Hexyl ions

The simplest primary alkyl cations, CHJ and C2H, are formed from methane and ethane, respectively, by SbPs—PHSO3 (Olah and Schlosberg, 1968 Olah et al., 1969) and by SbPs (Lukas and Kramer, 1971). In these cases, intermolecular electrophilic substitution of these ions at the precursor alkanes leads to oligocondensation products, e.g. tertiary butyl and hexyl ions. In the presence of carbon monoxide it has been found possible to intercept the intermediate CHJ and C2H quantitatively as oxocarbonium ions (Hogeveen et al., 1969 Hogeveen and Roobeek, 1972). The competition between the reactions of the ethyl cation with ethane and carbon monoxide, respectively, is illustrated by the following equations ... 

The behaviour of alkyl cations in SbFj solutions was used to establish the basic information needed to interpret the exchange studies with the norbornyl ion. The studies were conducted with butyl, amyl and hexyl ions but only the results with the C4 system are discussed here. 

Pentane and isopentane are ionized under the same conditions to the ferf-amyl cation. w-Hexane 6 and the branched C6 isomers ionize in the same way to yield a mixture of the three tertiary hexyl ions [Eq. (5.3)] as shown by their 1H NMR spectra. 

Rzad and Schuler" studied the radiation chemistry of a solution of " C-cyclopropane in hexane over the concentration range 10 " to 10 M. The main radioactive products, which appear to result from ion molecule reactions, are propane formed by H2 transfer (50 %) and by H transfer (20 %) and mixed nonanes (30 %) formed by the addition of CaHg unit to a hexyl ion. At the lower concentrations, very pronounced dose dependence of the yields was observed. This was ascribed to a competitive formation of olefins in the radiolysis. For cyclopropane-cyclohexane solutions the chemical processes seem to be considerably more complicated. The observed yield of total radioactive products extrapolated to zero concentration of cyclopropane are 0.05 and 0.11 G units for hexane and cyclohexane, respectively. These limiting yields are of the order of magnitude of and appear to be related to, the free ion yields in these systems. Since cyclopropane was found to react with hydrocarbon ions" it is used quite often as a scavenger for positive ions, as in the work of Davids and coworkers . 

The carbonium ion abstracts a tertiary hydrogen from the isoparaffin, yielding a hexyl ion which then abstracts a hydride ion from the unrearranged isoparaffin, saturates itself, and starts a new cycle. Carbonium ions are formed when unsaturated compounds are dissolved in proton acids or, in general, if the compound is sufficiently basic relative to the acid, i.e., alcohols, ethers, esters, acid anhydrides, alkyl-substituted aromatic compounds will form carbonium ions in sulfuric, hydrofluoric, and other acid solvents. 

For 2,3- and 1,2-dimethylnorbomyl ions the values of A5 are 287 and 274 ppm, respectively, lower than for classical 1,2-dimethylcyclopentyl and 1,2-dimethylcyclo-hexyl ions ( 350 ppm) being in a rapid degenerate equilibrium. These facts agree with the stmcture earlier ascribed by Olah to the l,2-dimethyl-2-norbomyl ion — a structure of partially c-delocalized, rapidly equilibrated ions (Wagner-Meerwein rearrangement) The same applies to Saunders s data on the 1,2-dimethylnor-bornyl cation in which one CHj is replaced by CDj (see below)... 

It is interesting to compare the 2-norbornyl cation with its lower homologue — the 2-bicyclo[2,l,l]hexyl cation. Calculations with the MINDO/3 method show that the steric strain in the bicyclo[2,l,l]hexyl ion makes it a particularly favourable model for exhibitingCT-participation... 

The difference between the sum of the chemical shifts of all the atoms of the 2-bicyclo[2,l,l]hexyl ion and the corresponding hydrocarbon is 297 ppm, i.e. by 100 ppm less than in the case of the 2-methyl-2-bicyclo[2,l,l]hexyl cation this points to a considerable nonclassical delocalization in the secondary ion... 

According to Sorensen, the bicyclo[2,1, l]hexyl ion is a nonsymmetrical nonclassical ion ... 

The partial cr-bridging seems to be a general phenomenon in organic chemistry and is suggested by quantum chemical calculations Olah et al. tried to record the a-participation in stable bicyclo[2,l,l]hexyl ions by increasing the electron demand of the cation rentre. When they studied the NMR... 

Without hydrogen shift the corresponding values for reactions 48 and 49 are identical (140kJmol ). Methylalkanes have been discussed by Holmes and coworkers " concerning the loss of Me. The authors conclude that in the metastable Me loss from 2-methylpentane, a mixture of s- and t-pentyl ions are formed, in contrast to 2-methylhexane, where only s-hexyl ions appear to be formed, because the kinetic energy release is small and independent of the observation time. 

Scheme 2.2.9 Reactivity of carbocations exemplified for a hexyl ion carding the cationic charge at carbon number 3 (C3).

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