Carbonium-ion

The general features of the cracking mechanism involve carbonium ion formation by a reaction of the type... 

The C2H5 is a carbonium ion (cf. ammonium NH4) and reacts with the benzene ... 

The benzene nucleus is then attacked by the carbonium ion (I) (. attack by +... 

The mechanism of the Schiemann reaction is not known with certainty. Two schemes, which have been proposed, are given below. One involves carbonium ion formation ... 

A mixture of an acid anhydride and a ketone is saturated with boron trifluoride this is followed by treatment with aqueous sodium acetate. The quantity of boron trifluoride absorbed usually amounts to 100 mol per cent, (based on total mola of ketone and anhydride). Catalytic amounts of the reagent do not give satisfactory results. This is in line with the observation that the p diketone is produced in the reaction mixture as the boron difluoride complex, some of which have been isolated. A reasonable mechanism of the reaction postulates the conversion of the anhydride into a carbonium ion, such as (I) the ketone into an enol type of complex, such as (II) followed by condensation of (I) and (II) to yield the boron difluoride complex of the p diketone (III) ... 

The conjugate acid (II) may also be formed by the addition of hydrazoic acid to an acylium (carbonium) ion ... 

Penta- (or higher) coordinate ( nonclassical carbonium ions contain five or (higher) coordinate carbon atoms. They cannot be described by two-electron two-center single bonds alone but also neces-... 

Whereas the differentiation of trivalent carbenium and pentacoor-dinated carbonium ions serves a useful purpose in defining them as limiting cases, it should be clear that in carbocationic systems there always exist varying degrees of delocalization. This can involve participation by neighboring -donor atoms, 7r-donor gronps, or [Pg.148]

Penta (or higher) coordinate "nonclassical" ions (carbonium ions) CH5 is parent... 

Concerning carbocations, previous usage named the trivalent, planar ions of the type carbonium ions. If the name is considered anal-... 

Trivalent carbenium ions are the key intermediates in electrophilic reactions of Tt-donor unsaturated hydrocarbons. At the same time, pen-tacoordinated carbonium ions are the key to electrophilic reactions of cr-donor saturated hydrocarbons through the ability of C-H or C-C single bonds to participate in carbonium ion formation. 

On the basis of my extensive study of stable, persistent carbocations, reported in more than 300 publications, I was able to develop the general concept of carbocations referred to in Chapter 9. Accordingly, in higher-coordinate (hypercoordinate) carbonium ions, of which pro-... 

The reverse reaction of the protolytic ionization of hydrocarbons to carbocations, that is, the reaction of trivalent carbocations with molecular hydrogen giving their parent hydrocarbons, involves the same five-coordinate carbonium ions. 

Friedel-Crafts and Related Reactions, Vols. I-IV (ed), 1963-1964. Carbonium Ions (ed. with Schleyer), Vols. I-V, 1968-1972. Friedel-Crafts Chemistry, 1973. 

The selectivity relationship merely expresses the proportionality between intermolecular and intramolecular selectivities in electrophilic substitution, and it is not surprising that these quantities should be related. There are examples of related reactions in which connections between selectivity and reactivity have been demonstrated. For example, the ratio of the rates of reaction with the azide anion and water of the triphenylmethyl, diphenylmethyl and tert-butyl carbonium ions were 2-8x10 , 2-4x10 and 3-9 respectively the selectivities of the ions decrease as the reactivities increase. The existence, under very restricted and closely related conditions, of a relationship between reactivity and selectivity in the reactions mentioned above, does not permit the assumption that a similar relationship holds over the wide range of different electrophilic aromatic substitutions. In these substitution reactions a difficulty arises in defining the concept of reactivity it is not sufficient to assume that the reactivity of an electrophile is related... 

Newly formed ions (CH4 +) collide several times with neutral molecules (CH4) to give carbonium ions (Figure 1.2b). The substance (M) to be investigated is vaporized into... 

Still under vacuum but at higher pressure (typically KT mbar), the initially formed ions collide with neutral molecules to give dilferent kinds of ions before they are injected into the analyzer. As an example, at low pressure, methane gas (CH4) is ionized to give molecular ions (CH4 ) but, at higher pressures, these ions collide with other CH4 molecules to give carbonium ions (CH5+). 

One of the side reactions that can complicate cationic polymerization is the possibility of the ionic repeat unit undergoing the well-known carbonium ion rearrangement during the polymerization. The following example illustrates this situation. 

Dual Function Catalytic Processes. Dual-function catalytic processes use an acidic oxide support, such as alumina, loaded with a metal such as Pt to isomerize the xylenes as weH as convert EB to xylenes. These catalysts promote carbonium ion-type reactions as weH as hydrogenation—dehydrogenation. In the mechanism for the conversion of EB to xylenes shown, EB is converted to xylenes... 

Lower alkanes such as methane and ethane have been polycondensed ia superacid solutions at 50°C, yielding higher Hquid alkanes (73). The proposed mechanism for the oligocondensation of methane requires the involvement of protonated alkanes (pentacoordinated carbonium ions) and oxidative removal of hydrogen by the superacid system. 

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