Carbonium ion structures

Effects of Carbonium Ion Structural Changes on Ionization Equilibrium... 

An explanation not easily distinguishable from the one involving resonance with a carbonium ion structure in the transition state is that the reactive species is an ion pair in equilibrium with the covalent molecule. This is quite likely in a solvent insufficiently polar to cause dissociation of the ion pairs. Examples of second order nucleophilic displacements accelerated by the sort of structural change that would stabilize a carbonium ion are of fairly frequent occurrence. Allyl chloride reacts with potassium iodide in acetone at 50° seventy-nine times as fast as does -butyl chloride.209 Another example is the reaction of 3,4-epoxy-1 -butene with methoxide ion.210... 

The acid condensation reaction of the aromatic and phenolic units is a typical reaction of lignin. The presence of acids results in resonance stabilized carbonium ion structures formed in the lignin macromolecule. These car-bonium ion structures react further, e.g., with unsubstituted positions in the lignin macromolecule. Thus, thermal treatment of powdered wood in acidic conditions causes condensation, the coniferyl aldehyde and coniferyl alcohol groups being especially reactive. In addition, other inter- and/or intramolecular condensations may occur. 

Evidence has been obtained by Speyer and Dickman (43) that this reaction proceeds through a common intermediate resonating carbonium ion structure, which is coordinated to iron in such a manner that the iron is always bound to the same ligands, regardless of the direction in which the reaction is proceeding (Figure 3). 

The enzyme aconitase. which contains the Fe2+ ion at the reactive center, catalyzes the interconversion of citric, isocilric, and aconilic acids. The reaction has been shown to occur through the formation of a single intermediate carbonium ion structure in which the Fc2+ ion is always bound to the same donor aloms. while the interconversion of the substrate occurs through the migration of only protons and electrons. 

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. ... 

Yoshida and co-workers have carried out a normal coordinate analysis for the in-plane and out-of-plane vibrations of thiopyrylium and pyrylium cations, in order to elucidate their infrared spectra (74T2099). The difference between the IR spectra of thiopyrylium and pyrylium has been attributed first to the mass effect of the heteroatom and second to the smaller contribution of the carbonium ion structures in the former ion than in the latter. 

Criticism of these conclusions by Farcajiu (1976, 1978) inspired Schleyer et al. (1980) to study [200] and [201] by C-nmr spectroscopy under stable-ion conditions. The spectra of [201] confirmed its classical static carbonium ion structure at low temperature. At 30°C an average of the C(l), C(2) and C(3) signals and the signals of the CHg-groups attached to these positions, respectively, were observed due to the degenerate rearrangement via mechanism (127). 

Treatment of geraniol, nerol, linalool, and their acetates and phosphates with 85% phosphoric acid results in essentially similar mixtures of predominantly cyclic hydrocarbons (a-terpinene, y-terpinene, isoterpinolene, limonene, p-cymene, and p-menth-3-ene) the results are rationalized in terms of carbonium ion structures and stabilities with the interesting observations (i) that limonene can neither be derived from the a-terpinyl cation nor from (67), into which the... 

Although many people write carbonium ion structures like the above in terms of free ions, it must be kept in mind that the catalyst (anion) is... 

All of these data may be reconciled with the formation of substituted allylic (alkenyl) carbonium ions. Structures of this type can be derived from any substituted olefin by removal of a hydride ion from a carbon atom adjacent to the double bond (a-hydrogen). Since an olefin may be derived by proton removal from an alkyl carbonium ion, a substituted alkenyl ion may be derived from any alkyl carbonium ion precursor containing at least four carbon atoms and hence the spectra of their solutions should be very similar. Sulfur dioxide evolution from sulfuric acid solutions of olefins would accompany the formation of the alkenyl... 

With Robert s data one should consider the criticism of his results by Deno According to the latter the equalization of carbon and hydrogen atoms that precedes the reaction of the carbonium ion with the solvent results from intramolecular rearrangements so it should be regarded neither as a proof of the equivalence of carbon atoms in carbonium ion structures nor as a confirmation of resonance structures. Hence, Robert s evidence that on conversion of 2-norbornyl tosylate into norbomyl acetate all the atoms except C become nearly equivalent does not favour the structure of the nonclassical ion 5 because the experimental data on isotopic equalization indicate only relative rates of carbonium ion rearrangements. 

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. ... 

It is not possible to examine alkyl cations such as er -butyl cation under similar conditions because of the intervention of a myriad of condensation, cyclization, and rearrangement reactions. In 96% sulfuric acid, fer -butanol is converted within minutes to a mixture containing 50% alkanes and 50% cyclopentenyl cations. " One of the major developments in organic chemistry during the decade of the 1960 s was the application of NMR spectroscopy in so-called superacid media to probe the structure of carbonium ions. The most obvious use of this technique is in examining alkyl cations and other less stable ions, the p s of which are not readily measured. In fact, the method is so versatile and the information gained so much more valuable than simple stability measurements that it is now the method of first choice in probing carbonium ion structure. 

The evidence discussed to this point, both for and against the nonclassical structure, rests on indirect evidence derived from interpretation of kinetic results and stereochemical features of the substitution reactions. With the development of the techniques for directly observing carbonium ions, structural studies on the ion became possible. The norbornyl cation was subjected to intense scrutiny by George Olah at Case Western Reserve University. These spectroscopic investigations constituted a new approach to the problem. 

The NMR chemical shift of the trivalent carbon is a sensitive indicator of carbonium ion structure. Given below are the data for three carbocations with varying aryl substituents. Generally the greater the chemical shift the lower the electron density at the carbon atom. 

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