T-Butyl ion

The primary cation is an intermediate in the isomerization to the t-butyl ion, which accounts for the relatively slow rate of this process. 

This was proved by showing that the reciprocal of the life-time of the pivaloyl ion, t5co+> is independent of the concentration of t-butyl ion. A mechanism based on equation (7) would have resulted in t5,co+ being proportional to [t-C4H ]. The rate constant of decarbonylation of t-C4H9CO+ in HF—SbFg (equimolar) and in FHSO3—SbFs (equimolar) was determined to be sec. This... 

Solvolysis of t-butyl chloride in the presence of methylcyclopentane yields a t-butyl ion which is quantitatively converted to isobutane before exchanging hardly any protons with the acid. 

The ionization energy of the t-butyl radical is lower than that of the pentyl radical. Thus, the t-butyl ion is preferentially observed in the top spectrum. The same holds true for the comparison between the allyl and methyl radicals. 

The t-butyl ion can then t-butylate the Cg olefin yielding the first generation products ... 

Results that suggest that the silicon analogue of the t-butyl ion, Me3Si +, can be produced as the perchlorate have been reported27. Finally, the iron-substituted material Cp(R3P)2Fe-SiR2 also has been reported as a possible product, obtained as usual by hydride abstraction from the analogous silyl hydride33. 

Comparison of these results with those from the methylation of toluene under similar conditions shows that more effective fragmentation channels are available to the excited arenium ions from t-butylbenzene. In particular, the loss of the very stable t-butyl ion, albeit endothermic by some 18 kcal mol is largely allowed by the excitation energy (some 85 kcal mol" ) available to the arenium ion, and is strongly supported by the high yield of toluene. Equation 16. 

One sees from Fig. 1 that the intensity of the mje = 56 ion (C4H8 ) initially increases and then decreases. We have no direct evidence as to the identity of the reactions occurring, but we guess that the ion is initially formed by an H2 transfer reaction and that the subsequent consumption reaction is an H transfer reaction to produce t-butyl ion. One also sees from Fig. 1 that the mje = 39 ion (C3H3 ) is, within experimental error, unreactive... 

The s c-C4H9 ion is also definitely known to exist, with a classical secondary carbonium ion configuration. The NMR spectrum of sec-C Rg has been obtained in n-butane in strong acid solution at temperatures between —110 and —40° above —40°, conversion of s c-butyl to t-butyl ion was observed ... 

In a recent investigation " of the isomerization reactions of the isobutyl ion formed in the deamination of isobutylamine, it was demonstrated that the isobutyl ion rearranges primarily to the t-butyl ion, but also forms the s c-butyl ion, and a small amount (< 1%) of protonated cyclopropane ... 

When n-hexane is irradiated in the presence of a reactive deuterated additive compound, one observes not only CH3CH2CHDCH3 and (CH3)3CD formed in reactions (33) and (34) of the sec-butyl and t-butyl ions, respectively, but also a small amount of an n-butane product labeled at the primary position ... 

Fig. 3. Log of the rate constant for hydride transfer to a t-butyl ion from (lowest to highest point) 2-methyl butane, 2-methyl pentane, 2-methyl hexane, and 2-methyl heptane, as a function of the exothermicity of reaction. The heat of reaction is given on an arbitrary scale, since exact values for the heats of formation of the product ions are unknown.

There are problems with this kind of experiment, however. For one thing, the reactions are carried out in a solvent, which then solvates the carbocations formed at the transition state of the reaction. The difference in the solvation energies between the methyl- and t-butyl ions may be substantial, and this would upset the comparison. But what seems to be even more important is this. This experiment only measures the difference in the amount of hyperconjugation in the two cases (methyl vs. t-butyl), not the absolute value. It is certainly true that CH3 is a very unstable ion. But H is not all... 

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