Alkyl carbenium ions

Tertiary Alkyl Alcohols. Tertiary alkyl alcohols generally undergo facile reduction when treated with acids in the presence of organosilicon hydrides.127,136 This comparative ease of reduction reflects the enhanced stability and ease of formation of tertiary alkyl carbenium ions compared with primary and secondary carbenium ions. Thus, treatment of 1-methylcyclohexanol with mixtures of triethylsilane and aluminum chloride in dichloromethane produces near quantitative yields of methylcyclohexane with or without added hydrogen chloride in as little as 30 minutes at room temperature, in contrast to the more vigorous conditions needed for the reduction of the secondary alcohol cyclohex-anol.136... 

Intermolecular hydride transfer (Reaction (6)), typically from isobutane to an alkyl-carbenium ion, transforms the ions into the corresponding alkanes and regenerates the t-butyl cation to continue the chain sequence in both liquid acids and zeolites. 

We concluded, therefore, that in sufficiently pure alkyl halide solvents the tert-alkyl tetrahaloaluminates are stable electrolytes and that previous failures to produce them, and the consequent legend of the instability of tcrt-alkyl carbenium ions, arose from the use of inappropriate and insufficiently rigorous experimental techniques. On this basis it seems highly probable that in the polymerised solutions the cations R+ partaking in reaction (viii) were also original ions, i.e., 2 at the end of a live chain and 3 and 4 formed by alumination of a terminal double bond, and not derived ions formed by degradative reactions of monomer or polymer. 

Prins reaction, heteropolyacid catalysis, 41 156 Probe molecules, 42 119 acidic dissociation constant, 38 210 NMR solid acidity studies, 42 139-140 acylium ions, 42 139, 160 aldehydes, 42 162-163 alkyl carbenium ions, 42 154-157 allyl cation, 42 143-144 ammonia, 42 172-174 arenium ions, 42 150-154 carbonium ions, 42 157-160 chalcogenenonium ions, 42 161-162 cyclopentenyl cations, 42 140-143 indanyl cations, 42 144-147 ketones, 42 162,163-165 nitrogen-containing compounds, 42 165-170... 

Typical alkylation reactions are those of propane, isobutane, and n-butane by the ferf-butyl or sw-butyl ion. These systems are somewhat interconvertible by competing hydride transfer and rearrangement of the carbenium ions. The reactions were carried out using alkyl carbenium ion hexafluoroantimonate salts prepared from the corresponding halides and antimony pentafluoride in sulfuryl chloride fluoride solution and treating them in the same solvent with alkanes. The reagents were mixed at —78°C warmed up to — 20°C and quenched with ice water before analysis. The intermolecular hydride transfer between tertiary and secondary carbenium ions and alkanes is generally much faster than the alkylation reaction. Consequently, the alkylation products are also those derived from the new alkanes and carbenium ions formed in the hydride transfer reaction. 

According to the reaction scheme shown in Figure 5 both hydroisomerization and hydrocracking of the n-alkanes (except n-hexane) proceed via branched alkyl carbenium ions. In the range of medium degrees of conversion (40 % <,X <, 90 %) both reactions may be investigated simultaneously. A relationship between the products of both types of reaction will be discussed in the present section. 

The first step in double-bond isomerization (DBI) is chemisorption of I-butene on the catalytic surface. Transition state intermediates can be generated at the active sites allowing the H-addition and elimination reactions of DBI to proceed. Alkyl carbenium ions (I) form on Bronsted acid sites and alkenyl carbenium ions (II) form on Lewis acid sites. 

Three major types of cationic species that can be derived from saturated hydrocarbons are alkyl carbenium ions (R+), alkane radical cations (RH +) and alkyl carbo-nium ions (RH2+). The term carbocations is usually reserved to denote alkyl carbenium and carbonium ions only. Pentacoordinated alkyl carbonium ions (proton-ated alkanes) are the species that result from protonation of alkane molecules they are of paramount importance as reactive intermediates/transition states in the initiation of (Br0nsted) acid-catalyzed conversions of saturated hydrocarbons. Upon dissociation of alkyl carbonium ions, trivalent alkyl carbenium ions are formed and these are responsible for the further progression of acid-catalyzed conversions of alkanes. Alkyl carbenium ions may also be formed by ionization of neutral alkyl radicals and by proton addition to olefins. In both carbenium and carbonium ions, the positive charge is very much located on a particular part of the cation. 

As became apparent from our experiments, only secondary C-H protonation leads to effective chemical transformation in y-irradiated CCl3F/alkanes (see below). As secondary C-H protonated alkanes are characterized by negative dissociation energies, protonation is followed immediately by dissociation of the C-H protonated alkanes into alkyl carbenium ions and molecular hydrogen. 

In this system, planar chain-end C-H bonds in pentane radical cations from which proton donation takes place only come into close contact with secondary C-H bonds at the irmer (C5) position in decane, as well as with primary C-H bonds the latter have a much lower protonation energy than secondary C-H bonds, however, and thus cannot compete effectively as acceptor in the protonation process. Experiments show a marked predominance of 5-chlorodecane over more lateral secondary chlorodecanes, in accordance with the restricted accessibility of secondary C-H bonds in decane to planar chain-end C-H bonds in pentane radical cations. Perhaps even more importantly, no substantial preference is observed for the penultimate position relative to the C3 and C4 positions. This shows unequivocally that the preference for the penultimate position in the experiments with other systems described above is not due to the transformation of alkyl carbenium ions by hydride transfer, i.e., reactions such as... 

The structures, stabilities, and Si NMR chemical shifts for pairs of silabicyclo[2.2.2]octyl and a pair of silanor-bornyl derivatives, 50/51, 52/53, and 54/55, all show that introduction of the /3-SiMe group stabilizes the cation and decreases pyramidalization at the cation center, consistent with the well-known stability of /3-silyl versus /3-alkyl carbenium ions <2003JOC1827>. 

Figure 12.9 Schematic representation of the primary (16a-e), secondary (17a-e), and tertiary (18a-e) alkyl carbenium ions.

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