Carbenium ions carbocations

PHYSICAL ORGANIC CHEMISTRY NO-MENCATURE CARBOCATION CARBENIUM ION CARBONIUM ION BRIDGED CARBOCATION CARBOHYDRATE MODIFICATION REACTIONS... 

The fundamental step in acid-catalyzed hydrocarbon conversion processes is the formation of the intermediate carbocations. Whereas all studies involving isomerization, cracking, and alkylation reactions under acidic conditions (Scheme 5.1) agree that a trivalent carbocation (carbenium ion) is the key intermediate, the mode of their formation of this reactive species from the neutral hydrocarbon remained controversial for many years. 

Examples for frequently encountered intermediates in organic reactions are carbocations (carbenium ions, carbonium ions), carbanions, C-centered radicals, carbenes, O-centered radicals (hydroxyl, alkoxyl, peroxyl, superoxide anion radical etc.), nitrenes, N-centered radicals (aminium, iminium), arynes, to name but a few. Generally, with the exception of so-called persistent radicals which are stabilized by special steric or resonance effects, most radicals belong to the class of reactive intermediates. 

Table 17.1 Carbocations carbenium ions and carbonium ions ...

The major carbon centered reaction intermediates in multistep reactions are carbocations (carbenium ions), carbanions, free radicals, and carbenes. Formation of most of these from common reactants is an endothermic process and is often rate determining. By the Hammond principle, the transition state for such a process should resemble the reactive intermediate. Thus, although it is usually difficult to assess the bonding in transition states, factors which affect the structure and stability of reactive intermediates will also be operative to a parallel extent in transition states. We examine the effect of substituents of the three kinds discussed above on the four different reactive intermediates, taking as our reference the parent systems [CH3], [CHi]", [CHi] , and [ CH2]. 

A carbon radical is a trivalent species containing a single electron in a p orbital. A carbanion is viewed as a tetrahedral species containing a pair of electrons in an orbital (1). We have viewed a carbocation (carbenium ion) as an sp hybridized, trigonal planar carbon with an empty p orbital (2). A radical, which contains one electron in an orbital, can be tetrahedral, planar, or in between, with properties of both a carbanion and a carbocation. As shown in 3, a reasonable in between structure is a flattened tetrahedron (the actual structure of radicals will be discussed below). In terms of its reactivity, radical 3 could be considered electron rich or electron poor. In most of its reactions, the electron-deficient characterization is the most useful for predicting products. 

G. Olah proposed to name the classical carbocations carbenium ions (cf. and those with penta-coordinated carbon atoms carbonium ions . In keeping with his recommendations the authors d ing with the above ion groups in their English-language publications now use the term arenium ions more often than the one used earlier ( arenonium ions ). 

During the activation of the initiator consisting of a heterocyclic, aryl substituted, or aryl-ring fused sulfonium salt, a carbon-sulfur bond is broken via a ring-opening reaction, leading to formation of a sulfide and a carbocation (carbenium ion) within the same molecule. The functionality of... 

A carbocation is a structure with a positive charge that is associated primarily with a carbon center or, in the case of delocalized systems, a collection of carbons. Heteroatom substitution is allowed, and it will inevitably lead to some positive charge being on the heteroatoms. There are two types of carbocations—carbenium ions and carbonium ions. Carbenium ions are trivalent species with a formula of RaC. Carbonium ions encompass two related types of structures. First are pentavalent species of the general formula R5C. While not common in solution phase chemistry, such pentavalent carbons are common in the gas phase. Also referred to as carbonium ions are carbocations that have an important contribution from three center—two electron bonding. The nomenclature will become more clear as we present examples below. 

Carbocations (carbenium ions) are important intermediates in organic chemistry, and have been intensely studied since as long ago as 1900. The analogy between carbon and silicon is often stressed, but the isolation of a silylium ion, RaSi, took much longer than for carbon chemistry, although as silicon is more electropositive than carbon it should form cations more easily. But it was not until 1986 that Lambert first reported the synthesis of a free silylium cation, by the following reaction [52] ... 

Trivalent ( classical carbenium ions contain an sp -hybridized electron-deficient carbon atom, which tends to be planar in the absence of constraining skeletal rigidity or steric interference. The carbenium carbon contains six valence electrons thus it is highly electron deficient. The structure of trivalent carbocations can always be adequately described by using only two-electron two-center bonds (Lewis valence bond structures). CH3 is the parent for trivalent ions. 

The ionization mechanism for nucleophilic substitution proceeds by rate-determining heterolytic dissociation of the reactant to a tricoordinate carbocation (also sometimes referred to as a carbonium ion or carbenium ion f and the leaving group. This dissociation is followed by rapid combination of the highly electrophilic carbocation with a Lewis base (nucleophile) present in the medium. A two-dimensional potential energy diagram representing this process for a neutral reactant and anionic nucleophile is shown in Fig. 

When feed contacts the regenerated catalyst, the feed vaporizes. Then positive-charged atoms called carbocations are formed. Carbo-cation is a generic term for a positive-charged carbon ion. Carbocations can be either carbonium or carbenium ions. 

The stability of carbocations depends on the nature of alkyl groups attached to the positive charge. The relative stability of carbenium ions is as follows [2] with tertiary ions being the most stable ... 

Thus, in the systems under consideration, MeX may form haionium ions with growing carbenium ions. Since the stability of haionium ions depends on the polarizability of ttie halogen38 —I > —Br > —Cl, Mel should form the most stable haionium ions, le., have most pronounced poisoning effect, followed by MeBr and MeCl. Indeed, Mel may even compete for the carbocation with highly nucleophilic counterions. 

Kolbe electrolysis is a powerful method of generating radicals for synthetic applications. These radicals can combine to symmetrical dimers (chap 4), to unsymmetrical coupling products (chap 5), or can be added to double bonds (chap 6) (Eq. 1, path a). The reaction is performed in the laboratory and in the technical scale. Depending on the reaction conditions (electrode material, pH of the electrolyte, current density, additives) and structural parameters of the carboxylates, the intermediate radical can be further oxidized to a carbocation (Eq. 1, path b). The cation can rearrange, undergo fragmentation and subsequently solvolyse or eliminate to products. This path is frequently called non-Kolbe electrolysis. In this way radical and carbenium-ion derived products can be obtained from a wide variety of carboxylic acids. 

Operando DRIFTS examination of the working zeolite catalysts shows adsorbed hexane but do not support the presence of bound alkoxide/olefin/carbenium ion species. Data substantiate that alkanes may be activated without full transfer of zeolite proton to the alkane, i.e., without generation of any kind of real carbocation as transition state or surface intermediate. 

Several reaction pathways for the cracking reaction are discussed in the literature. The commonly accepted mechanisms involve carbocations as intermediates. Reactions probably occur in catalytic cracking are visualized in Figure 4.14 [17,18], In a first step, carbocations are formed by interaction with acid sites in the zeolite. Carbenium ions may form by interaction of a paraffin molecule with a Lewis acid site abstracting a hydride ion from the alkane molecule (1), while carbo-nium ions form by direct protonation of paraffin molecules on Bronsted acid sites (2). A carbonium ion then either may eliminate a H2 molecule (3) or it cracks, releases a short-chain alkane and remains as a carbenium ion (4). The carbenium ion then gets either deprotonated and released as an olefin (5,9) or it isomerizes via a hydride (6) or methyl shift (7) to form more stable isomers. A hydride transfer from a second alkane molecule may then result in a branched alkane chain (8). The... 

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