Carbonium cation formation

Acid-treated clays were the first catalysts used in catalytic cracking processes, but have been replaced by synthetic amorphous silica-alumina, which is more active and stable. Incorporating zeolites (crystalline alumina-silica) with the silica/alumina catalyst improves selectivity towards aromatics. These catalysts have both Fewis and Bronsted acid sites that promote carbonium ion formation. An important structural feature of zeolites is the presence of holes in the crystal lattice, which are formed by the silica-alumina tetrahedra. Each tetrahedron is made of four oxygen anions with either an aluminum or a silicon cation in the center. Each oxygen anion with a -2 oxidation state is shared between either two silicon, two aluminum, or an aluminum and a silicon cation. 

Novak et al. <1998JA1643> devised a novel approach to amino-substituted tctrazolo[ 1,5-tf Jpyridine which provides a really unique pathway (Scheme 34). These authors studied the possibility of formation of nitrenium ions from the pivaloylhydroxylamine 143 and found that if azide anion is present in the main reaction route is the formation of tetrazolo[l,5-tf]pyridine 146. The authors concluded that the first intermediate is the formation of the carbonium cation 144 which captures the azide anion to yield 2-azidopyridine 145, that is, the valence bond isomer of the product 146. 

Dowden (27) considers the active centers for carbonium ion formation to be associated with surface cation vacancies. A proton, derived from water contained in the catalyst, is attracted to the anions surrounding the vacancy. A hydrocarbon molecule is assumed to be held by polarization forces above this lattice defect and the proton will be distributed between the hydrocarbon and the anions, forming a carbonium ion of a definite lifetime. 

O atoms, the formation of a carbonium cation due to cleavage of the C—O bond, and the stabilization of the product by heterolysis of a water molecule—there is still some discussion about the structure of the carbonium cation. This may be either cyclic or acyclic, depending on the primary site of protonation. Possibly, this discussion can be settled by assuming a partial protonation of both oxygen atoms caused by the structured nature of water (2), as shown in Figure 2. 

Corma and co-workers152 have performed a detailed theoretical study (B3PW91/6-31G level) of the mechanism of the reactions between carbenium ions and alkanes (ethyl cation with ethane and propane and isopropyl cation with ethane, propane, and isopentane) including complete geometry optimization and characterization of the reactants, products, reaction intermediates, and transition states involved. Reaction enthalpies and activation energies for the various elemental steps and the equilibrium constants and reaction rate constants were also calculated. It was concluded that the interaction of a carbenium ion and an alkane always results in the formation of a carbonium cation, which is the intermediate not only in alkylation but also in other hydrocarbon transformations (hydride transfer, disproportionation, dehydrogenation). 

The rearrangements of several twistane derivatives to adamantyl cations under the same conditions, on the other hand, appear to involve reversible, random carbonium ion formation, at least to a limited extent. Rearrangement of 2-twistanol-2-d (38) occurs with considerable intermolecular hydrogen scrambling (Eq. (15)) 40T Similar intermolecular rearrangements are observed when a 50 50 mixture of 1-adamantanol and l-adamantanol-3,5,7-d3 in S02 is treated with SbFs 40). 

Tri-0-acetyl-2-amino-2-deoxy-a-D-glucosyl bromide hydrobromide (LXVa)169 170 is more stable than the V-acyl analogs, possibly because heterolysis of the C—Br bond with the generation of a carbonium cation proceeds less readily as a result of the presence of the neighboring—NH3 center. /3-u-Glycoside formation proceeds normally with alcohols,169 and treatment with an equivalent of sodium ethoxide under anhydrous conditions liberates170 the free base (LXVb). 

Pursuing this idea further, it is seen that anhydride formation in the sugars resolves itself into an exchange of anions on a carbonium cation,... 

The rate at which a carbonium ion forms is dependent on its stability. It can be formed fastest if there are e-lectron-releasing groups to stabilize the positive charge. The rate of carbonium ion formation is 3>2>1>methyl. This is also the order of stability of carbonium ions. Since tert-butyl alcohol can form a 3° cation, whereas ethyl alcohol can only form a 1° carbonium ion, tert-... 

The apocamphyl case is an extreme one, and bridgehead carbonium ions in other systems are more accessible. Inclusion of more atoms in the bridge gives a more flexible molecule and allows carbonium ion formation to proceed with a somewhat lower activation energy. Thus, the relative solvolysis rates of the bridgehead bromides 1-bromoadamantane, l-bromobicyclo[2.2.2]octane, and 1-bromobicyclo[2.2.1]heptane in 80% ethanol at 25°C are 1,10", and Under the same conditions, the rate of solvolysis of tert-butyl bromide is 1000 times that of 1-bromoadamantane. The 1-adamantyl cation is sufficiently stable to be generated in antimony pentafluoride in concentrations sufficient for observation by NMR. ... 

Frey et al. used 1,1-dimethylcyclopropane as a probe. With sMMO from M. richosporium OB3b, this substrate is oxygenated to (1-methylcyclopropyl)methanol, 3-methyl-3-buten-l-ol, and 1-methylcyclobutanol, in the relative yields shown in eq. (14) [76]. The formation of the ring-opened product suggests a mechanism involving some intermediate rather than a concerted mechanism. Formation of 1-methylcyclobutanol is probably via not a radical intermediate but a carbonium cation. A carbon radical formed by the H-abstraction is oxidized to the carbonium cation by one-electron transfer to an electron deficient active species, probably a resultant diiron(III,IV) intermediate. The... 

The mechanism exhibited in Fig. 7 is also applicable to the sMMO-catalyzed oxygenation of terminal olefins. The formation of primary alcohol and epoxide from terminal olefin is reasonably explained by applying the reaction mechanism for the poly halogenated ethene as shown in Scheme 5. Formation of a carbonium cation at the tertiary carbon must be predominant, because a carbonium cation at the primary carbon is very unstable. The predominant intermediate gives both primary alcohol and epoxide. The minor intermediate, a carbonium cation at the primary carbon, gives only epoxide. Secondary alcohol having a terminal olefin can not be formed via these intermediates. Therefore, this mechanism explains the formation of the primary alcohol from terminal olefins better than the radical mechanism. 

The structural analysis of the samples PP ADMH = 2 1 obtained at 60 and 80°C in the absence of catalysts shows the formation of a branched structure at temperatures lower than those required for the formation of allophanate (120-140°C) and biuret (100°C) structures [1]. The ability of the ADMH to form hydrazinium cations leads to the assumption that the elastomer synthesis with the participation of ADMH involves formation of reactive centers of ionic nature, similarly to the Ritter reaction, where the synthesis of the N-substituted amides of carboxylic acids passes through a stage of carbonium ion formation... 

---