On carbonium ion stability

Because of their stability and ease of generation, triarylmethyl cations have been the subject of numerous quantitative studies aimed at determining the effects of structure on carbonium ion stability. Most of these studies have utilized ultraviolet spectroscopy as the probe and have taken advantage of the difference in electronic spectra between the carbonium ion and a covalent precursor, usually the corresponding triarylcarbinol. This permits determination of the equilibrium constant for the... 

This serious limitation of the utility of C-nmr has been recognized (Olah et al., 1970), but dismissed by recourse to the results of extended Hiickel M.O. calculations by Hoffmann (1964). These calculations indicated that the charge on the cationic centre of t-butyl is, in fact, more positive than in the isopropyl ion. Hoffmann noted that his results are at odds with the widely held assumption that a methyl group is a better electron donor than hydrogen but showed that the correct order of carbonium ion stability could be predicted even if a methyl group is electron withdrawing with respect to hydrogen. 

The reactivity of the organosilanes65 and organostannanes66 towards electrophiles is dependent on the characteristics of the organic ligands. Typically, the alkylsilanes and alkylstannanes are unreactive, which is a consequence of the weakly polarized carbon-silicon and carbon-tin cr-bonds (C8-—Ms+). However, allylsilanes67 and allylstannanes are highly reactive to electrophiles because of extensive ct-tt (C—Si or C—Sn) conjugation in the ally metals and the 0-carbonium ion stabilization effect of the metal center. Consequently, electrophiles add exclusively with allylic transposition. 

This is the reverse sequence to that found for carbonium ion stabilities sequence (5) is explainable in terms of the electron-releasing properties of the various alkyl groups, the greater the electron release ( + 1 character) of the alkyl group the more destabilised is the negative charge on the carbanion. 

Enzymes catalyze the formation of carbon-carbon bonds between allylic and homoallylic pyrophosphate species by mechanisms that are very different from those for carbonyl compounds. Here, carbonium ions, stabilized as ion pairs and generated from allylic pyrophosphates, are likely to be the intermediates that add to the TT-electron density of carbon-carbon double bonds to form new carbon-carbon single bonds. Reaction patterns are consistent with model systems and the mechanisms are based on analogies with the models, stereochemical information (which is subject to interpretation), and the structural requirements for inhibitors. Detailed kinetic studies, including isotope effects, which provide probes in the aldolase and Claisen enzymes discussed in Section II, have not yet been performed in these systems. The possibility for surprising discoveries remains and further work is needed to confirm the proposed mechanisms and to generalize them. 

A number of different types of kinetic evidence have been used to show that the dehydration reactions proceed via carbonium-ion intermediates in an El process. Simple rate studies show that the order of reactivity decreases along the series tertiary alcohol > secondary > primary, the order of decreasing carbonium-ion stability. Skeletal rearrangements typical of carbonium ions have also been observed " On a number of occasions, the rate of olefin formation has been shown to be a slower process than the exchange of the hydroxyl function with the reaction medium " -. This can conveniently be demonstrated by studying the reaction in water enriched with " OH2 or, in the case of an optically active alcohol, by comparing the rates of racemisation and dehydration - . These observations indicate that exchange occurs before (184), and not simultaneously with, elimination. 

The interrelationships among catalyst activity, carbonium-ion stability, and positional selectivity have been studied in detail, with the use of substituted benzyl halides and a wide variety of Friedel-Crafts catalysts. These data indicate that no single mechanistic description can encompass all Friedel-Crafts alkylations. With very reactive catalysts, there is little selectivity with respect to competing aromatic substrates. In less reactive systems, substrate selectivity increases. Quantitative description of catalyst activity has not been achieved, but a number of Lewis acids have been grouped into four broad categories, based on their activity in catalyzing the benzylation of benzene. Some of the catalysts are listed in Table 7.1. 

One of the most important and general trends in organic chemistry is the increase in carbonium ion stability with additional alkyl substitution at the carbonium ion site. This stability relationship is fundamental to understanding many aspects of reactivity, including nucleophilic substitution. In recent years it has been possible to put the stabilization effect on a quantitative basis. One approach has been gas phase measurements which determine the proton affinity of alkenes resulting in carbocation formation. From these data the hydride affinity of the carbonium ion can be obtained. These data provide a thermodynamic basis for comparison of the... 

The general terms regioselective and regiospecific have been introduced to describe addition reactions that proceed selectively or exclusively in one direction with unsymmetrical alkenes. Markownikoff s rule then describes a general case of regioselectivity that operates because of the stabilizing effect of alkyl and aryl groups on carbonium ion centers. 

The view was advanced in Sec. IIIC, 3(b) that a hyperconjugative isotope effect on the stability of radicals should occur, even though that on the stability of fully formed carbonium ions might be negligible. The reason given was that only the difference in Hs C and D C overlap would affect radical stability, whereas carbonium ion stability is affected in the opposite sense by the isotopic electropositivity difference. 

Hydroxypyrroles. Pyrroles with nitrogen-substituted side chains containing hydroxyl groups are best prepared by the Paal-Knorr cyclization. Pyrroles with hydroxyl groups on carbon side chains can be made by reduction of the appropriate carbonyl compound with hydrides, by Grignard synthesis, or by iasertion of ethylene oxide or formaldehyde. For example, pyrrole plus formaldehyde gives 2-hydroxymethylpyrrole [27472-36-2] (24). The hydroxymethylpyrroles do not act as normal primary alcohols because of resonance stabilization of carbonium ions formed by loss of water. 

This method is suitable only for the preparation of 4-substituted and/or 3,4-disubstituted derivatives, the substituents being only alkyl, aryl or heteroaryl groups. The presence of electron-withdrawing groups in the unsaturated side chain prevents the cyclization step. This is understandable if the influence of such groups on the stability of the intermediate carbonium ion is considered. Of more limited application is the analogous cyclization of diazotized o-aminophenylpropiolic acids, the reaction being referred to as the Richter synthesis (Scheme 70). A related synthesis (also referred to as the Neber-Bossel synthesis)... 

On the basis of the above, the rate acceleration afforded by lysozyme appears to be due to (a) general acid catalysis by Glu (b) distortion of the sugar ring at the D site, which may stabilize the carbonium ion and the transition state) and (c) electrostatic stabilization of the carbonium ion by nearby Asp. The overall for lysozyme is about 0.5/sec, which is quite slow (Table... 

In this paper, we study the stabihty of the carbonium ion intermediate formed in the cleavage of a glycosidic bond by lysozyme. It is found that the electrostatic stabilization is an important factor in increasing the rate of the reaction step that leads to the formation of the carbonium ion intermediate. Steric factors, such as the strain of the substrate on binding to lysozyme, do not seem to contribute significantly. 

The acyl residue controls the formation and stability of the carbonium ion. If the carbonium ion is destabilized (by electron withdrawing groups), then cyclization to the phenanthridine nucleus will be sluggish. The slower the rate of cyclization, the greater the chance of side reactions with the cyclization reagent. Therefore, the yield of the phenanthridine will depend on the relative rates of cyclization and side reactions, which is controlled by the stability of the carbonium ion. 

Carbonium ions are likewise known to be stabilized by solvation in strongly acidic media (Olah and Pittman, 1966), and consequently the reduction potential of carbonium ions, the ease of formation of a carbonium ion by the oxidation of a substrate and the products from these reactions would be expected to depend on the acidity of the electrolysis... 

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