Carbonium ion formation, from

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

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. 

Jerina DM. Lehr RE. Yagi H. et al. 1976. Mutagenicity of B[a]P derivatives and the description of a quantum mechanical model which predicts the ease of carbonium ion formation from diol epoxides. In de Serres FJ. Fouts JR. Bend JR. et al. eds. In vitro metabolic activation in mutagenesis testing. Amsterdam. The Netherlands Elsevier/North Holland. 159-178. 

The question of carbonium ion formation from saturated hydrocarbons was considered in (,1) by the writer when the possibility of participation by olefins from thermal cracking was mentioned. However, it was only somewhat later that this suggestion was seriously adopted ( ). Then it was postulated that even traces of unsaturated hydrocarbons can activate saturated hydrocarbons by first forming a carbonium ion by proton addition. This ion can then extract a hydride ion by hydrogen transfer from the paraffin or cycloparaffin. This initiates a sort of chain reaction in which new carbonium ions are formed by hydrogen transfer with a steady-state population of ions on the catalyst surface. 

D. M. Jerina, P. E. Lehr, H. Yagi, O. Hernandez, P. Dansette, P. G. Wislocki, G. Wislocki, A. W. Wood, R. I. Chang, W. Levin, and A. H. Conney, Mutagenicity of benzo(a)-pyrene derivatives and the description of a quantum mechanical model which predicts the phase of carbonium ion formation from diol epoxides, in In-Vitro Activation in Mutagenesis Testing, F. J. De-Seres, J. R. Bond, and R. M. Philpot (Eds.), Elsevier, Amsterdam, 1976. 

Methane is the simplest hydrocarbon, and basic features of solid acid catalysis can be elucidated by considering carbonium ion formation from CH4. At present, reliable quantum-chemical calculations are only feasible by using small clusters. 

All the reactions discussed so far have not involved spatial reorientation of hydrogen atoms during the reaction, and so the isotope effects on them have been of the second kind. The only effect of the first kind observed to date in a similar equilibrium has been the effect on carbonium ion formation from benzhydrols in aqueous sulfuric acid. 

More detailed are the results of Shiner and Cross (154) on the hydrolysis of diethyl ketals, which involves slow carbonium ion formation from the conjugate acid of the substrate ... 

A final example of a reaction of this type is the 11% reduction in the rate of racemization of S-phenyl-/3-hydroxypropionic-adi acid in 40-47% H2SO4 at 45°C., observed by Noyce and Lane (155). They present evidence that racemization proceeds via carbonium ion formation from the conjugate acid ... 

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... 

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]. 

The relative rate data closely parallel the results obtained in the solvolysis studies. Such a result might be expected from reactions proceeding through similar transition states. The observed order of relative rates may result from better overlap as the size of the central metal atom and the polarizability of its electron shell increase. This would result in increased stabilization and therefore ease of formation of the carbonium ions, proceeding from lighter to heavier metal complexes. 

The above transformations and reactions can be used to explain the formation, as an example, of different heptane isomers when alkylating isobutane with propylene. As the first step in the mechanism, a tertiary butyl carbonium ion derived from isobutane reacts with a propylene molecule to form a carbonium ion of seven carbon atoms as outlined in reaction 2. This ion may then react directly with a molecule of isobutane as in reaction 6 to form the expected heptane molecule and to convert the isobutane molecule to a tertiary butyl carbonium ion. This seven-carbon carbonium ion, however, may undergo isomerization by the mechanism outlined in reaction 3 or 4 before reacting with isobutane to form an isomeric heptane molecule. 

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. 

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...

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. 

Equation 26. Electron-deficient flavins will also oxidize nitroalkane anions in model reactions (12). The observation (11) that nitromethane anion and FloXEt yield a stable 4a-adduct is evidence that 4a-adducts are not on the reaction path for nitroalkane oxidation. That the blocking of the N(5)-position of flavin (i.e., FloxEt) prevents oxidation of nitromethane would, however, be in accord with the requirement for an N(5)-adduct (11). The nitroalkane reaction with flavoenzyme has been used to implicate N(5)-adducts as intermediates in the oxidation mechanism of amino acid oxidases. However, it must be understood that nitroalkane anions differ significantly from the carbanions generated from a normal substrate. The nitroalkane anion on loss of its pair of electrons would provide an impossibly unstable carbonium ion, whereas in the case of the amino acid anion an internal electron release obviates carbonium ion formation. 

In the NMR experiments carried out by Wenthe and Cordes [187] with methyl orthobenzoate and methyl orthocarbonate in CD3OD—D20 solutions, the rate coefficients for the disappearance of orthoester and those for the formation of CH3OD and of carboxylic ester have been found identical within experimental error (Table 15). This indicates that there is no exchange of methoxy groups prior to hydrolysis. The same result has been obtained from product analysis studies of the carboxylic esters formed. Consequently, the rate-determining step must be carbonium ion formation or a previous step. The findings do not support an A2 mechanism, for the following reason. As the nucleophilic reactivities of water and methanol are similar, the A2 reaction with attack of water... 

In F, benzyl alcohol is the hydrogen-rich product from hydride transfer to benzaldehyde in the Cannizzaro reaction the hydrogen-deficient species is benzoic acid. Hydride transfer to the carbonium ion derived from benzyl alcohol also results in the formation of toluene, an even more reduced species. As shov n in Equations 11 and 12, these Cannizzaro-like transformations are essentially redox type, at least in respect to hydrogen balance. 

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