Carbonium formation from

Winstein, one of the most brilliant chemists of his time, concluded that it is attractive to account for these results by way of the bridged (non-classical) formulation for the norbornyl cation involving accelerated rate of formation from the exo precursor [by anchimeric assistance His formulation of the norbornyl cation as a cr-bridged species stimulated other workers in the solvolysis field to interpret results in a variety of systems in similar terms of rr-delocalized, bridged carbonium... 

Equilibrium constants for carbonium ion formation from triphenylcarbinols in aq H2SO4 25 -3.64... 

The intermediate in the second mechanism is identical with that postulated by Skell and Starer (1959) in the formation of carbonium ions from alkoxide and carbenes. 

As has been suggested in the previous section, explanations of solvent effects on the basis of the macroscopic physical properties of the solvent are not very successful. The alternative approach is to make use of the microscopic or chemical properties of the solvent and to consider the detailed interaction of solvent molecules with their own kind and with solute molecules. If a configuration in which one or more solvent molecules interacts with a solute molecule has a particularly low free energy, it is feasible to describe at least that part of the solute-solvent interaction as the formation of a molecular complex and to speak of an equilibrium between solvated and non-solvated molecules. Such a stabilization of a particular solute by solvation will shift any equilibrium involving that solute. For example, in the case of formation of carbonium ions from triphenylcarbinol, the equilibrium is shifted in favor of the carbonium ion by an acidic solvent that reacts with hydroxide ion and with water. The carbonium ion concentration in sulfuric acid is greater than it is in methanol-... 

The formation from neutral substances (triphenylcarbinol) of coloured, salt-like reaction products which are more or less easily decomposed by water is a phenomenon called halochromism . The halo-chromic salts of triphenylcarbinol are regarded as carbonium salts this follows at once from the above discussion. A quinonoid formula, by which various authors explain the colour, seems less probable. Recently the attempt has been made to attribute complex formulae to the carbonium salts (Hantzsch), in accordance with Werner s scheme for ammonium salts. Such formulae express the fact that, in the ion, the charge is not localised at the methane carbon atom, but spread over the field of force of the whole radicle. The simplest carbonium salt of the group, the yellow perchlorate (K. A. Hofmann), would accordingly have the following structural formula ... 

Mechanism for protonation of alkenes was previously discussed in Section 13.5.1. In general, protonation of alkenes is an exothermic process. Protonation of alkanes was discussed in Section 13.5.2. There wiU be further discussion on this step in Section 13.8.4 within the context of alkane cracking mechanisms. The formation of a penta-coordinated carbonium ion from alkane protonation is typically an endothermic process, the reverse being true for deprotonation. 

Of particular interest are certain ionic graft copolymerizations in which the polymerization reaction is initiated only on the macromolecular framework and no homopolymer is formed. An example is provided by the formation of polymeric carbonium ions from chloride-containing polymers, such as poly(vi-nylchloride), in the presence of diethylaluminum chloride ... 

The hydroxymethylphosphonium ion first formed changes into mono-hydroxymethylphosphine by releasing a proton. This phosphine reacts further in the same way as phosphine itself until finally the quarternary phosphonium ion is formed. For a bimolecular reaction mechanism, the first stage must be assumed to be the formation of a carbonium ion from the aldehyde molecule and a proton. This ion then reacts with phosphine. 

Effects of structure on reactivity have been studied several times. The sulphides are more stable than the thiols [248,250], In both series of thiols and of sulphides, the reactivity increases with the inductive effect of the alkyl group [248,251,252], in accordance with other elimination reactions. A linear relation between the logarithm of the rate coefficient and the enthalpy change on carbonium ion formation from the corresponding alkanes has been observed [248]. As Fig. 9 shows, linear correlation of the same rate data by means of the Taft equation is also possible. 

These results indicate that n-butylvinylether forms the cation radicals through positive charge transfer rather than by capturing an electron to form the anion radical and suggests that the ionization potential of n-butylvinylether is lower than that of 3-methylpentane (according to the measurements by the present authors, this is the case) and its electron affinity is negative. The observed behavior of n-butylvinylether seems to coincide with its cationic nature in the radiation-induced polymerization. Though the formation of carbonium ions from the cation radicals has not yet been elucidated, the cation radicals may play an important role in the initiation process of polymerization. 

The catalytic activity for the aniline formation from chlorobenzene and ammonia of the Y zeolites with various cations was studied at 395° C (Table I). It is clear that the transition metal-exchanged zeolites have the catalytic activity for the reaction, while alkali metal and alkaline earth metal zeolites do not. The fact that alkaline earth metal-exchanged zeolites usually have high activity for carbonium ion-type reactions denies the possibility that Bronsted acid sites are responsible for the reaction. Thus, catalytic activity of zeolites for this reaction may be caused by the... 

The model is able to predict with reasonable accuracy the experimental data if the following hypotheses are made (1) the pre-exponential factors of the rate constants for the formation of the carbonium ions from any of the isomers are the same (2) the pre-exponential factors for the disappearance of the carbonium ions only differ by a statistical factor, which takes into account the fact that in forming the 1-pentene the 2-carbonium ion can lose any of the three hydrogens of Ci, while in forming the 2-pentenes there is the possibility of losing only one hydrogen from the C3 (3) the steady-state approximation is valid for the concentration of adsorbed carbonium ions. The same assumptions were made for the butenes. 

Flayon E, Simic M (1973) Addition of hydroxyl radicals to pyrimidine bases and electron transfer reactions of intermediates to quinones. J Am Chem Soc 95 1029-1035 Flaysom FIR, Phillips JM, Scholes G (1972) Formation of carbonium ions from dihydropyrimidyl radicals in they-radiolysis of aqueous solutions of dihydropyrimidines. J Chem Soc Chem Commun 1082-1083... 

Table 16. Heats of formation of carbonium ions from gaseous carbon and hydrogen atoms...

Ritter9,17 has expressed the view that the reactions involve the formation of a carbonium ion from the alcohol or alkene which subsequently A-alkylates the nitrile. The intermediate product was thought to be the imidol hydrogen sulfate (25) from which the hydrogen sulfate residue is displaced nucleophilically by the electron-rich group Y. 

The formation and subsequent decomposition of a carbonium ion from pinacol ... 

In many cases studied, the predominant contribution to the propagation process is apparently made by a carbonium ion. The details of the formation of a normal carbonium ion from the originally formed carbonium ion radical is not yet clear. The fate of the ejected electron is likewise unclear. 

The failure of difluoramine to appear among the final products is not particularly surprising. In the presence of nitric acid and/or nitrogen oxides, it might easily be oxidized and may well constitute the source of the silicon tetrafluoride. The formation of a carbonium ion from trityl-difluoramine would be favored by resonance stabilization. In the tert-butyl case, on the other hand, this driving force is not present and formation of the ion would be expected to occur less readily. In addition, both the tert-butyl carbonium ion and the difluorammonium ion from which it is derived would be more subject to a variety of side reactions than the corresponding trityl species. 

In fact, the rate of formation of the carbonium ions from the 8-decay is simply proportional to the first power of the concentration of the tritiated compound in the system. On the other hand, the rate of formation of labeled radiolytic products varies with a higher power of the concentration of tritiated molecules which determines, in the first place, the intensity of the 3-radiation (and therefore the total rate of the radiolytic reactions), and in the second place, the probability that the radiolytic processes affect, in particular, a tritiated molecule. 

A number of functionalization reactions in which C—N bonds are formed depend on the initial formation of a carbonium ion from the alkane. This cation is quenched by the acetonitrile solvent and an amide or related species is obtained after hydrolysis. In the example shown in equations (49) to (51) Br2 was used to generate the carbonium ion. Adamantane is a particularly favorable substrate as the carbonium ion is so easily formed and resists elimination. A 92% yield of amide was obtained in this process. In a related reaction, HCN gives amine products (equation 52). ... 

The skeletal isomerization of tetrabydrodicyclopentadiene into adamantane is an example of a very complex rearrangement diat is commercially carried out over strong Lewis acids with a hydride transfer initiator. The reaction can be catalyzed by rare earth (La, Ce, Y, Nd, Yb) exchanged faujasites (Scheme 1) in a Hj/HCl atmosphere at 25(yX3. Selectivities to adamantane of up to 50% have been reported, when a metal fimction, such as Pt, capable of catalyzing hydrogenation is added [54]. Initially acid catalyzed endo- to exo- isomerization of tetrahydro-dicyclopentadiene takes place and then a series of 1,2 alkyl shifts involving secondary and tertiary carbonium ions leads eventually to adamantane[55]. The possible mechanistic pathways of adamantane formation from tetrahydro-dicyclopentadiene are discussed in detail in ref [56]. 

The alkylation of the naphthenic cation causes formation of complex aliphatic carbonium ions. Transformation of such intermediates according to Poustma [30] gives the molecules of light saturated hydrocarbons and aromatics. It is generally accepted that the formation of condensed aromatic rings being a coke precursors is difficult in the pores of ZSM-5 zeolite. The fact that the products of the toluene transformation reaction in all cases contained 1 - and 2-methylnaphthalene seems to prove their formation from the olefinic or naphthenic carbocations. Transformation of the naphthenic carbocations occuring in zeolite pores and on the external zeolite surface is the most probable source of methyinaphthalene isomers [23]. 

The carbonium ion is formed by dissociation of the protonated alcohol this involves separation of a charged particle, R, from a neutral particle, H2O. It is obvious that this process requires much less energy than would formation of a carbonium ion from the alcohol itself, since the latter process involves separation of a positive particle from a negative particle. Viewed in another way, the carbonium ion (a Lewis acid) releases the weak base, water, much more readily than it... 

In listing carbonium ions in order of their ease of formation from alkenes, we find that once more (compare Sec. 5.21) we have listed them in order of their stability (Sec. 5.18). 

Jerina DM. Lehr RE. Yagi H. et al. 1976. Mutagenicity of B[a]P derivatives and the description of a quantum mechanical model which predicts the ease of carbonium ion formation from diol epoxides. In de Serres FJ. Fouts JR. Bend JR. et al. eds. In vitro metabolic activation in mutagenesis testing. Amsterdam. The Netherlands Elsevier/North Holland. 159-178. 

Since cation-radical formation in the chemisorption of hydrocarbons has not previously been considered in the catalytic literature, the nature, reactions, and mechanism for formation of such species should be of considerable importance to the elucidation of catalytic reaction mechanisms particularly in view of the fact that Webb (20) has found spectral evidence for the formation of species other than carbonium ions from butene-2 adsorbed on silica-alumina. It is not possible at the present time to define either the role of cation-radicals in acid catalysis or the chemical nature of the electrophilic surface sites involved in their formation. 

The complex then splits beta to the point of complexing to produce an olefin and a new hydrogen deficient entity. However, superimposed on this basic cracking reaction are the simultaneous and consecutive reactions which produce the characteristic catalytic cracking product distribution. The relative stability of carbonium ions is tertiary > secondary > primary. There is, then, either a preferential formation of tertiary and secondary ions, or else isomerization to these preferred forms. The property of beta fission results in the formation from secondary ions of no olefins smaller than propylene, and from tertiary ions of no olefins smaller than isobutylene. Cycli-zation and hydrogen transfer reactions result in the large amounts of aromatic hydrocarbon formed. The sum total of these described reactions lead to the desirable product distribution characteristic of catalytic cracking. 

The generally accepted mechanism for the acid polymerization of olefins is the one proposed by Whitmore involving the formation of carbonium ions from catalyst and olefin ... 

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