Carbonium ions relative stability

Deno el a/.20, on the basis of their equilibrium studies, have concluded that substituents which stabilize carbonium ions also stabilize RC02H2+ relative to RCO,H, and RCO+ relative to RC02H2+. In fact the differences are small for simple aliphatic acids. There is, however, a marked effect in the expected direction for chloroacetic acid, which is half converted to the protonated form only in 100% H>S04, and remains predominantly in this form up to 65% S03 in H2S0420. 

Carbonium ion stabilities may then be compared by reference to pKr The more positive the piCR+ value, the more stable the carbonium ion. Stability in this sense refers to the carbonium ion in equilibrium with the parent alcohol, and does not imply kinetic stability. Some carbonium ions are easily generated, but undergo rapid, irreversible conversion to other materials such as polymers or rearranged products. Table 5.2 presents the p/Cr+ values of a number of relatively stable carbonium ions. The data clearly indicate the sensitivity of carbonium ions to stabilization by electron-releasing substituents. Substitution by para-methoxy groups increases the value of pl R+, substitution by para-nitro groups makes p R+ more negative. By... 

The acyl residue controls the formation and stability of the carbonium ion. If the carbonium ion is destabilized (by electron withdrawing groups), then cyclization to the phenanthridine nucleus will be sluggish. The slower the rate of cyclization, the greater the chance of side reactions with the cyclization reagent. Therefore, the yield of the phenanthridine will depend on the relative rates of cyclization and side reactions, which is controlled by the stability of the carbonium ion. 

It is also difficult to determine exactly the relative stabilities of vinyl cations and the analogous saturated carbonium ions. The relative rates of solvolysis of vinyl substrates and their analogous saturated derivatives have been estimated to be 10 to 10 (131, 134, 140, 154) in favor of the saturated substrates. These rate differences, however, do not accurately reflect the inherent differences in stability between vinyl cations and the analogous carbonium ions, for they include effects that result from the differences in ground states between reactants, as well as possible differences between the intermediate ions resulting from differences in solvation, counter-ion effects, etc. The same difficulties apply in the attempt to estimate relative ion stabilities from relative rates of electrophilic additions to acetylenes and olefins, (218), or from relative rates of homopropargylic and homoallylic solvolysis. 

As a result of the inductive and hyperconjugative effects it is to be expected that tertiary carbonium ions will be more stable than secondary carbonium ions, which in turn will be more stable than primary ions. The stabilization of the corresponding transition states for ionization should be in the same order, since the transition state will somewhat resemble the ion. Thus the first order rate constant for the solvolysis of tert-buty bromide in alkaline 80% aqueous ethanol at 55° is about 4000 times that of isopropyl bromide, while for ethyl and methyl bromides the first order contribution to the hydrolysis rate is imperceptible against the contribution from the bimolecular hydrolysis.217 Formic acid is such a good ionizing solvent that even primary alkyl bromides hydrolyze at a rate nearly independent of water concentration. The relative rates at 100° are tertiary butyl, 108 isopropyl, 44.7 ethyl, 1.71 and methyl, 1.00.218>212 One a-phenyl substituent is about as effective in accelerating the ionization as two a-alkyl groups.212 Thus the reactions of benzyl compounds, like those of secondary alkyl compounds, are of borderline mechanism, while benzhydryl compounds react by the unimolecular ionization mechanism. 

The stability of a carbanion (or ion pair) is increased by certain substituents and decreased by others. It is possible to rank the various structures in an order of increasing stability of the carbanion just as was done for carbonium ions. It will be recalled that our information about carbonium ions does not suffice for a prediction of the effect of temperature changes on the relative stabilities, and that it is unknown to what degree an increase in stability actually reflects a decrease in potential energy. The situation is similar in the case of carbanions the precise relationship of the stabilities is an unknown function of the temperature. It is also likely that the effects of structural changes are somewhat dependent on the solvent. Nevertheless it is possible to make valuable qualitative comparisionsof the various structures and to interpret them in terms of resonance and other potential energy quantities. 

Richards and Hill have recently obtained quantitative evidence of the stabilization of a -metallocenyl carbonium ions (38, 95). They have shown that sol-volyses of methylmetallocenylcarbinyl acetates proceed via a carbonium ion mechanism, and that these acetates solvolyze with rates greater than even tri-phenylmethyl (trityl) acetate. Further, the relative rates of solvolysis and therefore the order of carbonium ion stabilities increas.e, proceeding from the iron to the osmium acetate. A portion of these data is summarized in Table II. 

The relative rate data closely parallel the results obtained in the solvolysis studies. Such a result might be expected from reactions proceeding through similar transition states. The observed order of relative rates may result from better overlap as the size of the central metal atom and the polarizability of its electron shell increase. This would result in increased stabilization and therefore ease of formation of the carbonium ions, proceeding from lighter to heavier metal complexes. 

The equations derived on this basis (16) show that it is not necessary to make any assumption in regard to relative stability of both carbonium ions. This is because any difference in energy between them cancels in the selectivities calculation. 

The interest in this area stems from attempts to assess the relative stability of various possible structures of carbonium ions. Therefore absolute values for the heats of formation are not necessarily required. CNDO calculations can thus be used equally well to determine the relative stability of isomers. Such calculations have been performed by Wiberg 45> and are illustrated in Table 17. 

Ion lifetimes as long as milliseconds have been measured in a number of different ways. One of the earliest methods involved flight tubes some metres long along which the ions were passed at relatively low velocities and within which the decompositions studied occurred [817, 818, 878]. The trajectories were stabilized by electric radio frequency quadrupole fields. Ionization was by electron impact and decompositions of carbonium ions derived from alkanes were observed over the time range 1/is to 1 ms. 

Also from this study the difference in the rate of bromination of alkenes and alkynes is quite evident. Thus the bimolecular coefficient (k2) for styrene is 2 x 103 times that for phenylacetylene and k2 for 3-hexene is 1-4 x 105 times the value for 3-hexyne. Since the first two compounds are believed to react via carbonium ions and the latter two via bromonium ions there seems to be an extra factor of 102 in the stability of bromonium ions from alkenes relative to those from alkynes. 

The rate of hydrolysis of polysaccharides is affected by several factors. Because of substituent interaction effects, furanosides are hydrolyzed much more rapidly than the pyranoside analogues. Differences in the hydrolysis rates of diastereomeric glycosides are significant. For example, the relative hydrolysis rates of methyl-a-D-gluco-, manno-, and galactopyranosides are 1.0 2.9 5.0. This can be related to the stabilities of the respective conjugate acids, which are transformed into the half-chair carbonium ions at different rates. Also, substituents bound to the C-2 position obviously prevent the formation of the half-chair conformation. 

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