Stability of carbonium ions

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. 

For the determination of stabilizations of carbonium ions the equilibrium constants of carbonylation-decarbonylation have been used in previous Sections. For the ions discussed in this Seetion, however, the rate constants of decarbonylation are not known and, therefore, the rate constants of carbonylation will be used as a criterion for such stabilizations. This kinetic criterion is a useful indicator if there are no significant steric factors in the carbonylation step and if this step is indeed rate-determining in the overall process (Hogeveen and Gaasbeek, 1970). The following rate constants in Table 2 are of particular importance. 

Buell, McEwen, and Kleinberg have observed that weak acids such as hydrogen azide and acetic acid add readily across the double bond of vinylferrocene (XLI, M = Fe) (8). They have postulated that the mechanism of addition proceeds via intermediate formation of the a -ferrocenylcarbonium ion (X-LII, M = Fe), followed by conversion to the acetate (XLIII, M = Fe). Stabilization of carbonium ions of this type can result from overlap of filled metal orbitals with the vacant p -orbital of the carbonium ion. 

Radical cations that are produced by electrochemical oxidation are not stable in solvents with appreciable base character. This results because such radicals are subject to attack by available nucleophiles, and solvents that contain donor electron pairs are good nucleophiles. Cation radicals are most stable in solvents that are good Lewis acids and show negligible basic properties. Some of the solvent systems that have been employed to stabilize electrochemically produced cation radicals include nitromethane and nitrobenzene,21 dichloro-methane,22 trifluoroacetic acid-dichloromethane (1 9),23 nitromethane-AlCl3,24 and AlCl3-NaCl (1 l).25 Organic chemists should be familiar with the stabilization of carbonium ions by superacid media.26 These media usually contain fluorosulfuric acid, or mixtures of fluorosulfuric acid with antimony pen-tachloride and sulfur dioxide, and are potent solvents for the production and stabilization of organic cations. 

Warshel A, M Levitt (1976) Theoretical Studies of Enzymic Reactions - Dielectric, Electrostatic and Steric Stabilization of Carbonium-Ion in Reaction of Lysozyme. J. Mol. Biol. 103 (2) 227-249... 

Bond dissociation energies have already shown (Sec. 3.24) that the amount of energy required to form free radicals from alkanes decreases in the same order CH3 > r > 2° > 3°. If we combine these two sets of data—ionization potentials and bond dissociation energies—we see (Fig. 5.9) that, relative to the various alkanes concerned, the order of stability of carbonium ions is ... 

What we really want as standards for stability of carbonium ions are, of course, the kinds of compounds they are generated from alcohols at this particular point or, later, alkyl halides (Chap. 14). However, the relative stabilities of most ordinary neutral molecules closely parallel the relative stabilities of the alkanes, so that the relative order of stabilities that we have arrived at is certainly valid whatever the source of the carbonium ions. To take an extreme example, the difference in stability between methyl and /m-butyl cations relative to the alkanes, as we have just calculated it, is 71 kcal. Relative to other standards, the difference in stability is alcohols, 57 kcal chlorides, 74 kcal bromides, 78 kcal and iodides, 76 kcal. 

Figure 5.10. Molecular structure and rate of reaction. Stability of transition state parallels stability of carbonium ion more stable carbonium ion formed faster. (Plots aligned with each other for easy comparison.)...

Catalysis by acid suggests that here, as in dehydration, the protonated alcohol R0H2" is involved. The occurrence of rearrangement suggests that carbonium ions are intermediates—although not with primary alcohols. The idea of carbonium ions is strongly supported by the order of reactivity of alcohols, which parallels the stability of carbonium ions—except for methyl. 

Warshel A, Levitt M. Theoretical studies of enzymatic reactions dielectric, electrostatic and steric stabilization of carbonium ion in the reaction of lysozyme. J Mol Biol 1976 103 227-249. 

The complex then splits beta to the point of complexing to produce an olefin and a new hydrogen deficient entity. However, superimposed on this basic cracking reaction are the simultaneous and consecutive reactions which produce the characteristic catalytic cracking product distribution. The relative stability of carbonium ions is tertiary > secondary > primary. There is, then, either a preferential formation of tertiary and secondary ions, or else isomerization to these preferred forms. The property of beta fission results in the formation from secondary ions of no olefins smaller than propylene, and from tertiary ions of no olefins smaller than isobutylene. Cycli-zation and hydrogen transfer reactions result in the large amounts of aromatic hydrocarbon formed. The sum total of these described reactions lead to the desirable product distribution characteristic of catalytic cracking. 

It does this in a manner so that the most stcible carbonium is obtained. (Remember, the stability of carbonium ions is 3°>2°>1°>CH3+.) The abstraction of the hydride ion can be illustrated as follows ... 

The rate at which a carbonium ion forms is dependent on its stability. It can be formed fastest if there are e-lectron-releasing groups to stabilize the positive charge. The rate of carbonium ion formation is 3>2>1>methyl. This is also the order of stability of carbonium ions. Since tert-butyl alcohol can form a 3° cation, whereas ethyl alcohol can only form a 1° carbonium ion, tert-... 

Wieting, R. D., Staley, R. H., Beanchamp, J. L. (1974). Relative stabilities of carbonium ions in the gas phase and solution. Comparison of cyclic and acyclic alkylcarbonium ions, acyl cations and cyclic halonium ions. Journal of the American Chemical Society, 96, 7552-7554. 

Gassman, P. G., Fentiman, A. F. (1969). Aryl delocalization vs. neighboring gronp participation in the stabilization of carbonium ions. Journal of the American Chemical Society, 91(6), 1545-1546. 

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