Carbenium ions, carbocation

Step 1, the addition of a proton to the alkene, results in formation of a cationic intermediate. One carbon atom in this intermediate has only six electrons in its valence shell and carries a charge of +1. A species containing a positively charged carbon atom is called a carbocation (carbon + cation). Such carbon-containing cations are also called carbonium ions and carbenium ions. Carbocations are classified as primary (1°), secondary (2°), or tertiary (3°), depending on the number of carbon atoms bonded to the carbon bearing the positive charge. All carbocations are electrophiles as well as Lewis acids (Section 4.7). 

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... 

Carbocation property Carbenium ions Carbonium ions... 

The RC60+ cations are carbenium ions (trivalent carbocations), which are formally conjugated with the entire 7i-system of the fullerene cage. This chapter focuses on recent developments in the author s laboratory regarding the prepara-... 

Numerous studies suggest that alkyl-aluminumsilyl oxonium ions should be the real intermediates in hydrocarbon reactions over zeolite, whereas carbocations should be just transition states (J). Equilibrium between the alkyl-aluminumsilyl oxonium ion and the carbocation, although suggested in some cases, has never been experimentally or theoretically proven, but recent calculations indicated that the tert-butyl carbenium ion is an intermediate on some specific zeolite structures 6,7). 

Carbocations are central to hydrocarbon chemistry (/). Much of this chemistry is based on acid catalysis, which leads to generation of positive ions of carbon. The resulting intermediates are classified as carbenium and carbonium ions, as proposed by Olah (2-4). Carbonium ions are the penta- or higher coordinate carbocations that maintain 8 valence electrons via 2-electron/3-center bonding, quite different from carbenium ions that possess only 6 valence electrons. Figure 1 shows a systematic classification of carbocations. 

Figure 1. Classification of carbocations into carbonium and carbenium ions,...

In some of their publications Higashimura s group, and others using the same terminology, are close to our view when they write about the modifiers reducing the reactivity of the carbocation . However, since in our view there is no carbenium ion to be stabilised, we see these donors as reducing the polarity of the ester bond and the reactivity of the 0-protons, and they obstruct physically the transfer of a P-proton to the monomer or to any other base. 

However, acid-catalyzed isomerization attracts more attention, probably due to its connection with the recent intensive development of carbenium ion chemistry. It is common knowledge that effective methods for stabilization of reactive carbocations have been known since 1962 while base-catalyzed processes with the participation of carbanions were developed more than 100 years ago. 

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