Oxonium ion equilibria

Carbocation-oxonium ion equilibria are obvious complicating factors in studies of the kinetics of initiation of polymerisation and useful thermodynamic data for such equilibria involving Ph3C+ and a variety of linear and cyclic ethers have been reported by Slomkowski and Penczek (132). A dramatic increase in rates of initiation of polymerisation of THF induced by Ph3C+ salts is observed on addition of small amounts of epoxides such as propylene oxide (113a,b), which compete favourably with THF in the primary carbocation-oxonium ion equilibria and simplify the initiation reaction ... 

Table 5 Enthalpy of Carbenium-Oxonium Ion Equilibria in solution, CH2CI2, 25°C, AsF6", SbF6, PF6- Counterions...

The conductivity also increases in solutions of weak electrolytes. This second Wien effect (or field dissociation effect) is a result of the effect of the electric field on the dissociation equilibria in weak electrolytes. For example, from a kinetic point of view, the equilibrium between a weak acid HA, its anion A" and the oxonium ion H30+ has a dynamic character ... 

Polymerization Equilibria. As mentioned earlier, esters of strong acids, e.g. trifluoromethane sulfonic acid ("triflates"), are excellent initiators for the polymerization of THF. With such initiators, however, a complication arises. In addition to the normal propagation i depropagation equilibria of oxonium ions, Smith and Hubin postulated that the macroion ( ) may also convert into a corresponding nonpolar macroester ( ) by attack of the anion (14). ... 

Both XX and XXI undergo propagation (presumably through the corresponding oxonium ions). The two equilibria (Eqs. 7-38 and 7-39) do not proceed to the same extent, with the result that the copolymer structure deviates from that of the monomer [Szwarc and Perrin, 1979]. 

Plesch and collaborators have recently introduced this technique for the study of carbenium and oxonium ions and of Br nsted acid dissociation equilibria ° The ultimate aim of this work is of course to gain a better understanding of the mechanism of initiation in cationic polymerisation. For the moment however it is difficult to assess the real potential of polarography in that context and we feel that, althou it certainly possesses a hi sensitivity, the conditions required for meaningful measurements are still too distant from those employed in an actual polymerisation, in particular the high ionic strength of the medium. Its use is therefore still doubtful and one can only hope that more research will bridge the gap. 

Equilibria like (62) are strongly shifted to the side of oxonium ions e. g. the calculated heat of formation of the triethyloxonium ion from ethyl cation and diethyl ether is as high as 128 kcal mole" in the gas phase. This would give no chance for a single carbenium ion to exist at the usual polymerization conditions of the majority of heterocycles cyclic sulfides and amines are even stronger nucleophiles than cyclic ethers (cf., however, Ihe discussion on cyclic acetals below). 

In the polymerization of several cyclic acetals and ethers autoacceleration was observed. There might be a number of reasons for this behaviour, and some of these have already been discussed, namely the preinitiation equilibria and the inequality ki enhanced reactivity of the hydroxy end group in polyacetals toward active species (in comparison with acetal groups) is another, not yet considered, reason for induction periods. When the pol5mierization d ee increases with conversion the proportion of the active tertiary oxonium ions also increases, at the cost of the less reactive secondary oxonium ions (cf. Ref. 164), because the b k-biting or intermolec-ular transfer become more important than the end-biting. Thus, there b no need to make speculative assumptions about the nature of the active species and to propose the two stage polymerization of acetals in order to explain the induction... 

All chemical reactions can be regarded as equilibria. Multistep reactions are simply series of equilibria in which species occupying energy minima are separated by barriers, the tops of which are transition states. The SnI solvolysis reaction of er -butyl iodide in water makes a good example. Here we have three equilibria. The first is between r -butyl iodide and the /cr/-butyl cation and iodide ion. This equilibrium is highly unfavorable, because the cation-anion pair is far less stable than the covalent iodide. The second equilibration is the exothermic capture of the cation by water to give the protonated alcohol, an oxonium ion. Finally, the oxonium ion is deprotonated to give the ultimate product, /cr/-butyl alcohol (Fig. 8.2). 

When DXL was used instead of dimethoxymethane in the studies of equilibria, it was observed by NMR that the predominant structure was not the five-membered ring oxonium ion but the corresponding cyclic ion involving the seven-membered ring (Scheme 17). 

When the six-membered (1,3-dioxane) or the seven-membered (DXP) cyclic acetals were used in the studies of related equilibria, isomerization to the expanded ring stmc-tures (eight- or nine-membered, respectively) was negligible because the starting rings were less strained than the expanded ones. All these reactions proceed with high rates therefore, at any stage of polymerization, the equilibrium between alkoxycarbenium ions and various cyclic (formed by intramolecular reaction) and branched (formed by intermolecular reaction) oxonium ions is quickly established (Scheme 18). 

Scheme 18 Equilibria between alkoxycarbenium ions and cyclic or branched oxonium ions.

Which illustrates only one of the possible (limiting) forms actually only this form, in the shape given above or in connection with a counter-ion, represents an oxonium centre. In reality they exist in several forms (as esters, ion pairs, free ions) connected by equilibria. 

Propagation. The structure of the growing species (tertiary oxonlum ions) in ring-opening polymerization of several monomers was already characterized by NMR spectroscopy (Table III). Carbenium-oxonium equilibria were also evidenced and measured. 

The Nernst distribution law applies to metal complexes, but their distribution ratios are determined by several interrelated equilibria. As in the case of organic acids and bases, the efficiency of extraction of metal chelates is pH dependent, and for some ion-association complexes, notably oxonium systems (hydrogen ions solvated with ethers, esters or ketones), inorganic complex ions can be extracted from concentrated solutions of mineral acids. 

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