Isobutyl cation, primary

FIGURE 3.77 Relative stability of the / -/-butyl cation (tertiary) and the isobutyl cation (primary). 

The reaction scheme can be represented conveniently in the form of an energy diagram (Figure 10-10). The activation energy, AH ert for the formation of the tert-butyl cation is less than AHlrim for the formation of the isobutyl cation because the tertiary ion is much more stable (relative to the reactants) than the primary ion, and therefore is formed at the faster rate. The second step, to form the product from the intermediate cation, is very rapid... 

The processes are the monomolecular reaction through a protonated cyclopropane produced by the abstraction of H" over Lewis acid sites and the bimolecular mechanism where an olefin takes part in the reaction. The olefin is produced over Bronsted acid sites, in the case of butane in the monomolecular mechanism, isobutane is formed through protonated methylcyclopropane with an activation energy of 8.4kcalmoT followed by the formation of the primary isobutyl cation with high energy [134]. 

Taft and Cannell also determined thiocyanate isothiocyanate ratios for the deamination of benzhydrylamine and for the isobutyl fraction in the deamination of isobutylamine. The values of [RNCS] [RSCN] were 9 1 and 5-3 1, respectively. It is interesting that values for the benzhydryl- and t-butyl-amines bracket that for isobutylamine. It is most unlikely that the isobutyl cation is of intermediate selectivity, and the results may suggest that the reaction of primary alkyldiazonium ions is sufficiently near the mechanistic borderline for so strong a nucleophile as thiocyanate to force an Sif2 displacement on the isobutyldiazonium ion. 

Reaction coordinate diagram for the addition of to 2-methylpropene to form the primary isobutyl cation and the tertiary tert-butyl cation. 

The hydration reaction is regiospecific. It obeys Markovnikov s rule. r rr-Butyl alcohol forms rather than the isobutyl alcohol that would result from adding a proton to C-2 followed by reaction of water with a carbocation at C-1. Such a process would proceed via a much less stable primary carbocation (the isobutyl cation). 

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