Cations isopropyl

Besides the tert-butyl cation, we also succeeded in preparing and studying the related isopropyl and the terf-amyl cations. The isopropyl cation was of particular relevance. 

Figure 6.1, H NMR spectra of (a) the tert-butyl cation [trimethylcarbenium ion, (CH jj-jC ] [b) the tert-amyl cation [dimethyletbylcarbenium ion, (CH3)3C -C2H 5] and (c) the isopropyl cation [dimethylcarbenium ion, (CH3)2C Hj (60 MHz, in SbF5 S02CIF solution, —60°C).

Carbocations are classified according to their degree of substitution at the positively charged carbon The positive charge is on a primary carbon m CH3CH2" a secondary car bon m (CH3)2CH" and a tertiary carbon m (CH3)3C Ethyl cahon is a primary carbocation isopropyl cation a secondary carbocation and tert butyl cation a tertiary carbocation... 

Methyl cation Ethyl cation (primary) Isopropyl cation (secondary) tert Butyl cation (tertiary)... 

A rule of thumb is that a C=C substituent stabilizes a carbocation about as well as two methyl groups Al though allyl cation (H2C=CHCH2 ) is a primary carbocation it is about as stable as a typical secondary carbocation such as isopropyl cation (CH3)2CH-"... 

The tertiary-secondary 1,2-H shift O itlO is not rate-determining in the interconversion of 5 and 6, but may become so in a conformationally fixed system. It has been found for the interconversion of tertiary and secondary adamantyloxocarbonium ions that <10" sec at 70°C (Hogeveen and Roobeek, 1971a) as compared with k= 1-5 x 10 sec at 20°C for the reaction 5 6. The absence of interconversion between tertiary and secondary adamantyloxocarbonium ions is due to the circumstance that 1,2-H shifts do not occur in the tertiary adamantyl ion as a result of the effect of orbital orientation (Brouwer and Hogeveen, 1970 Schleyer etal., 1970). That the secondary adamantyloxocarbonium ion can lose CO is demonstrated by the reaction with isopropyl cation in SbFs—SO2CIF solution at 0°C with formation... 

Sigma nonbonded attraction also obtains in the isopropyl cation and, arguing as before, we conclude that sigma nonbonded attractive interactions are maximized in the Cee conformation. The 2-propyl cation constitutes a good example where pi and sigma nonbonded interactions reinforce each other. 

More recent work93,127-129 has shown that /1-hydrogen transfer takes place only if the cation corresponding to the N—R substituent is stable. Thus, CH2=N+(CH3)CH(CD3)2 and related species expel C3HD5, via specific D-transfer128 129, because isopropyl cation is stable with respect to a 1,2-H shift. In contrast, if the associated cation readily rearranges to a more stable isomer, there is actually a discrimination against... 

Isopropyl alcohol, photoxidation, 41 238-239 Isopropyl cation, 42 154, 155 chemical shift tensors, 42 124 geometries, 42 135, 136 Isopropylcyclopentane, aromatization, 30 54 Isopropylcyclopropane, oxidation, 41 324 Isoteric enthalpy of adsorption, 38 154, 156 Isothermal heat of adsorption, 38 154, 158 Isotopes... 

From the values in Table 2, it can be seen that the isopropyl cation... 

Some systems show NMR coalescence consistent with a rearrangement to an intermediate cation that is less stable. The NMR of the isopropyl cation 9, for example, shows a coalescence of the two proton signals between 0 and 40 °C. The activation barrier is 16 kcal/mol, close to the difference between a secondary and a primary cation. Calculations show that a comer protonated cyclopropane (11) is the only other minimum on the C3H energy surface. " The 1-propyl cation 10 is a transition state. 

The energies for transferring a proton from isopropyl cation to cyclopropane and cyclobutane are ... 

Fig. 3. Chemical shift tensor orientations for the isopropyl cation, propionylium ion, benzoylium ion, and benzenium ion obtained from MP2 chemical shift calculations.

In chemical shift calculations for acylium ions, it was not necessary to model the ionic lattice to obtain accurate values. These ions have tetravalent carbons with no formally empty orbitals, as verified by natural bond orbital calculations (89). Shift calculations for simple carbenium ions with formally empty orbitals may require treatment of the medium. We prepared the isopropyl cation by the adsorption of 2-bromopropane-2-13C onto frozen SbF5 at 223 K and obtained a 13C CP/MAS spectrum at 83 K (53). Analysis of the spinning sidebands yielded experimental values of = 497 ppm, 822 = 385 ppm, and (%3 = 77 ppm. The isotropic 13C shift, 320 ppm, is within 1 ppm of the value in magic acid solution (17). Other NMR evidence includes dipolar dephasing experiments and observation at higher temperature of a scalar doublet ( c-h = 165 Hz) for the cation center. 

Fig. 8. (a) MP2/6-311 + + G optimized geometry of the isopropyl cation with selected... 

The allyl cation has never been characterized as a persistent species in solution. If prepared, it would be the smallest carbenium ion universally accepted to have been formed in condensed media (this title has for many years been held by the isopropyl cation). No allyl cation derivatives, e.g., 10, were observed in a 1990 report of a CAVERN study of butadiene on HY and HZSM-5 (104). The same year, Hutchings reported a flow reactor study of allyl alcohol on HZSM-5 (105) and Gorte (106) reported a TPD study of allyl alcohol on the same zeolite. Hutchings and co-workers found that allyl alcohol had two reaction paths, one in which propanal was formed and another that formed hydrocarbons. The latter route was proposed to proceed through an allyl cation intermediate, but no claim for its persistence or spectroscopic observation was made or implied by Hutchings. Gorte... 

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