Carbenium ions nonclassical

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. ... 

Carbocations are a class of reactive intermediates that have been studied for 100 years, since the colored solution formed when triphenylmethanol was dissolved in sulfuric acid was characterized as containing the triphenylmethyl cation. In the early literature, cations such as Ph3C and the tert-butyl cation were referred to as carbonium ions. Following suggestions of Olah, such cations where the positive carbon has a coordination number of 3 are now termed carbenium ions with carbonium ions reserved for cases such as nonclassical ions where the coordination number is 5 or greater. Carbocation is the generic name for an ion with a positive charge on carbon. 

In this section we shall discuss carbocations in which at least one carbon-through a three-center bond (see Section 5.3) is coordinated to four orJive-atoxns. By Olah s terminology these are carbonium ions as opposed to tricoordinated carbenium ions. 61 By older terminology the more highly coordinated carbocations are called nonclassical carbonium ions to differentiate them from the tricoordinated classical carbonium ions. ... 

Clear, unequivocal experimental evidence has by now been obtained for nonclassical ions such as the norbomyl cation.38 10 The bonding concept required to define nonclassical ions is simply to consider them as penta(or higher)-coordinated carbonium ions involving at least one two-electron three-center (or multicenter) bond, of which CH5+ (themethoniumion-carbonium ion) is the parent, asCH3+ (the methenium ion, methyl cation, carbenium ion) is the parent for trivalent carbenium ions. An example of a hexacoordinate carbonium ion is the pyramidal dication of Hogeveen.41... 

When R—X is enantiomerically enriched exo-2-norbornyl 4-bromobenzenesulfonate (brosylate), 1 in Fig. 4.5, and SOH is aqueous ethanol, for example, solvolysis is accompanied by racemisation. However, the rate of racemisation is faster than the rate of product formation, and starting material isolated before completion of the solvolysis is partially racemised [14]. The most economical interpretation of these results (and much other evidence, see Chapter 7) is that R+ X- includes the achiral nonclassical carbenium ion (3 in Fig. 4.5), and the ion pair undergoes internal return faster than nucleophilic capture. In other words, k- > k2 in Scheme 4.4, the precise value of the ratio k- lk2 depending upon the particular solvent, and the greater this ratio, the closer the initial reversible process approaches a pre-equilibrium. 

For many decades chemists had been interested in whether the positively charged intermediate of the SN reaction in Figure 2.28 was a carbenium or a carbonium ion. Also the existence of a rapidly equilibrating mixture of two carbenium ions was considered. It is now known with certainty that this intermediate is a carbonium ion it is known as a nonclassical carboca-tion. In this carbonium ion, there is a bond between the centers Cl, C2, and C6, that consists of two sp2 AOs and one sp3 AO (see MO diagram, lower right, Figure 2.28). It accommodates two electrons. There are many examples of nonclassical carbocations. 

The olefin seems to disappear mainly in the second stage after the decomposition of 125. Probably, a nonclassical type of carbenium ion intermediate (128) is formed with the help of the olefin group located at an intramolecularly favorable position, and this intermediate obviously makes the polymer structure complicated by a possible cyclization-skeleton rearrangement prior to the nucleophilic attack of the monomer. 

The carbonium ion is a nonclassical molecule, because it contains a pentacoor-dinated carbon atom. It is very unstable and is therefore difficult to form. It is formed at temperatures of about 500°C. Once formed it readily decomposes to a carbenium ion by H2 formation or cracking (Fig. 4.70). 

Since our present interest is the reaction of r-bonded systems, the carbon-carbon double bond of alkenyl monomers, with electrophiles to give classical carbenium ions (or esters) as end products, we will concentrate on these entities. Whether nonclassical carbonium ions ( TT-complexes ) are involved in these reactions as hi -energy transition states, is still uncertain, and this interesting aspect will be discussed specifically in later sections (see Chaps. Ill and IV). 

In order to differentiate between these two possibilities the Raman and 13C NMR spectra of the ion were studied10. As Raman spectroscopy is a fast physical method (assuming that vibrational transition rates are faster than any of the hydrogen or alkyl shifts) and the question of possible equilibration versus bridged ion is unimportant. The previously discussed technique of average 13C NMR shifts should be also applicable in this case to differentiate a static nonclassical bridged ion from rapidly equilibrating classical carbenium ions. 

It can be concluded that the original views of Winstein208 on the nonclassical nature of the norbomyl cation, based on kinetic and stereochemical results, were fully substantiated through the direct spectroscopic studies of the long-lived ion10, which helped to develop our general understanding of carbonium ions, as distinct and well differentiated species from trivalent carbenium ions. 

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. 

Structure V would be ideally constituted (see Vb) for the construction of the second C-C bond required by structure II. One major roadblock, however, is found at this point. It is the formation of a very unusual bridgehead carbo-cation (Via) that violates Bredt s rule. Nevertheless, carbenium ions at bridgeheads of polycyclic structures are well accepted reaction intermediates.4 Furthermore, structure VI, being an anti norbomenyl cation, benefits from the additional potential stabilization of its nonclassical carbenium ion character.3 It would not sit around long before water converted it to the longifoiene precursor (II). 

Olah emphasizes that the division of cations into classical and nonclassical is frequently arbitrary, since in many cations there is an intenn liate range of delocalization ( partial carbonium-ion character ) as in the 2-methylnorbomyl ion. The author does not want to name classical ions carbonium because it is restricted to highest valeiK state carbocations this requirement is nwt by penta-and tetracoordinate carbocations but not trivalent ones. On the other hand, while in the formation of other onium ions the atom of the donor (nitrogen, oxygen etc.) increases its covalence by one unit upon addition of the acceptor (electrophile), in the formation of a classical ion the covalence of the carbon atom decreases from 4 to 3. As for the name carbenium ion, in the author s opinion it reflects the logical relationships between the carbene and the carbeiunm ion, between the alkene and the carbenium ion ... 

The Cg-Cj bond is situated at the rear side of the ionizable group and the a electrons attacks the carbon bearing the ionizable group, thus facilitating the ionization. This leads to a nonclassical carbenium ion, which reacts with acetic acid to yield the racemic mixture of acetates. No such anchimeric assistance is available to the endo-isomer, which undergoes slow acetolysis through classical carbenium ions. 

These results were interpreted as implying that the reaction of the exo-substrate occurred solely via a nonclassical carbocation, while the endo-substrate reacted by initial formation of a classical carbenium ion, which then rearranged to the nonclassical carbocation, but not before a small amount had reacted with solvent (attack being sterically directed to the exo-face). 

Stable carbocations. CXVIII. General concept and structure of carbocations based on differentiation of trivalent (classical) carbenium ions from three-center bound penta- of tetracoordinated (nonclassical) carbonium ions. Role of carbocations in electrophilic reactions. Olah, G.A. 

All electrophilic carbon atoms may be described as carbocations. These can be classified further as carbenium ions or carbonium ions. Carbenium ions are trivalent carbocations with electron-deficient centers, and have planar or almost planar 5/ hybridization. In contrast, carbonium ions are four- or fivefold coordinated nonclassical carbocations. In this case, the carbocation consists of three single bonds and a two-electron-two-center bond. Onium ions are formally formed by corresponding addition of heterocompounds to the carbocation, for example. 

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. ... 

Cations are kinetic chain carriers in cationic polymerizations. Such cations may be, for example, carbocations or oxonium ions. All electrophilic carbon atoms may be described as carbocations. These can be classified as carbenium ions (trivalent carbocations) and as carbonium ions (carbocations with coordination numbers of four or five). Carbenium ions such as, for example, R3C are classical carbocations. Carbonium ions such as, for example, R5C or R5C2 are nonclassical ions. 

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