Propyl carbenium ion

The secondary propyl carbenium ion formed in the reaction of propylene (3.4%) at 40 in 10 1 HF-TaFs, attacks methane (96.6%) to form isobutane (Scheme 5) with 60% selectivity. 

The rate-limiting step in this process is the propene formation via P-H shift (7.15). Moreover, as Ga203 and H" are on the catalyst surface, the propyl carbenium ion on Ga203 might readily exchange with a proton via surface migration reaction (7.18) [90] ... 

Other typical reactions of carbenium ions are alkene loss, provided sufficient chain length is available (Chap. 6.6.1), and dehydrogenation in case of the smaller ions such as ethyl, propyl, or butyl ion (Chap. 6.2.4.). 

Rapidly equilibrating (degenerate) carbenium ions are characterized by averaged carbon-13 shifts and carbon-proton coupling constants. 2,3-Dimethylbutyl in comparison with 2-propyl cation is an example [497]. Only two instead of four signals are observed at — 80 °C due to a rapid 1,2-hydride shift. 

Moreau et al.56 obtained unexpected results in the alkylation of naphtalene with 2-propanol over H-Beta in the liquid phase at 200°C. Here a cyclic compound 1 was formed with a selectivity around 40% at 28.5% conversion. When applying HY as the catalyst alkylation to di- and trialkylnaphthalenes was faster but the cyclic compound was not observed. These results illustrate the more confined space within the zeolites Beta channels. The cyclic compound is assumed to be formed through iso-propylation of naphthalene followed by a hydride abstraction giving a carbenium ion, reaction with a propylene and finally ring-closure. 

For carbenium ions with none or just one alkyl substituent attached to C+, internal stabilization is lowered and strong coordination of solvent molecules can take place. For example, when CH3F is dissolved in SbFs/S02, the solvent is methylated so that instead of CH3+ the complex H3COSO+ is formed. [104,105] This occurs also for ethyl and iso-propyl fluoride while use of SO2F2 instead of SO2 prevents dissociation of CH3F or other monoalkyl fluorides. [105] Obviously, there seems to be a competition between solvent and anion to coordinate to the carbocation. 

Protonation of cyclopropanes leads to propyl cations and their respective products. Bond opening occurs to produce the most stable (that is the most highly alkyl substituted) carbenium ion. Depending on its specific constitution, the latter can react in different ways. 

Special methods have been developed for such ionic compounds as sulfobutyl ethers and cationic starches.To identify the substituted position from typical shifts in mass spectra, it is important to avoid coincidence of the mass contribution of functional groups as, for example, with propyl and acetyl. For instance, glucan allyl ethers cannot be hydrolyzed directly, since the 2-O-allyl group forms an intramolecular 6-membered cyclic adduct with the carbenium ion formed during hydrolysis. 

As we discussed in Section 4.2.1, monomolecular n-butene to isobutene isomerization requires a relatively high activation energy because isomerization via the propyl cationic intermediate generates a primary carbenium ion in the transition state. 

The carbenium ion intermediate then eliminates the alkene by charge-induced cleavage of a C-C bond. The striking argument for a carbenium ion intermediate is presented by the influence of the y-substituent R on the competition of onium reaction and McL. If R = H, i.e., for propyl-substituted iminum ions, the products of both reactions exhibit similar abundance. If R = Me or larger or if even two alkyls are present, McL becomes extremely dominant, because then its intermediate is a secondary or tertiary carbenium ion, respectively, in contrast to a primary carbenium ion intermediate in case of R = H. The importance of relative carbenium ion stability for onium ion fragmentations (Chap. 6.11.2) will become more apparent when dealing with the mechanism of the onium reaction. 

Reaction scheme (bs basic site of the catalyst, R carbenium ion ethyl, R carbenium ion propyl ). Adapted from Alwahabi SM, Froment GF. Conceptual reactor design for the methanol-to-olefrns process on SAPO-34. Ind Eng Chem Res 2004 43 5112-22 Alwahabi SM, Froment GF. Single event kinetic modelingof the methanol-to-olefins process on SAPO-34. Ind Eng Chem Res 2004 43 5098- 111. 

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