The Formation of Carbonium Ions

Specifically there are several different starting points for the formation f carbonium ions. 

Similarly, for an electronically deficient molecule such as alumiuum chloride we have the equilibria 4, 5, or 6  

In each case the species formed contains a carbon atom with only six 1 Whitmore, Ind. Eng. Chem., Newt Ed., 26, 669 (1948). 

The starting materials can be recovered unchanged by the addition of water to the sulfuric acid solution. With olefins the situation is complicated by accompanying chemical reactions, but it is significant that aromatic hydrocarbons are soluble in liquid hydrogen fluoride while saturated hydrocarbons arc not.4 This fact suggests the electron donor capacity which equations 3 and 6 attribute to an unsaturated linkage. 

Additions to olefins (p. 135), acid-catalyzed olefin self-condensation reactions (p. 141), and addition to carbonyl groups (p. 156), appear to involve carbonium ions formed by tliis process. 

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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. 

The experimentally observed pseudo-first order rate constant k is increased in the presence of DNA (18,19). This enhanced reactivity is a result of the formation of physical BaPDE-DNA complexes the dependence of k on DNA concentration coincides with the binding isotherm for the formation of site I physical intercalative complexes (20). Typically, over 90% of the BaPDE molecules are converted to tetraols, while only a minor fraction bind covalently to the DNA bases (18,21-23). The dependence of k on temperature (21,24), pH (21,23-25), salt concentration (16,20,21,25), and concentration of different buffers (23) has been investigated. In 5 mM sodium cacodylate buffer solutions the formation of tetraols and covalent adducts appear to be parallel pseudo-first order reactions characterized by the same rate constant k, but different ratios of products (21,24). Similar results are obtained with other buffers (23). The formation of carbonium ions by specific and general acid catalysis has been assumed to be the rate-determining step for both tetraol and covalent adduct formation (21,24). 

Silica-alumina catalysts are acidic and presumably can furnish protons for the formation of carbonium ions (Kazanskii and Rozen-gart, 21). 

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. 

Treatment of a 5-methoxy-2-furyl carbinol (106) with zinc chloride in acetone-water converts it into two products, the 4-ylidenebutenolide (111) and the 4-oxo-2-enoic acid methyl ester (112) in approximately 3 1 ratio (80T3071). The key step in this conversion is the formation of carbonium ion (107) which subsequently adds water to furnish (108). Intermediate (108) is in prototropic equilibrium between (109) and (110). The latter then break down to furnish the final products (Scheme 26). The overall scheme provides a new synthetic route to the 4-ylidenebutenolides, a class of compound that includes many natural substances of biological importance. 

The reactions studied so far are confined to those indicative of the formation of carbonium ion intermediates. For these reactions the Bronsted acid sites usually have high catalytic activity. Thus, it might be difficult to obtain information on the catalytic properties of metal ions since the catalysis by acid sites may mask the catalysis by metal ions. Therefore, to investigate catalytic properties of metal ions, it is desirable to avoid the carboniogenic reactions and to poison the Bronsted sites. 

Olefins are formed by dehydrogenation of the n-paraffin feed over the metallic hydrogenation-dehydrogenation function and are adsorbed on the acidic surface of the catalyst as carbonium ions by proton addition. After skeletal isomerization they are desorbed as isoolefins and subsequently hydrogenated to the corresponding isoparaffins. The net result (i.e., the formation of carbonium ions) of the action of metal and acid in dual function catalysis is, on pure Friedel-Crafts type catalysts, described by the scheme ... 

A central feature of the mechanism that accounts for the catalytic cracking of hydrocarbons by appropriately cation exchanged zeolites is the formation of carbonium ions (also designated carbocations and alkylcarbenium ions) as intermediates. Many other reactions for which aluminosilicates, be they clay-or zeolite-based, also predicate (320) the existence of carbonium ion intermediates, formed usually by proton donation from Bronsted acid sites, have been discussed earlier (Section III,K). 

All side-chain isomers are formed in acid-catalyzed isomerization. Carbonium ions are the intermediates here. Over dual-function catalysts, such as platinum-on-alumina and platinum-on-silica-alumina, platinum increases the rate of isomerization by dehydrogenating alkanes to olefins. This facilitates the formation of carbonium ions. 

The protolytic activation of the alkane is, however, only the apparent part of the reaction as long as the alkane or the acid is not isotopically labeled. When HF is replaced by DF and the isobutane-CO mixture is bubbled through the DF-SbF5 acid (6 1 molar ratio) at — 10°C, the apparent conversion based on ester or H2 formation is only 4% but the 1H/2H NMR analysis of the apparently unreacted isobutane (96%) shows extensive H-D exchange (18 atom% in the tertiary position and 9 atom% at each primary position).30 The most plausible rationalization of hydrogen exchange is via the formation of carbonium ions (here pentacoordinate transition states or intermediates) as described in Eq. (5.15). 

As described in the case of the 1-adamantyl cation, protosolvation of carbon-carbon and carbon-hydrogen sigma bonds can lead to cationic 2e-3c bonding and the formation of carbonium ion centers. The role of this interaction in the chemistry of superelectrophiles is typihed by protosolvation of the tert-butyl cation (1) in superacid to provide the dicationic... 

To continue with the Kolbe reaction, it has been shown that carbon anodes strongly favour the carbonium ion pathway (Koehl, 1964) at least for simple alkanecarboxylic acids. Also, for phenyl-acetic acid and 1-methylcyclohexylacetic acid the same tendency towards carbonium ion formation on carbon anodes was observed, the phenomenon being explained as due to the presence of paramagnetic centres in carbon. These would bind the initially formed radicals, impede their desorption and hence promote the formation of carbonium ions via a second electron transfer (Ross and Finkelstein, 1969). However, cases of Kolbe oxidations in which no dependence on anode material was noticeable have been found more recently (Brennan and Brettle, 1973 Eberson and Nilsson, 1968a Sato et al., 1968). Actually, the nature of the carbon material determines the yield of products formed via the radical versus carbonium ion pathway (Brennan and Brettle, 1973). Yields of the... 

Activation of unreactive alkanes only occurs at relatively high temperature, even with strong protons, as present in the zeolite [129]. It proceeds via the formation of carbonium ions (Fig. 4.69). 

Consistent with this explanation are the observations that tertiary alcohols react much faster than primary and secondary alcohols and that even when linear alcohols are carbonylated the predominant products are the branched tertiary acids. Both of these are consistent with the formation of carbonium ions, which then rearrange to form the favored tertiary structure, for example,... 

The process observed is reminiscent of the NIH shift , observed previously in iron hydroxylases, in which a reactive iron-oxy species (with an as yet undetermined identity) is an electrophile, attacking an arene substrate. This results in hydroxylation-induced migrations, due to the formation of carbonium ion intermediates and retention of heavier... 

E. Conclusions Relevant to the Formation of Carbonium Ions in Gases at... 

One cannot distinguish between the analogous copper intermediates involved in oxidative electron-transfer and ligand-transfer reactions. In each the ionization of the ligand to copper(II) has an important role in the formation of carbonium ion intermediates. A reaction analogous to the copper-catalyzed decomposition of peroxides is the copper-promoted decomposition of diazonium salts (178). The diazonium ion and copper(I) afford aryl radicals which can undergo ligand-transfer oxidation with copper(II) halides (Sandmeyer reaction) or add to olefins (Meerwein reaction). 

The addition of secondary and tertiary amines to aryl-substituted alkenes also occurs in acidic medium via the formation of carbonium ions, as shown in the following transannular cyclization6. 

Mechanisms involving the formation of carbonium ions have been postulated, but it is thought likely that the reaction takes place by a front-side displacement. ... 

Lemieux has given a detailed account of a possible mechanism for this reaction. It is suggested that absorption of a 1,2-cis-poly-O-acetylglycosyl bromide on the silver chloride facilitates the formation of carbonium ions which are stabilized (immediately they are formed) by assuming a 1,2-cyclic structure. Reaction of this cyclic intermediate with chloride ion then yields the 1,2-[Pg.220]

The reaction of a-ketodiazonium ions is of interest because there is considerable evidence that loss of nitrogen can occur by an 8 2 mechanism (p. 337-347). If this is generally true, the possibility arises of a comparison between the reactions of diazonium ions and those of alkyl halides and tosylates under conditions that do not lead to the formation of carbonium ion intermediates. In the discussion of the molecularity of the rate-determining step, the reaction of ketodiazonium ions was supposed to proceed with simple substitution by an external nucleophile. Product analyses, on the reactions of diazoketones with acids and the deamination of aminoketones, show, however, that extensive rearrangement and molecular fragmentation can occur in suitable alkyl structures. The simplest of these reactions have the following stoichiometric form (Baumgarten and Anderson, 1961) ... 

At ordinary graphite electrodes in aqueous solutions, the reaction products are those derived from the formation of carbonium ion intermediates,... 

Lewis acids initiate polymerization through the formation of carbonium ions, and Plesch (17) has proposed that a suitable coinitiator is necessary to produce these ions. The mechanism for the initiation of the homopolymerization of epoxy resins by BF3 complexes has been proposed by Arnold (11) to proceed as follows ... 

That the observed spectrum was the result of a chemical reaction between the hydrocarbon and the catalytically active centers of the silica-alumina surface (chemisorption), and not due to a general sur-fatochromic spectral shift, was demonstrated from the spectrum of this compound adsorbed on a nonacidic or very weakly acidic silica gel (29). The spectrum (Fig. 30, Curve B) of silica gel exposed to triphenylmethane vapor for 1000 hours at 100°C was identical to the spectrum (Curve A) of an alcoholic solution of this hydrocarbon. The close agreement between these spectra suggested that on silica gel the triphenylmethane was physisorbed. This was further evidenced by the marked loss of spectral intensity (Curve C) attendant to a four hour evacuation at 100°C. In contrast, on silica-alumina where the hydrocarbon was chemisorbed as the carbonium ion, no decrease in absorbance was noted even after 48 hr evacuation at 275°C. These data constituted the first direct demonstration of the formation of carbonium ions as a consequence of chemisorption of a tertiary hydrocarbon on the surface of a cracking catalyst by a reaction involving the rupture of an aliphatic C-H bond. The generality of this process of carbonium ion... 

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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