Carbenium ion mechanisms

Hence, the rate depends only on the ratio of the partial pressures of hydrogen and n-pentane. Support for the mechanism is provided by the fact that the rate of n-pentene isomerization on a platinum-free catalyst is very similar to that of the above reaction. The essence of the bifunctional mechanism is that the metal converts alkanes into alkenes and vice versa, enabling isomerization via the carbenium ion mechanism which allows a lower temperature than reactions involving a carbo-nium-ion formation step from an alkane. 

Since the discovery of alkylation, the elucidation of its mechanism has attracted great interest. The early findings are associated with Schmerling (17-19), who successfully applied a carbenium ion mechanism with a set of consecutive and simultaneous reaction steps to describe the observed reaction kinetics. Later, most of the mechanistic information about sulfuric acid-catalyzed processes was provided by Albright. Much less information is available about hydrofluoric acid as catalyst. In the following, a consolidated view of the alkylation mechanism is presented. Similarities and dissimilarities between zeolites as representatives of solid acid alkylation catalysts and HF and H2S04 as liquid catalysts are highlighted. Experimental results are compared with quantum-chemical calculations of the individual reaction steps in various media. 

The isobutene oligomerization is a highly exothermic reaction, carried out via the carbenium ion mechanism, which is thermodynamically favoured at low temperature. The kind of products obtained as well as the conversion and stability at constant temperature and pressure will depend on the reaction GHSV, which determine the intermediate carbenio ion formed during the first steps. 

The 13C NMR study of the polymerization of pent-l-ene with 95% H2SO4 points to the carbenium-ion mechanism, which involves the formation of sulfuric acid esters and their further heterolytic dissociation to generate aliphatic carbenium ions, whose steady-state concentration is low. By contrast, polymerization in 60-70% H2SO4 proved to occur via oxonium (rather than carbenium) ions.38... 

A comparison of the reactivity of SbF5-treated metal oxides with that of HS03F-, SbCl5-, and HS03F-SbF5 (magic acid)-treated catalysts showed that the former was by far the best catalyst for reaction of alkanes (31, 32). Tracer studies of conversion of alkanes catalyzed by the superacids were performed it was suggested that the reactions proceeded by carbenium ion mechanisms in which the reactions were initiated by abstraction of H from the reactants (33). 

This view appeared to support the carbenium ion mechanism suggested for the olefin formation in liquid phase191. 

Under very mild conditions (0-20°C, 200 Torr ethylene pressure), ethylene was shown to be selectively dimerized to n-butenes over RhY (140). As shown in Fig. 14, 1-butene was formed initially but further isomerized to an equilibrium composition of -butenes with increasing reaction time. In a comparative experiment using HY as a typical solid-acid catalyst, no ethylene conversion was measurable up to 200°C, and at higher temperatures unselective polymerization and cracking reactions occurred. This provided good evidence that the selective dimerization over RhY did not proceed via a carbenium ion mechanism. 

Although the reactions have been generally described in terms of a carbenium ion mechanism, this does not altogether explain the catalytic behavior of the alkali metal ion-exchanged zeolites or the selectivity behavior. An ionic mechanism of the type previously described for cyclopropene dimerization would seem to be more appropriate for the alkali metal ion-exchanged zeolites, where the activity does seem to correlate qualitatively with the electrostatic field (e/r) exerted by the cation. 

In the alkylation of arene systems with olefins, the above chemistry proceeds via a carbenium ion mechanism. Alkylation of an aromatic compound with an olefin occurs by the interaction of a Bronsted acid site of any of the catalysts with a participating olefin, creating a carbenium ion, via protonation of the double bond, and thus a polarized complex is formed, as shown below ... 

Volatile products derived from cracking PE with solid acid catalysts can be rationalized by carbenium ion mechanisms. Under steady-state conditions, hydrocarbon cracking processes that yield volatile prodncts can be represented by initiation, disproportionation, P-scission, and termination reactions [72, 73]. Initiation involves the protolysis of PE with Bronsted acid sites (H+ S ) to yield paraffins and surface carbenium ions ... 

Again the results are reasonable in view of known carbenium ion mechanisms. An important conclusion is that after protonation these highly branched olefins are prone to crack and the smaller carbenium iem fragments then are able to cationate the larger olefins leading to higher olefins. For example ... 

Simpler a-chloro ketones have also been examined, and exhibit negligible selectivity. The mixtures of products shown in equations (91) and (92) may be the result of formation of both cis- and rranj-halo-hydrin isomers in the addition step or, less likely with the chloride, a carbenium ion mechanism. The effect of varying the halide in such reactions has not been systematically investigated. 

The effects of epoxide structure on rates and product distributions indicate that LiC104 reactions occur by a carbenium ion mechanism. Conversely, the LiBr-catalyzed reactions involve bromohydrin salts as precursors to rearrangement products. These are sharp distinctions, and provide the cleanest examples of the two extreme mechanisms for epoxide-[Pg.760]

Since Pt-Re/Al203 is not sulphided, a high level of cracked products is expected to be formed. They are mainly produced by the metal function via hydrogenolysis, and further crackates can be also generated by the acid function of the bifunctional catalyst. With the addition of ZSM-5, the major products are C3 and C4 species resulting from the acidic nature of the zeolite, which can be accounted for by tbe carbenium ion mechanism [13]. 

Scheme 1 Carbenium ion mechanism of ring contraction and methyl side-chain generation...

Mechanistic Interpretation - Although the nature of the interactions between the polymers and the catalysts is not exactly known, the products that are formed and the changes that occur when an acid catalyst is used as compared to thermal degradation are consistent with a carbenium ion mechanism commonly used to account for products observed for conversion of small hydrocarbons over acid catalysts. Recent work suggests that the actual species on the surface of the catalyst is an alkoxide, but carbenium ion chemistry can still be used as a model of the transformations that occur. ... 

Figure 2 Adsorption mechanisms on Brensted and Lewis acid sites (a) carbenium ion mechanism (adsorption on Brensted acid sites), (b) ethoxy structure mechanism (adsorption on Brensted acid sites), (c) cationic mechanism (adsorption on Lewis acid sites)...

As already mentioned, another attractive reaction would be the homologation of alcohols, in particular that of methanol to ethanol. This reaction is described in some early as well as recent patents. The most frequently suggested catalystis a complex of Co, sometimes with iodine as a promoter. There are several mechanisms suggested in the literature (carbenium ion mechanism, CO insertion, etc examples of which can be found in the literature quoted. " One can speculate that the role of iodine is the same... 

True hydrocracking (Figure 6) is mediated over dual-functional metal / acid catalysts and proceeds via adsorbed carbenium ion intermediates where cleavage is most likely on central C-C bonds and, from the fourth C-position, occurs with almost equal probability [5, 10, 11]. Moreover, due to the carbenium ion mechanism, the intermediate to cracked products is typically an isomerised carbenium ion with the result that products are generally highly branched. 

Alkylation was very rapid in methyl chloride solvent, somewhat slower in cyclopentane, but irrespective of the diluent, proceeded without disturbing side reactions (2S). Table 2 shows representative results with A1(CH3)3 and tertiary alkyl chlorides. Table 3 compiles data obtained with A1(CH3)3 and various primary, secondary and allylic chlorides, and Table 4 shows the results with various trialkylaluminums and tert-butyl chloride. These findings can be explained by assuming a carbenium ion mechanism. For example, the reaction between AlICHsls and rm-butyl chloride to form neopentane can be visualized as follows ... 

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