Carbenium ions, formation

A mixture of water/pyridine appears to be the solvent of choice to aid carbenium ion formation [246]. In the Hofer-Moest reaction the formation of alcohols is optimized by adding alkali bicarbonates, sulfates [39] or perchlorates. In methanol solution the presence of a small amount of sodium perchlorate shifts the decarboxylation totally to the carbenium ion pathway [31]. The structure of the carboxylate can also support non-Kolbe electrolysis. By comparing the products of the electrolysis of different carboxylates with the ionization potentials of the corresponding radicals one can draw the conclusion that alkyl radicals with gas phase ionization potentials smaller than 8 e V should be oxidized to carbenium ions [8 c] in the course of Kolbe electrolysis. This gives some indication in which cases preferential carbenium ion formation or radical dimerization is to be expected. Thus a-alkyl, cycloalkyl [, ... 

Fig. 1. a) Standard protonation enthalpy in secondary carbenium ion formation on H-(US)Y-zeolites with a varying Si/Al ratio, b) Effect of the average acid strength for a series of H-(US)Y zeolites experimental (symbols) versus calculated results based on the parameter values obtained in [11] (lines) for n-nonane conversion as a function of the space time at 506 K, 0.45 MPa, Hj/HC = 13.13 (Si/Al-ratios 2.6, 18, 60)... 

Examples are reactions initiated by the protonation, as for example carbenium ion formation from — for example—olefins (Bronsted acidic centres) or deprotonation reactions like base-catalysed aldol condensation. 

Acylation [reaction (3 A)] is supported hy R being a small electron-withdrawing group, R destabilizing carbenium ion formation. 

Alkylation [reaction (3B)] is supported by R being a sterically demanding electron-with-drawing group, R supporting carbenium ion formation. 

For PtCu/NaY, however, most of the coke is presumably not in intimate contact with metal clusters, its oxidation therefore takes place at a higher temperature. Since splll -over of active oxygen from the oxidized metal over finite distance can be excluded, the oxidation of this coke will not be catalyzed by Pt or Cu. In the reduced form of the catalysts the Ft ensembles at the surface of the encaged bimetal particles are diluted with Cu and additional protons of hi Brensted acidity were formed in the reduction of each Cu ion to Cu°. The combination of both phenomena, small Pt ensembles and hl concentration of protons of strong Brensted acidity, results in an Increase of the RE/RO activity ratio as observed and reported(S). It is therefore reasonable to attribute the TPO peak at higher temperature to the coke deposited on the acid sites of the zeolite via carbenium ion formation and polymerization. The results are thus quite similar to those observed and... 

Earlier mention has been made of the use of Lewis acid and Friedel-Crafts reagents as initiators of carbenium ion formation. Another versatile device is to employ a metal to assist in generation of the carbenium ion. In its simplest form, addition of silver(I) ion to an alkyl halide is an excellent technique for encouraging reaction by means of the 5n 1 pathway. This process was first applied to the Ritter reaction by Cast and Stevens, but yields obtained were modest. A recent elegant application of this technique is the two-step conversion of dodecahedrane into its acetamide derivative (Scheme 45). ... 

The ring contraction of monofunctionalized four-membered rings (i.e. halo- or tosyloxycyclobutanes), involving carbenium ion intermediates, usually affords mixtures of cyclobutyl, cyclopropylmethyl and but-3-enyl compounds, but are of limited synthetic applicability (Section 4.1.2.1.). On the other hand, the ring eontraction of vicinally difunctionalized cyclobutanes appears easier and does not necessarily require carbenium ion formation. 

The ease of formation of carbenium ions from substrates such as tertiary alcohols means these are ideal starting materials for the Ritter reaction. However, formation of such an ion at a less substituted centre is much less facile, if not impossible. There has been a large amount of work that focused on the stabilisation or promotion of carbenium ion formation at such centres. 

Various heteroatoms have been used to facilitate carbenium ion formation at less substituted centres, including halogens,9"" nitrogen,12 selenium,13 and sulfur.14 These reactions generally proceed via activation of the alkene, through formation of a cyclic cationic intermediate 25.15... 

The linear free-energy relationship indicates that 213 reacts similarly to AdOTs by rate-limiting carbenium ion formation through the SN1/E1 mechanism illustrated by equation 103. The substitution/elimination products can be formed from all intermediate species shown in equation 103a. 

The noted increase of propane conversion and selectivity can be explained in the framework of the accepted mechanism of the lower alkanes aromatization. As is known carbenium ion formed on a zeolite is then either disintegrated or added to the formed unsaturated hydrocarbons [2, 3]. Due to the higher concentration of unsaturated hydrocarbons in the used mixtures in composition with pure propane, the probability of addition products formation grows, and hence, the first reversible stage of reaction moves towards carbenium ion formation. 

The data of Table 5 show that propane activation with the formation of aromatic hydrocarbons and propylene is possible at appreciably lower temperatures. The possible mechanism of benzene dehydroalkylation by propane with the formation of IPB and propylene occurs with the carbenium ion formation by the scheme ... 

Of the two OH groups, the one which forms the more stable carbocation is protonated preferentially. This factor takes precedence over the migratory aptitude factor. Further evidence for carbenium ion formation in the pinacol rearrangement has been obtained by oxygen-exchange experiments. Partial rearrangement of pinacol to pinacolone has been carried out in acidic solutions containing H20 (Scheme 2.27). 

As an example, consider the relative rates of solvolysis shown below. The all-alkyl system is indeed the fastest, yet when the electron withdrawing cyano is placed directly on the site of carbenium ion formation, the rate is faster than for an adjacent cyano. 

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