Isobutane protonation state

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

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. 

The dehydrogenation reaction proceeds through the simultaneous elimination of the zeolitic proton and a hydride ion from the alkane molecule, giving rise to a transition state which resembles a carbenium ion plus an almost neutral H2 molecule to be formed. For the linear alkanes, the TS decomposes into an H2 molecule and the carbenium ion correspondent alkoxide. However, for the isobutane molecule the reaction follows a different path, the TS producing isobutene and H2. Most certainly the olefin elimination is flavored to the alkoxide formation due to steric effects as the t-butyl cation approaches the zeolite framework. The same mechanism is expected to be operative for other branched alkanes. 

The alkylation of isobutane with C3-C5 olefins involves a series of consecutive and simultaneous reactions with carbocation species as the key intermediates. Scheme 6.10.2 shows the reaction of 2-butene and isobutane as a typical example. In the initial step, proton addition to 2-butene affords a sec-butyl cation. This sec-butyl cation can either isomerize or accept a hydride from a molecule of isobutane, giving n-butane and the thermodynamically more stable fert-butyl cation. These initiation reactions are required to generate a high level of ions in the start-up phase of alkylation but become less important under steady state conditions. 

---