselective and result in both Markovnikov and anti-Markovnikov ring-opened products from unsymmetrical thiiranes. The nature of the products in terms of stereochemistry and mode of addition are reflective of cyclic (thiiranium or episulfonium ion) and open-chain (carbocation) intermediates. The generation and chemistry of episulfonium ions has been reviewed in detail and is not elaborated further here thiiranes with electrophiles. 

The cationic polymerization of styrene has been known since the 1960s using Lewis acids or strong protic acids as initiators leading to fast, uncontrolled polymerization due to extensive proton transfer reactions to, for example, counteranion, solvent, monomer, or polymer. In addition, chain transfer occurs as a result of an intramolecular Friedel-Craft reaction of the carbocation with the penultimate monomer unit, resulting in an indane chain end and the release of a proton that can reinitiate a new polymer chain as illustrated in Scheme 8.13. 

Fragmentation of branched-chain alkanes leads preferentially to the formation of secondary and tertiary carbocations, and because these cations are more easily formed than methyl and primary carbocations, extensive fragmentation is likely. For this reason, the molecular ion of branched-chain hydrocarbons is often very weak or absent entirely from the spectrum. The molecular ion corresponding to miz 114 is not observed, for example, in the mass spectrum of the highly branched... 

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