Carbonium ions formation

The general features of the cracking mechanism involve carbonium ion formation by a reaction of the type... 

The mechanism of the Schiemann reaction is not known with certainty. Two schemes, which have been proposed, are given below. One involves carbonium ion formation ... 

Trivalent carbenium ions are the key intermediates in electrophilic reactions of Tt-donor unsaturated hydrocarbons. At the same time, pen-tacoordinated carbonium ions are the key to electrophilic reactions of cr-donor saturated hydrocarbons through the ability of C-H or C-C single bonds to participate in carbonium ion formation. 

Equilibrium constants for carbonium ion formation from triphenylcarbinols in aq H2SO4 25 -3.64... 

The catalysts generally used in catalytic reforming are dual functional to provide two types of catalytic sites, hydrogenation-dehydrogenation sites and acid sites. The former sites are provided by platinum, which is the best known hydrogenation-dehydrogenation catalyst and the latter (acid sites) promote carbonium ion formation and are provided by an alumina carrier. The two types of sites are necessary for aromatization and isomerization reactions. 

Acid-treated clays were the first catalysts used in catalytic cracking processes, but have been replaced by synthetic amorphous silica-alumina, which is more active and stable.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. 

TTie solvolysis of propargylic substrates (199) and formation of alkynylcarbonium ions (200) has been extensively investigated. Particularly good evidence for the formation of alkynylcarbonium ions comes from the nuclear magnetic resonance spectra of alkynyl alcohols in strong acid media (200, 201). The downfield shifts of 4ppm for the proton of HC=C— and 1 ppm for CH3C=C- relative to their neutral precursors is indicative of carbonium-ion formation and shows the importance of the allenyl resonance contribution. 

From the above discussion it follows that the probability of carbonium ion formation during decomposition of RTIX2 compounds by a Type 5 process is low when X is carboxylate, but significantly higher when X is nitrate, sulfate, perchlorate, or fluoroborate. The important role played by the anion of the metal salt in oxymetallation has in fact been recognized only very recently for both oxymercuration 11, 12) and oxythallation (92). The... 

The relative rates of oxidation of phenylmethanes cover too small a range to be compatible with carbonium ion formation cf. the discussion on chromic acid oxidation of diphenylmethane, p. 295), and an initial reaction to give a radical plus Cr(V) followed by rapid transfer of a second electron to form Cr(IV) is more... 

The factor of seven variation between k2 for the ordinary and benzylic tertiary hydrogen is too small to be associated with carbonium ion formation. The observed degrees of retention and 0 transfer imply a caged radical rapidly reducing Mn(VI) to Mn(V) by accepting oxygen... 

Hydrolysis of optically pure bromide 100 a in dioxane/water gives the optically pure alcohol. This is consistent with the transannular p-xylylene ring participating in carbonium ion formation only through tz-g charge delocalization (iz-c resonance) of the type 104 rather than by direct participation in replacement of bromide via a transannularly bridged ion such as 102a. In the latter case, racemization would be expected to take place ... 

Many hydroxy compounds would not survive such harsh treatment therefore other methods must be used. Some alcohols were hydrogenolyzed with chloroalanes generated in situ from lithium aluminum hydride and aluminum chloride, but the reaction gave alkenes as by-products [605], Tertiary alcohols were converted to hydrocarbon on treatment at room temperature with triethyl- or triphenylsilane and trifluoroacetic acid in methylene chloride (yields 41-92%). Rearrangements due to carbonium ion formation occur [343]. 

Substitution at immediate vicinity of bay region t if Facilitate metabolic activation and carbonium ion formation... 

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. 

An increase in the electronegativity of R is expected to result in a decreased rate of carbonium ion formation, a relationship shown to hold for substituted 1 l-diaryl-ethanes (84),... 

Effects of structure on reactivity have been studied several times. The sulphides are more stable than the thiols [248,250], In both series of thiols and of sulphides, the reactivity increases with the inductive effect of the alkyl group [248,251,252], in accordance with other elimination reactions. A linear relation between the logarithm of the rate coefficient and the enthalpy change on carbonium ion formation from the corresponding alkanes has been observed [248]. As Fig. 9 shows, linear correlation of the same rate data by means of the Taft equation is also possible. 

There is a correlation between the chemical reactivity of diol epoxides and relative carcinogenicity. The index of chemical reactivity has been taken as the ease of carbonium ion formation via spontaneous opening of the epoxide to the Zwitterion (Table XI). 

Reversible, random carbonium ion formation is not required to explain the rearrangements of both 2 and 37 to adamantane in the highly acidic media. Sequential 1,2 alkyl shifts coupled with the well documented 5316,2- and 3,2-hydride shifts of the norbomyl system permit a rearrangement pathway analogous to that discussed earlier as the most likely route for the Lewis acid catalyzed rearrangement of 2 to adamantane. 

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). 

Fig. 23 The location of transition states from values of p x. The limiting value of px x for carbonium ion formations is - 2. We assume that px x = 0 for r = 1. Hence from equations (121) and (124) we can locate the transition states in the shaded area...

Whenever an acid is brought into the vicinity of a compound which can furnish a pair of electrons, carbonium ion formation is possible. 

The results obtained are shown in Table III (see p. 26). The mutarotation of the product indicated, in each case, that the /3-D-modification predominated for the acetyl sugars formed from reaction with water. The data show that the reaction was only slightly affected by change of solvent or of temperature, and was free of orthoester formation. These facts, and the almost exclusive formation of 1,2-trans products from the 1,2-cis-bromide, was interpreted as evidence that the reaction proceeds by way of the Sw2 mechanism. It was suggested that the increase of O-acetyl-a-D-glucoside formed at 50° (over that at 20°) may be due to racemization through carbonium-ion formation. 

---