Primary Alkyl Cations

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 h 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 SbFs— FHSOg (Olah and Schlosberg, 1968 Olah et al., 1969) and by SbFs (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 C2H5 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  

From the product distribution at various CO—C2He ratios it was concluded that the dimerization step (21) has a rate constant of the same order of magnitude as that of the carbonylation step (20). 

Although a value for the rate constant of earbonylation 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 earbonylation 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 earbonylation of primary alkyl cations—and even more so for the methyl cation— will exceed that of secondary al l cations by more than a factor of 10 , so that h 10 litre mole sec, which means that the reaction is diffusion controlled. 

It should be emphasized that, in contrast to the cases of secondary al l cations (Olah et al., 1964 Saunders et al., 1968) and tertiary alkyl cations (Olah et al., 1964 Brouwer and Mackor, 1964), the evidence for the existence of primary alkyl cations as distinct species has been only indirect, because they have escaped direct spectroscopic observation so far. 

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Kinetic data on the carbonylation of vinyl cations have not been obtained so far, but it is likely to be a diffusion-controlled reaction as in the case of primary alkyl cations (Section IV, A). 

R H) is much faster than alkylation, so that alkylation products are also derived from the new alkanes and carbocations formed in the exchange reaction. Furthermore, the carbo-cations present are subject to rearrangement (Chapter 18), giving rise to new carbocations. Products result from all the hydrocarbons and carbocations present in the system. As expected from their relative stabilities, secondary alkyl cations alkylate alkanes more Teadily than tertiary alkyl cations (the r-butyl cation does not alkylate methane or ethane). Stable primary alkyl cations are not available, but alkylation has been achieved with complexes formed between CH3F or C2H5F and SbFs-212 The mechanism of alkylation can be formulated (similar to that shown in hydrogen exchange with super acids, 2-1) as... 

Saunders and Rosenfeld (1969) extended their H-nmr investigation to temperatures above 100°C and discovered another, slower process which exchanges the two methylene protons with the nine methyl protons, resulting in coalescence of these bands above 130°C. The band shape analysis gave an activation energy of 18.8 1 kcal mol" for this new process. Since any mechanism involving primary alkyl cations is expected to have a barrier of ca 30 kcal mol" (the enthalpy difference between tertiary and primary carbo-cations), the formation of a methyl-bridged (corner protonated cyclopropane)... 

CH3)2CH+> H2C==CH—CH+ CH3CH+ > H2C=CH > Ph > CHj. The stabilities of various carbocations can be determined by reference to the order of stability for alkyl carbocations, 3 > 2° > 1 > CH3. The acetyl cation has a stability similar to that of the r-butyl cation. Secondary carbocations, primary benzylic cations, and primary allylic cations are all more stable than primary alkyl cations. Vinyl, phenyl, and methyl carbocations are less stable than primary alkyl cations. 

Primary alkyl cation (1°) CH3CH2 Stabilized by one alkyl group... 

The green hydrogen migrates to allow a secondary rather than a primary alkyl cation to be formed, and rso-propylbenzene results. This leaves us with a problem how can you add primary alkyl groups to benzene rings ... 

Primary alkyl chlorides do not easily undergo S l reactions because primary alkyl cations are unstable and because primary alkyl derivatives easily undergo reactions. Neopentyl chloride is exceptional in that the reaction is sterically hindered. Moreover, Ag promotes S l reactions of chlorides by effectively solvating the resulting chloride ion, AgCl having a very negative heat of formation. 

A vinylic cation is less stable than a similarly substituted alkyl cation because a vinylic cation has a positive charge on an sp carbon. An sp carbon is more electronegative than the sp carbon of an alkyl cation and is, therefore, less able to bear a positive charge (Section 2.6). By similarly substituted, we mean that a primary vinylic cation is less stable than a primary alkyl cation, and a secondary vinylic cation is less stable than a secondary alkyl cation. 

Nitrous acid reacts with primary aliphatic amines in the presence of mineral acids with the formation of volatQe nitrosamines, which isomerise to stable diazohydroxides. Diazohydroxides are split into nitrogen and a primary alkyl cation, which may partly isomerise to a secondary cation and both cations are eliminated, which yields the corresponding alkene. Substitution reactions then yield primary and secondary alcohols and symmetrical and unsymmetrical secondary amines. I n the presence of hydrohalogen acids, both alkyl... 

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