Carbonium ions isobutane protonation

In the case of zeolites, an additional pathway has been proposed involving the formation of a non-classical carbonium ion by protonation of isobutane, which can alkylate the olefin with formation of a protonated cyclopropane intermediate (60,61) ... 

The direct protonation of isobutane, via a pentacoordinated carbonium ion, is not likely under typical alkylation conditions. This reaction would give either a tertiary butyl cation (trimethylcarbenium ion) and hydrogen, or a secondary propyl cation (dimethylcarbenium ion) and methane (37-39). With zeolites, this reaction starts to be significant only at temperatures higher than 473 K. At lower temperatures, the reaction has to be initiated by an alkene (40). In general, all hydrocarbon transformations at low temperatures start with the adsorption of the much more reactive alkenes, and alkanes enter the reaction cycles exclusively through hydride transfer (see Section II.D). 

Figure 27. H-carbonium ion-like transition states for methane, ethane, propane and isobutane protonation by HF/SbFs.

Reaction of the ierf-butyl carbonium ion with octene (ethylene tetramer), for example, will produce isobutane and an unsaturated carbonium ion which may form a diene by loss of a proton or which may cyclize to yield an ethylcyclohexyl carbonium ion ... 

It will be noted from the above examples that the tertiary butyl carbonium ions required for the reaction are constantly being replenished to establish a chain reaction. It is assumed that the reaction is initiated by olefin molecules accepting protons from the catalyst to form carbonium ions which react with isobutane to produce the necessary active tertiary butyl carbonium ions. 

For this reason when Magic Acid (HS03F-SbF5,1 1 molar ratio) is used under the same experimental conditions as above at room temperature, isobutane undergoes very slow ionization and the formation of the ferf-butyl ion can be monitored. However, recovered isobutane shows no exchange because the reversible protonation via carbonium ion transition state does not take place and because the ferf-butyl ion, stable in this solution at room temperature, does not deprotonate. 

Undoubtedly, it seems plausible that the large difference observed in the CH3T yield from the protonation of butane (15%) and isobutane (23%) reflects the higher probability that a carbonium ion protonated on a primary carbon is formed from the attack of HeT+ on isobutane. The decomposition of the intermediate according to equation 52 represents a direct route to tritiated methane ... 

Chain Initiotion. The theory pxKtulated by a number of investigators (Cupit etal., 1961, Schmerling, 1955) is that carbonium ions are generated by addition of a proton (H+) to an olefin molecule in the presence of HF. Albright and Li, 1970, and Hofmann and Schriesheim, 1962, indicate that initiation steps with H2SO4 catalyst may involve red oil hydrocarbons. However, only the tertiary butyl carbonium ion performs the chain carrying function in isobutane alkylation. Reactions follow ... 

The most striking product result is the extensive formation of propane over very active catalysts. Venuto et al. (99) reported analogously that dealkylation of rf-butylbenzene over rare earth-exchanged X zeolite at 260° gave isobutane as the major gaseous product. Such paraffin formation is presumably the result of hydride transfer reactions to the car-bonium ions formed by initial electrophilic cleavage of the alkylbenzene 100) or by protonation of the olefin. Reasonable hydride donors are cumene and propylene the resultant hydrogen-deficient species are then precursors of residue formation (32, 89). Parafiin formation by treatment of alkylbenzenes with aluminum halides in the presence of cyclohexane or decalin has been known for 30 years 47), and there is ample evidence for hydride transfer between carbonium ions and hydrocarbons 10, 22, 27,53). 

A similar regioselective exchange pattern was found later in triflic acid. Furthermore, the deuterium distribution observed in isobutane recovered after short contact times with DF-SbFs at 0°C (in contrast to -78°C) showed a fast deuterium exchange at all C-H bonds. " Three isomeric carbonium ions in equilibrium are formed (Scheme 6.5) and the relative concentration of these prevalent reaction intermediates depends only on the relative basicity of the proton-accepting bonds in accord with the Olah o-basicity concept. 

The mechanisms of alkylation reactions appear to be very complex. Analyses of typical alkylation products show that on basis of known reactions, secondary reactions of isomerization, cracking, and disproportionation, hydrogen transfer and polymerization must occur in the reaction. All these reactions are almost certain to occur by means of carbonium ion complexes including formation, addition, rearrangement, and proton and hydride ion transfer. The following reactions are at present beheved to occur as the main reactions in the alkylation of butene-1 with isobutane ... 

The trimethylethylethylene may undergo polymerization, copolymerization, or alkylation, or it may add a proton and then be converted to isopentane by means of the hydrogen exchange reaction. The ethyl carbonium ion loses a proton to yield ethylene which may then react with isobutane (via t-butyl carbonium ion) to yield 2,3-dimethylbutane. 

When isobutane was alkylated with 1- or 2-butene in the presence of aluminum chloride monomethanolate, very little or no n-butane was formed despite the fact that appreciable amounts of 2,2,4-trimethylpentane were produced (Schmerling, 14d). Similarly, no n-butane or n-pentane, respectively, was obtained by the alkylation of isobutane with 2-butenc and with 2-pentene in the presence of sulfuric acid although trimethylpentanes were formed in both cases (McAllister et al., 12 cf. Marschner and Carmody, 24). This apparent discrepancy in the alkylation mechanism may be explained readily. The n-alkylcne is converted not into n-alkane, but into isoalkane. The resulting isobutane cannot, of course, be differentiated from that charged the resulting isopentane, on the other hand, can be and was actually found in substantial yield. In other words, the proton transfer reaction is accompanied by rearrangement of the carbon skeleton of the carbonium ion. 

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