Carbenium ions controversy

The literature is more controversial concerning the relation between the bridging of the intermediate and the stereochemistry of the 1,4-addition. It has been suggested15 that if 1,4-addition occurs via a bridged intermediate, an Sn27 process should be involved and the stereochemistry of the product should be completely syn. Alternatively, an increase in the amount of anti 1,4-addition would be expected when an open carbenium ion is involved15. Furthermore, the lack of any 1,4-product has been considered as evidence for an at least weakly bridged bromonium ion, and therefore the dependence of 1,4- vs... 

Since many NMR studies of solid acids entail the observation of a chemical shift change in a probe molecule, reactant, intermediate, or product upon complexation with an acid site, there is an opportunity to fundamentally impact the application of NMR to solid acids by making better use of this information. We therefore review the salient parts of the chemical shift in some detail. Since some of the more visible controversies in NMR studies of solid acids regard carbenium ions and related electrophilic species, this treatment will use such ions as examples wherever possible. 

In contrast to the rather well-defined trivalent ( classical ) carbenium ions, nonclassical ions 26 have been more loosely defined. In recent years, a lively controversy centered on the classical-nonclassical ion problem.27-37 The extensive use of dotted lines in writing carbonium ion structures has been (rightly) criticized by Brown, 31 who carried, however, the criticism to question the existence of any o-delocalized (nonclassical) ion. For these ions, if they exist, he stated ... a new bonding concept not yet established in carbon structures is required. ... 

A controversial issue of heteroatom-stabilized cations is the relative stabilization of carbocationic centers adjacent to oxygen and sulfur.541 In solution studies, a-O-substituted carbocations were found to be stabilized more than a-iS -substituted carbocations.677 Gas-phase studies reached an opposite conclusion,678 679 whereas subsequent theoretical studies (high-level ab initio methods) supported the findings of solution chemistry. Recent results, namely, basicities of various vinylic compounds (365-370) measured in the gas phase also support this conclusion.680 Although monoheteroatom-substituted compounds 365 and 366 were found to have similar proton affinities, an additional a-methyl group increased the stability of the carbenium ion derived from 367 more than that of the sulfur counterpart 368. Even larger differences were found between proton affinities of the bis-heteroatom-substituted compounds 369 and 370. 

The fundamental step in acid-catalyzed hydrocarbon conversion processes is the formation of the intermediate carbocations. Whereas all studies involving isomerization, cracking, and alkylation reactions under acidic conditions (Scheme 5.1) agree that a trivalent carbocation (carbenium ion) is the key intermediate, the mode of their formation of this reactive species from the neutral hydrocarbon remained controversial for many years. 

There is considerable controversy with regard to the formation of persistent (long-lived) carbenium ions in zeolite solid acids (8). The conventional view has been that stable carbenium and even carbonium ions are readily formed from even weakly basic molecules such a propene. However, there is increasing evidence that the formation of persistent species requires a strongly basic precursor, and is thus only possible in select cases (25). Consequently, we feel that any positive claims of persistent carbenium species in zeolites warrant exceptional proof. 

If physical-organic chemistry as a whole has profited substantially from these studies, it is legitimate to ask whether the specific field of cationic polymerisation has been equally affected in terms of a better comprehension of at least some of its many controversial aspects. The answer to this question is both yes and no. Yes, because at last ions sought for a long time, such as (CH3)3C and Ph(CH3)CH, are now known to exist and have been characterised spectroscopically. No, because the specific conditions in which these carbenium ions have been prepared, i.e., under the stabilising environment of superacid media, are not those used in cationic polymerisation. Thus, the stability, reactions, and general behaviour of these species cannot be extrapolated to the much milder situations typical of a polymerisation. It is notorious that chain carriers have seldom been seen during a pdymerisation, and this implies that either the concen-... 

For many years, a lively controversy centered over the actual existence of nonclassical carbocalions. " The focus of argument was whether nonclassical cations, such as the norbornyl cation, are bona fide delocalized bridged intermediates or merely transition states of rapidly equilibrating carbenium ions. Considerable experimental and theoretical effort has been directed toward resolving this problem. Finally, unequivocal experimental evidence, notably from solution and solid-state C NMR spectroscopy and electron spectroscopy for chemical analysis (ESCA), and even X-ray crystallography, has been obtained supporting the nonclassical carbocation structures that are now recognized as hypercoordinate ions. In the context of hypercarbon compounds, these ions will be reviewed. 

As we have seen earlier, the propagating species in cationic chain polymerization is a positively charged carbon species. The older term for this trivalent, trigonal, positively charged carbon ion is carbonium ion which we have used up to this point. Olah [10] proposed that the term carbenium ion be used instead of carbonium ion, the latter being reserved for pentavalent, charged carbon ions, and the term carbocation for both carbonium and carbenium ions. Since the term carbenium ion is not universally followed, to avoid the controversy we will henceforth refer to the propagating species as carbocations. Most text and journal references use the term carbocation and the term carbocation polymerization is used synonymously with cationic polymerization in the literature. 

There is some controversy (ref. 10) about the extent to which ethylene participates in the quasi-equilibrium, since ethylene is the least reactive olefin in cationic oligomerization and the least readily obtained by a carbenium-ion cracking mechanism. 

Perhaps the "classic" example of a nonclassical carbocation is the 2-norbornyl cation, which was at the center of what has been called "the most heated chemical controversy in our time." In Chapter 8 we will review the experimental evidence, largely based on solvolysis reactions, that led to the proposal of the nonclassical carbonium ion structure shown in Figure 5.48. However, this description was not accepted by all researchers, and an alternative model for the 2-norbomyl cation was a pair of rapidly equilibrating classical (carbenium) ions, as shown in Figure 5.49. Many papers relating to the development of contrasting ideas in this area were published in a reprint and commentary volume by Bartlett. ... 

The last two contributions show the controversy about the mechanism and involvement of acid sites in the hydride transfer reactions over zeohtes. If the hydride transfer activity depends on the hydrophobicity, the rate should correlate to the acid strength of the sites, but not to the acid site density, i.e., the concentration of adjacent active sites. If the reaction is expected to proceed via two carbenium ions adsorbed on adjacent sites or one adsorbed carbenium ion on one site and a feed molecule influenced by the second adjacent site, the acid site density would be probed, but the information about the acid strength would be less obvious. However, in both cases it does not seem that these test reactions can be applied to compare large, medium, and small pore zeolite structures, due to the large (bimolecular) transition state proposed for hydride transfer reactions (unless very small molecules are used [203]). 

Althou the mechanism of aromatics (and concurrent parafiin) formation from the intermediate olefins can readily be understood in terms of classic carbenium ion theory, the mechanism of formation of the initial C-C bond from Cl precursors remains a mystery and a matter of controversy at present. Unlike other alkanols, methanol lacks a b-H and therefore cannot under the expected elimination reaction yielding water and a parent olefin. Much of the controversy is centered on the genesis, nature, and fate of the reactive "Cl moiety." For a discussion... 

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