T-Pentyl cation

From the values in Table 2, it can be seen that the isopropyl cation is more stable than the n-propyl cation by some 26 kcal mole . Similarly the t-butyl cation is substantially more stable than any of the other butyl cations, as is the t-pentyl cation compared with any of the other pentyl cations. These considerations are based upon the energetics of ions in the... 

The barrier to the hydride and methyl shifts that interconvert the methyl groups in the t-pentyl cation is 10-15 kcal/mol. This shift Involves the formation of secondary ions as transient intermediates. ... 

Calculation of the second-order rate constant of carbonylation, kg, and the equilibrium constant, K = [t-C4H9CO+]/[t-C4H ][CO] = A c/fcD> requires knowledge of the concentration of CO. The constant a in Henry s law Pco = [CO] was determined to be 5-3 litre mole atm in HF—SbFs (equimolar) and 53 litre mole atm in FHSOs—SbFs (equimolar) at 20°C. From the ratio [t-C4HBCO+]/[t-C4HJ"] at a known CO pressure, values for k and K were obtained. The data are listed in Table 1, which includes the values for the rate and equilibrium constants of two other tertiary alkyl cations, namely the t-pentyl and the t-adamantyl ions (Hogeveen et al., 1970). 

In discussing the elFect of structure on the stabilization of alkyl cations on the basis of the carbonylation-decarbonylation equilibrium constants, it is assumed that—to a first approximation—the stabilization of the alkyloxocarbonium ions does not depend on the structure of the alkyl group. The stabilization of the positive charge in the alkyloxocarbonium ion is mainly due to the resonance RC = 0 <-> RC = 0+, and the elFect of R on this stabilization is only of minor importance. It has been shown by Brouwer (1968a) that even in the case of (tertiary) alkylcarbonium ions, which would be much more sensitive to variation of R attached to the electron-deficient centre, the stabilization is practically independent of the structure of the alkyl groups. Another argument is found in the fact that the equilibrium concentrations of isomeric alkyloxocarbonium ions differ by at most a factor of 2-3 from each other (Section III). Therefore, the value of K provides a quantitative measure of the stabilization of an alkyl cation. In the case of R = t-adamantyl this equilibrium constant is 30 times larger than when R = t-butyl or t-pentyl, which means that the non-planar t-adamantyl ion is RT In 30= 2-1 kcal... 

Olah et al. (1966) have recently carried out a detailed U.V. investigation of 10 to 10 M solution of alkyl cations in FSOsH-SbFs solution at — 60° and were able to confirm that they have no U.V. absorption above 210 my. Solutions of t-butyl, t-pentyl and t-heptyl cations in FSOsH-SbFs are stable for hours at —60°, and N.M.R. studies have demonstrated that the ions are quantitatively formed from their corresponding alcohols in these systems (Olah et al., 1965). 

Brouwer and Mackor (1964) found that concentrated and stable solutions of a series of tertiary alkyl cations can be prepared in HF-SbFs and their proton magnetic resonance spectra were recorded. The t-butyl, t-pentyl and t-hexyl cations were observed in this solvent system. The spectra were identical with those obtained previously in SbFs and FSOsH-SbFs solvent systems. 

The most stable of all alkyl cations is the t-butyl cation. Even the relatively stable f-pentyl and f-hexyl cations fragment at higher temperatures to produce the t-butyl cation, as do all other alkyl cations with four or more carbons so far studied.21 Methane,22 ethane, and propane, treated with super acid, also yield r-butyl cations as the main product (see 2-18). Even paraffin wax and polyethylene give f-butyl cation. Solid salts of t-butyl and f-pentyl cations, e.g., Me3C SbF4, have been prepared from super-acid solutions and are stable below -20°CP... 

Thermodynamic Information indicates that 2-butenes would be the predominant olefins released. The resulting 2-butenes presumably react in this process with a t-butyl cation to produce a trimethyl pentyl ion. Hydride transfer from isobutane or more likely an acid-soluble hydrocarbon would result In the production of a trimethylpentane. 

When propylene is in the feed, it will react with t-C4Hg to form dimethyl pentyl cations this reaction is very similar to Reaction G shown in Table II. In addition, propylene reacts with sulfuric acid to form sec-propyl sulfates similar to Reaction 1-1. Goldsby (21,22) has described the production of sec-propyl sulfates In some cases, d1-sec-propyl sulfate Is formed. These propyl sulfates are more thermally stable than butyl sulfates. 

The resulting propylene will then react primarily with t-butyl cations and heavier cations forming dimethyl pentyl and still heavier cations. 

Propyl, butyl, and pentyl fluorides with antimony pentafluoride gave the isopropyl, t-butyl and t-amyl cations (as their fluoroantimonate salts) 2,1 and 3. 

The protolysis occurs following the direct protonation of the different conditions, protolysis of a C—H bond occurs yielding rearranged t-pentyl ion (t-amyl cation, equation 31). 

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