Carbonium and carbenium ions

The reaction mechanism involves carbonium and carbenium ion intermediates. The first and difficult step is the generation of carbonium ions from alkanes ... 

The hydrocarbon catalytic cracking is also a chain reaction. It involves adsorbed carbonium and carbenium ions as active intermediates. Three elementary steps can describe the mechanism initiation, propagation and termination [6]. The catalytic cracking under supercritical conditions is relatively unknown. Nevertheless, Dardas et al. [7] studied the n-heptane cracking with a commercial acid catalyst. They observed a diminution of the catalyst deactivation (by coking) compared to the one obtained under sub-critical conditions. This result is explained by the extraction of the coke precursors by the supercritical hydrocarbon. 

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

Direct evidence for the presence of carbonium and carbenium ions in the molecular sieve pores is scarce. Experiments point to such species only in the presence of very strong acid sites provided relatively basic reactant molecules are used [60]. Even in such cases the interpretation of the experimental data does not seem to be unequivocal [61]. Most results suggest that the true cation exists only in the transition state resulting in a quite complex reaction coordinate. The course of the reaction is determined by the chemical nature of the leaving and the substituting group, the acidbase properties of the molecular sieve, the influence of co-reactants and the availability of space for the reaction to take place. The majority of the nucleophilic substitutions involve the replacement of an -OH group with an -NH, -S, -SH,... 

The elementary reaction steps of the hydrocarbons considered in this section are summarized in Fig. 8. Tlie occurrence of monomolecular reactions with linear hydrocarbons that produce hydrogen and alkane fragments was first demonstrated by Haag and Dessau [94], For convenience, the zeolite lattice to which the proton is attached is not explicitly shown in the scheme. However, it will become clear later that proton activation cannot be understood properly without explicitly taking into account the interaction of the carbonium and carbenium ion intermediates with the negatively charged zeolite wall. 

As we have seen earlier, the propagating species in cationic chain polymerization is a positively charged carbon species. The older term for this trivalent, trigonal, positively charged carbon ion is carbonium ion which we have used up to this point. Olah [10] proposed that the term carbenium ion be used instead of carbonium ion, the latter being reserved for pentavalent, charged carbon ions, and the term carbocation for both carbonium and carbenium ions. Since the term carbenium ion is not universally followed, to avoid the controversy we will henceforth refer to the propagating species as carbocations. Most text and journal references use the term carbocation and the term carbocation polymerization is used synonymously with cationic polymerization in the literature. 

In conclusion, it has been shown that zeolites are largely stabilised by the presence of rare earths in extra-framework positions. The acidity generated during the exchange yields catalysts that are active and selective, either by themselves or in combination with metals, for acid catalysed processes involving carbonium and carbenium ions. They are of practical importance in processes related with oil refining petrochemistry and also in the production of chemicals. 

In zeolite catalysis, carbenium- or carbonium-ion intermediates are energetically located at the top of the reaction energy barriers. In contrast, in superacid solutions, these protonated intermediates are ground-state reactants. The zeolite carbonium- and carbenium-ion transition state concepts are illustrated for C-C activation and olefin isomerization reactions below. ... 

For cracking reactions, combinations of zeolites, alumina, clay, and silica are used as the catalyst. These acidic materials, which contain both Br0nstead and Lewis acidic sites, initiate a complex set of carbonium- and carbenium ion-based reactions. Note that carbonium ions are protonated alkyl groups (e.g., C Hg ), while carbenium ions refer to alkyl cations (e.g., To enhance the acidic properties, rare... 

Ans The reactions differ in boiling points because the number of carbon atoms of the hydrocarbons present in each fraction is different. In catalytic cracking reactions, solid acid catalysts (clay, rare earth exchanged zeolites, etc.) promote carbonium- and carbenium ion-based C-C bond cleavage, while in thermal aacking it is homolytic cleavage. 

Protons and carbenium ions always add to C=C double bonds via carbenium ion intermediates simply because no energetically favorable onium ions are available from a reaction with these electrophiles. An onium intermediate formed by the reaction of a proton would contain a divalent, positively charged H atom. An onium intermediate produced by the reaction of a carbenium ion would be a carbonium ion and would thus contain a pentavalent, positively charged C atom. Species of this type are generally rare, but an excellent example is the nor-bornyl cation (Figure 2.29). 

This section describes elementary reaction steps and reaction chemistry of proton activated alkane reactions as understood mainly from studies in superacids. The reaction steps and reaction intermediates are also useful to consider in zeolite catalysis. However there is an important difference. Whereas carbonium-ion and carbenium-ion in superacids are usually stable intermediates, in zeolites they are highly activated states often corresponding to transition states [53]. 

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). 

It is speculated that the mechanism concerning pentacoordinated carbonium ion intermediates (eg. Ri-CH3 -CH2-R2, Ri-CH2 =CH-R2, CcHy" ) occurs at temperatures above 500°C with the intermediates undergoing ft-scission to smaller paraffins and carbenium ions. As well, the carbonium ions are converted to carbenium ions through the loss of hydrogen, present as molecular hydrogen in the cracking products. This mechanism is also favoured by low conversion, low hydrocarbon partial pressure and high constraint indexed zeolites (Scherzer, 1989). 

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. 

The carbonium ion s charge is not stable and the acid sites on the catalyst are not strong enough to form many carbonium ions. Nearly all the cat cracking chemistry is carbenium ion chemistry. 

The unstable carbonium ion decomposes to a carbenium ion [CnH2 3r [CnH2n.ir+H2 and, in a cracking step... 

There are substantial differences between gas-phase and liquid-phase hydride transfer reactions. In the latter, the hydride transfer occurs with a low activation energy of 13-17 kJ/mol, and no carbonium ions have been detected as intermediates when secondary or tertiary carbenium ions were present (25). 

These differences were explained by solvation effects in the liquid phase. The carbenium ions are more efficiently stabilized by solvation than carbonium ions, because the former have unsaturated trivalent carbon atoms. In this way, the energy barrier between the (solvated) carbenium ion and the carbonium ion transition state increases. 

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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