Hogeveen

Charton s at values for SOPh and S02Ph are based on the pKa values for phenylsulfinyl- and phenylsulfonyl-acetic acids137, which we have already discussed in connection with the transmission of electronic effects by SO and S02 (Section III.C). These groups have also been the subject of a detailed study by Hogeveen and Montanari172, who measured the pKa values of some 3-phenylthio-, 3-phenylsulfinyl- and... 

In previous studies (Hogeveen, 1970) the reactivity of long-lived carbonium ions towards molecular hydrogen has been investigated and interesting differences between secondary and tertiary alkyl cations have been observed. Tliis reaction is too slow, however, to be extended to other types of carbonium ions. The reactivity of carbonium ions towards carbon monoxide is much higher (about six powers of ten) than towards molecular hydrogen, which enabled us to determine not only the rate of reaction (3) for some tertiary and secondary alkyl cations, but also the rate of carbonylation of more stabilized carbonium ions. 

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). 

An example of thermodynamic control of oxocarbonium ion formation has been found (Hogeveen and Roobeek, 1970) in the dehydration of the pentylcarboxylic acids 1, 2, 3 or 4 in FHSO3—SbFg at 40-60°. 

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... 

The isomerization of 5 to 7 and 8 involves a chain-branching type rearrangement (lOis ll) (Brouwer and Oelderik 1968) and has a free-enthalpy of activation of about 22 kcal mole . This result, combined with the data of Fig, 2, the free-enthalpy of activation of 17 kcal mole for the rearrangement 9-i ll (Brouwer and Hogeveen, 1972), and an estimated difference in free-enthalpy of about 0-8 kcal mole between 10 and 11 constitutes the basis for the free-enthalpy diagram in Fig. 3. 

From Fig. 4 it is seen that the free-enthalpy of activation for the rearrangement of tertiary butyl to secondary butyl cation is 30-4 — 3.9 = 26-5 kcal mole . As the reverse rearrangement has been found by direct observation to have JG cl7-18 kcal rnole" (Saunders et al., 1968), it follows that the difference in stabilization between tertiary and secondary butyl cations is indeed 9 + 1 kcal mole . This value is in excellent agreement with a previous experimental value of 10 + 1 kcal mole (Brouwer and Hogeveen, 1972). 

The difference in behaviour between pentyl and butyl cation systems (Figs. 3 and 4) has also been encountered in trapping experiments with carbonium ions, primarily formed from alkanes and SbFs, by CO (Hogeveen and Roobeek, 1972). In the case of n-butane the secondary butyloxocarbonium ion is the main product, whereas in the case of n-pentane only the tertiary pentyloxocarbonium ion is found. 

Although carbonylation of the 2-norbomyl ion at or below room temperature leads to exclusive formation of the 2-ea o-norbomyloxo-carbonium ion, reactions at higher temperatures have shown that the 2-cwdo-norbornyloxocarbonium ion is just as stable as the exo-isomer (Hogeveen and Roobeek, 1969). This means that at low temperatures the carbonylation is kineticaUy controlled, and at high temperatures thermodynatnically controlled. The detailed free-enthalpy diagram in... 

Fig. 5 for the equilibrium (19) is based on the rates of interconversion of the 2-exo- and 2-ewdo-norbomyloxocarbonium ions, the above-mentioned value of K for the 2-exo-norbornyloxocarbonium ion, and the limits of the rates of formation and decarbonylation of the latter ion (Hogeveen and Roobeek, 1969). 

In Sections II and III it was shown that secondary and tertiary alkyl cations can be formed by decarbonylation of the corresponding oxo-carbonium ions. This has been found impossible in the case of primary alkyl cations (Hogeveen and Roobeek, 1970) the oxocarbonium ions 13 and 14 were unchanged after one hour at 100°C k < 1-3 x 10 sec ), whereas ion 15 is fragmented by a j3-fission under these circumstances ... 

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 ... 

Although a value for the rate constant of carbonylation of primary alkyl cations has so far not been obtained experimentally, it can be shown that the reaction must be diffusion-controlled. The difference in stabilization between secondary and tertiary alkyl cations in solution (9 + 1 kcal mole Section III, C) shows up as a difference in rate of carbonylation of a factor of 10 (Section III, A). As the difference in stabilization between primary and secondary alkyl cations is certainly larger than that between secondary and tertiary alkyl cations (various estimates have been summarized by Brouwer and Hogeveen, 1972), the rate constant of carbonylation of primary alkyl cations— and even more so for the methyl cation—will exceed that of secondary alkyl cations by more than a factor of 10, so that k> 10 litre mole sec, which means that the reaction is diffusion controlled. 

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