Alkylcarbenium ions

Trapped by a suitable compound, a transient intermediate can be converted into a more stable species for unequivocal identification. Stepanov and Luzgin (82) investigated the reaction of acetonitrile with 1-octene or tert-butyl alcohol on acidic zeolite HZSM-5 ( 2si/ Ai = 49) at 296 K by in situ MAS NMR spectroscopy under batch reaction conditions. Upon coadsorption of acetonitrile and 1-octene, a C MAS NMR signal at 108 ppm was observed, indicative of TV-alkylnitrilium ions 2 in Scheme 3. As depicted in Scheme 3a, the formation of these cations was explained by trapping the chemically unstable alkylcarbenium ions (formed from the adsorbed... 

A central feature of the mechanism that accounts for the catalytic cracking of hydrocarbons by appropriately cation exchanged zeolites is the formation of carbonium ions (also designated carbocations and alkylcarbenium ions) as intermediates. Many other reactions for which aluminosilicates, be they clay-or zeolite-based, also predicate (320) the existence of carbonium ion intermediates, formed usually by proton donation from Bronsted acid sites, have been discussed earlier (Section III,K). 

Since ozone is a strong 1,3-dipole,635 or at least has a strong polarizability (even if a singlet biradical structure is also feasible), it is expected to be readily protonated in superacids, in manner analogous to its alkylation by alkylcarbenium ions. Protonated ozone HC>3+, once formed, should have a much higher affinity (i.e., be a more powerful electrophile) for cr-donor single bonds in alkanes than neutral ozone. 

While the rate of cleavage is given by temperature, acidity of the catalyst and concentration of i-alkyl cations, the rate of desorption is assumed to be enhanced by the steady state concentrations of n-alkenes, i. e., a high dehydrogenation activity of the catalyst favors hydroisomerization. This is the concept of competitive chemisorption which in ideal bifunctional catalysis keeps the residence times of the alkylcarbenium ions low. 

An unusual class of gitonic superelectrophiles (having vicinal -dicationic structure) are the protosolvated alkylcarbenium ions (of carbenium-car-bonium dication nature). Several studies have shown that alkylcarbenium... 

According to the reaction scheme shown in Figure 5 both hydroisomerization and hydrocracking of the n-alkanes (except n-hexane) proceed via branched alkylcarbenium ions. In the range of medium degrees of conversion (40 % 90 %) both... 

When using tertiary C5 or Ce alkanes, considerable amounts of secondary carboxylic acids are produced by the reaction of CO with secondary alkylcarbenium ions. Such cations are formed as transient intermediates by skeletal isomerization of the initially formed tertiary cations (eq. (10)) [43]. 

For the reaction of ozone with alkanes under superacidic conditions, two mechanistic pathways may be considered. The first is the formation of an alkylcarbenium ion via protolysis of the alkane prior to quenching of the ion by ozone (Scheme 6.26, route a). Alkylcarbenium ions may also be generated via initial transformation of the alkane to an alcohol followed by protonation and ionization (Scheme 6.26, route b). There have already been a number of reports of ozone reacting with alkanes to give alcohols and ketones. °" In both cases, intermediary alkylcarbenium ions would then undergo nucleophilic attack by ozone as mentioned earlier. 

Brouwer, D. M., Reactions of alkylcarbenium ions in relation to isomerization and cracking of hydrocarbons, NATO-ASI Ser. E39, in Chemistry and Chemical Engineering of Catalytic Processes (R. Prins, and G. C. A. Schuit Eds.), pp. 137-160. Sijthoff Noordhoff, Alphen-aan-den-Rijn (1980). 

Examples of mechanisms of skeletal isomerization of alicyclic alkylcarbenium ions are presented in Scheme III.4 [14k]. 

Tertiary C-H tonds show the highest reactivity. C-C bonds are generally more reactive than secondary or primary C-H bonds, leading to preferential nitrolysis of n-alkanes(cleavage nitratlcn) The nitronium ion, which is linear itself, does not seem to exercise excessive steric hindrance in the transition states of the reactions, where it is substantially bent (indicated also from its behavior in electrophilic aromatic substitutions). Side products of the nitrolysis are methyl, ethyl, and isopropyl fluoride (formed by the reaction of PFg with the cleaved alkylcarbenium ions) or secondary alkylation products, themselves capable of undergoing reaction with the nitronium salt. 

Comparison of the IR and Raman spectra of the trimethylcarbenium ion with those of its isoelectric analogue — trimethylboron — has conflrmed the planar structure of alkylcarbenium ions. The planar structure of the carbenium fragment —C < was also concluded from the IR spectra of salts of the triphenylmethyl cation (CgH5)3C 449-451) analogues having different substituents at the para positions of the... 

These data on the C—C stretching vibration of various enyl carbenium ions are summarized in Table 38. For comparison, the frequency of the C —C stretching vibrations of alkylcarbenium ions is included. 

A similar extremely low sensitivity to the acid m ium is demonstrated for the rearrangements of the 6-hydroxy-1,1,2,3,4,5-hexamethylbenzenium ion >, substituted cyclopentyl cations and alkylcarbenium ions (see also These observations allow us to compare the carbenium ion rearrangements obtained in different acid media. 

Figure 5. -scission mechanisms of alkylcarbenium ions (adapted from ref.35). 

Figure 7. Cjj alkylcarbenium ion trapped in hypothetical zeolite cages and expected carbon number distribution of its cracked products.

Formation of an intermediate alkylcarbenium ion which is the key step in superacid-catalyzed reaction of ozone with alkanes is considered to proceed by two mechanistic pathways as illustrated in Scheme 12. The carbenium ions subsequently undergo nucleophilic reaction with ozone as discussed previously. Reactions of ozone with alkanes giving ketones and alcohols as involved in mechanism b have been reported in several instances " . The products obtained from isobutane and isoalkanes (Table 6) are in accordance with the mechanism discussed above. 

The conversions of n-alkanes, cyclo-alkanes and dieir mixtures over Pt/USY zeolites under vapor phase reaction were successfully modeled using independently determined adsorption equilibria and fundamental reaction networks based on alkylcarbenium ion chemistry [9-12]. In diese models, the intrinsic reaction rate of an individual reactitm step... 

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