Carbenium ions disproportionation

As an example for aromatic transformation the mechanism for meta-xylene disproportionation to toluene -i- trimethylbenzene is illustrated in Figure 13.46. In the first step the zeolite extracts a hydride from meta-xylene to form a carbenium ion at one of the methyl groups, presumably the rate-controlling step. This mechanism is likely to involve a Lewis acid site. The carbenium ion then adds to a second... 

In a subsequent study Devynck and co-workers81,82 studied the electrochemical oxidation of alkanes and alkenes in triflic acid monohydrate. The acidity of CF3SO3H H20 was found to be intermediate between that of aqueous acid media and superacidity. Alkanes undergo two-electron oxidation, whereas alkenes are protonated to yield carbenium ions in this medium. In addition to various transformations characteristic of carbenium ions [Eqs. (5.36)—(5.38)], they undergo a reversible disproportionation to give an alkane and an aldehyde [Eqs. (5.40)]. 

Corma and co-workers152 have performed a detailed theoretical study (B3PW91/6-31G level) of the mechanism of the reactions between carbenium ions and alkanes (ethyl cation with ethane and propane and isopropyl cation with ethane, propane, and isopentane) including complete geometry optimization and characterization of the reactants, products, reaction intermediates, and transition states involved. Reaction enthalpies and activation energies for the various elemental steps and the equilibrium constants and reaction rate constants were also calculated. It was concluded that the interaction of a carbenium ion and an alkane always results in the formation of a carbonium cation, which is the intermediate not only in alkylation but also in other hydrocarbon transformations (hydride transfer, disproportionation, dehydrogenation). 

The formation of C6 and C7 acids along with some ketones was reported in the reaction of isopentane, along with methylcyclopentane and cyclohexane with CO in HF-SbF5 at ambient temperatures and atmospheric pressure.406 Yoneda et al.407 have also found that other alkanes can be carboxylated as well with CO in HF-SbF5. Tertiary carbenium ions, which are produced by protolysis of C—H bonds of branched alkanes in HF-SbF5, undergo skeletal isomerization and disproportionation before reacting with CO. Formation of the tertiary carboxylic acids in the... 

The other cause is the catalytic disproportionation of two normal adjacent carbenium ions to yield a paraffin and a site-spanning di-ion which deactivates two sites. This is a minor catalytic process it can form coke, but at the same time it yields valuable complex products, notably aromatics. In fact, this process can in principle be engineered not to form coke if an appropriate reaction environment can be arranged. 

From these considerations we can see an outline of the kinetics and mechanism of catalyst decay. While the catalyst remains in the presence of the reactant-product stream, on each active site the processes which dominate are the "fiuitful" processes of the attached carbenium ions, involving protolysis, P-cracking, disproportionation, and the reversible adsorption-desorption of product olefins. These events, in combination, constitute the dKiin mechanism of cracking 4) and yieldthe major products of the "cracking" reactioa None of these processes results in an irreversible reduction of catalyst activity, although the various carbenium ions present will undergo various mainline reactions at different rates. 

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 ... 

Propagation reactions involve disproportionation between feed molecules and surface carbenium ions to yield paraffins ... 

Disproportionation reaction rates depend on carbenium ion reactivities, which are determined by catalyst site acid strength. Carbenium ions produced at strong acid sites are less likely to undergo P-scission or desorption. Compared with HY, the smaller pores in HZSM-5 inhibit bimolecular disproportionation reactions. In contrast, the low paraf-fin/olefin volatile product ratio for the PE-MCM-41 sample is likely due to the low acidity of the catalyst. Although the MCM-41 pore size is large enough to facilitate disproportionation, catalytic site acidity is too low for this reaction pathway to be dominant. 

The results obtained in this study indicate that in Al-ffee H-boralite (BOR 1) only weak BrOnsted acid sites (Si—OH—B) are present. They are active only in cyclohexanol dehydration. Their catalytic activity is, however, relatively low. The insertion of A1 into the framework results in the creation of strong Bronsted acid sites. Most probably they are Si—OH—Al, the same as in zeolites. The IR band which could be characteristic of such Si—OH—Al (at about 3610 cm ) was not seen in the spectrum because of the very low concentration of these hydroxyls. The catalytic activity of Si—OH—Al is much higher that of Si—OH - B. Contrary to Si—OH -B, Si—OH— A1 are active in consecutive reactions of cyclohexene (isomerization and disproportionation). Cyclohexene isomerization (to methylcyclopentenes), a typical carbenium ion reaction is catalysed by strong Brdnsted acid sites even at temperatures as low as 450 K. The same strong Bronsted acid sites catalyse also cyclohexene disproportionation (to cyclohexane, methylcyclopentane and coke). Our earlier... 

Since the early 1960s, superacids have been known to react with saturated hydrocarbons to yield carbocations, even at low temperature [41]. This discovery initiated extensive studies devoted to electrophilic reactions and conversions of saturated hydrocarbons. Thus, the use of superacidic activation of alkanes to their related carbocations allowed the preparation of alkanecarboxylic acids from alkanes themselves with CO. In this respect, Yoneda et al. have found that alkanes can be directly carboxylated with CO in an HF-SbFs superacid system [42]. Tertiary carbenium ions formed by protolysis of C-H bonds of branched alkanes in HF-SbFs undergo skeletal isomerization and disproportionation prior to reacting with CO in the same acid system to form carboxylic acids after hydrolysis (eq. (9)). 

The lifetime of the carbenium ion formed will be limited by transferring a proton back to the zeolite, thus completing the dehydrogenation ofthe hydrocarbon. Hydride abstraction from xylene is assumed to be the initial step in its disproportionation into toluene and trimethylbenzene [9]. The parent compound (7, Fig. 22.9) ofthe carbenium ion formed (6) has such a high proton affinity (1031 kj mohh Table 22.1) that proton transfer back to the zeolite does not occur at all. However, the lifetime of carbenium ions in zeolites is not only limited by proton transfer, but also formation of a C-O bond between the carbenium ion and a framework oxygen atom, yielding an alk-oxide, needs to be considered. In ferrierite (FER) the alkoxide of 6 is found to be 50 to 60 kJ mofi more stable than the carbenium ion [9]. 

Localization of stationary points along the reaction path for reactions taking place inside the zeolite pores is one of the greatest challenges in zeolite modeling. The reactions of hydrocarbons are particularly difficult to model since the hydrocarbon...zeolite interaction can be dominated by the dispersion interaction that is not properly accounted at the DFT level. Only one example is presented here. Clark et al. investigated the role of benzenium-lype carbenium ion in the bimolecular w-xylene disproportionation reaction in zeolite faujasite.163] The benzenium-type carbenium ion 1 was identified in zeolite catalyst for the... 

Cracking. Bond breakage during cracking may be by B-scission of a tri-coordinated carbenium ion (for carbon numbers much greater than 6) or via a penta-coordinated carbonium ion in either case, a shorter-chain olefin is formed. Subsequent hydrogen transfer (HT) reactions may form the shorter-chain paraffin. Isomerization, aromatization, disproportionation and dealkylation are other... 

The second, which explains the disproportionation of xylenes, occurs through benzylic carbenium ions and diarylmethane intermediates. 

An example is the disproportionation of /w-xylene to toluene and trimethyl-benzenes in the wide-pored zeolite Y (Fig. 7-5 c). In the large zeolite cavity, bulky diphenylmethane carbenium ion transition states can be formed as precursors for methyl group rearrangement, whereby the less bulky carbenium ion B is favored. Thus the reaction product consists mainly of the imsymmetrical 1,2,4-trimethylben-zene rather than mesitylene (case A). In contrast, in ZSM-5, with its medium sized pores, monomolecular xylene isomerization dominates, and the above-mentioned disproportionation is not observed as a side reaction. 

The anodic oxidation of alkanes in anhydrous hydrogen fluoride has been studied at various acidity levels from basic medium (KF) to acidic medium (SbFs) to establish optimum conditions for the formation of carbenium ions . The oxidation potential depends on the structure of the hydrocarbon methane is oxidized at 2.0 V, isopentane at 1.25 V vs Ag/Ag. Three cases of oxidation can be distinguished. In basic medium, direct oxidation of the alkane to its radical cation occurs. In a slightly acidic medium, the first-formed radical cation disproportionates to cation, proton and alkane. The oxidation is, however, complicated by simultaneous isomerization and condensation reactions of the alkane. In strongly acidic medium, protonation of the alkane and its dissociation into a carbenium ion and molecular hydrogen occurs. In acidic medium n-pentane behaves like a tertiary alkane, which is attributed to its isomerization to isopentane. The controlled potential electrolysis in basic medium yields polymeric species. 

Just as deprotonation adjacent to a carbenium ion can form an olefin, similarly removal of H adjacent to a free radical will form an olefin (Eq. 10.95). As we noted in Section 10.10.4, the process of Eq. 10.95 is referred to as radical disproportionation when the radicals are the same. Unimolecular elimination from a radical is the simple reverse of the addition of a radical to an alkene (Eq. 10.96). Since the addition is typically exothermic, it takes heat to reverse the addition. One example is the depolymerization of polystyrene, which will occur at temperatures of 300 °C (Eq. 10.97 see Chapter 13 for a discussion of the polymerization reactions). Strain in an adjacent ring will favor elimination, as shown in Eqs. 10.98 and 10.99. These two examples convert one radical to another, and such reactions will be discussed in more detail in Section 11.11. 

Formation of ethylene via alkyl cyclopentyl carbenium ion. Adapted from Tsai T-C, LiuS-B, Wangl. Disproportionation and transalkylation of alkylbenzenes over zeolite catalysts. AppI Catal A 1999 181 355-98. 

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