Hydronium ion formation

The rate retarding role of water in radiation induced ionic polymerizations was visualized as proton extraction by water from the growing carbonium ion and hydronium ion formation (101). The formation of solvated hydronium ions is greatly favored energetically. 

In an aqueous solution of formic acid, compare the concentrations of hydroxide ions, hydronium ions, formate ions (CH02 ), and formic acid molecules. 

The salt of a weak base ionizes to form equal amounts of B and HsO (H if we disregard hydronium ion formation as was done previously). We can therefore solve for the hydrogen ion concentration (by assuming Cbh+ >100 Ka) -... 

According to this mechanism, the reaction rate is proportional to the concentration of hydronium ion and is independent of the associated anion, ie, rate = / [CH3Hg][H3 0 ]. However, the acid anion may play a marked role in hydration rate, eg, phosphomolybdate and phosphotungstate anions exhibit hydration rates two or three times that of sulfate or phosphate (78). Association of the polyacid anion with the propyl carbonium ion is suggested. Protonation of propylene occurs more readily than that of ethylene as a result of the formation of a more stable secondary carbonium ion. Thus higher conversions are achieved in propylene hydration. 

In studies of the hydration and dehydration of pteridine and the methylpteridines, but not levelled out as solutions were made more acid. This was explained by assuming that hydronium ion catalysis of the reactions proceeded only by the formation of the cations of HY+ and HX+, respectively. This effect is strikingly shown by 1,3,8-triazanaphthalene, for which the pH-rate profile of is V-shaped between pH 6.82 and 10.29 but levels out and remains constant from pH 5.3 down to, at least, 2.4. ... 

Extension of these studies to formic acid media (containing 4 vol. % ethylene glycol and 1.3 vol. % water) showed that for protodeboronation of 4-methoxy-benzeneboronic acid at 25 °C) rates were invariant of a tenfold variation in acidity produced by adding sodium formate (0.05-0.20 M) to the medium (Table 194), and in this range the concentration of molecular formic acid is essentially constant. This was, therefore, assumed to be the reactive species. At higher acidities the rate increased, which was attributed to the increase in concentration of hydronium ions and protonated formic acid ions which bring about reaction more readily625. 

The small size of the proton relative to its charge makes the proton very effective in polarizing the molecules in its immediate vicinity and consequently leads to a very high degree of solvation in a polar solvent. In aqueous solutions, the primary solvation process involves the formation of a covalent bond with the oxygen atom of a water molecule to form a hydronium ion H30 +. Secondary solvation of this species then occurs by additional water molecules. Whenever we use the term hydrogen ion in the future, we are referring to the HsO + species. 

Fig. 5 Logarithmic plots of rate-equilibrium data for the formation and reaction of ring-substituted 1-phenylethyl carbocations X-[6+] in 50/50 (v/v) trifluoroethanol/water at 25°C (data from Table 2). Correlation of first-order rate constants hoh for the addition of water to X-[6+] (Y) and second-order rate constants ( h)so1v for the microscopic reverse specific-acid-catalyzed cleavage of X-[6]-OH to form X-[6+] ( ) with the equilibrium constants KR for nucleophilic addition of water to X-[6+]. Correlation of first-order rate constants kp for deprotonation of X-[6+] ( ) and second-order rate constants ( hW for the microscopic reverse protonation of X-[7] by hydronium ion ( ) with the equilibrium constants Xaik for deprotonation of X-[6+]. The points at which equal rate constants are observed for reaction in the forward and reverse directions (log ATeq = 0) are indicated by arrows.

Figure 8.5. Top formation of hydronium ion. Bottom hydronium ion surrounded by water molecules.

Acetonitrile, acetone and dimethylformamide—these non-aqueous solvents exert a greater differential in the protophillic properties of many substances than in the corresponding aqueous solutions, due to the levelling effect of water in the latter solutions. Hence, the most acidic substance in aqueous solutions of a number of acids is the formation of the hydronium ion as shown below ... 

Water enhances the acidic or basic properties of dissolved substances, as water itself can act as either an acid or a base. For example, when hydrogen chloride (HCl) is in aqueous solution, it donates protons to the solvent (1). This results in the formation of chloride ions (Cr) and protonated water molecules (hydronium ions, H3O+, usually simply referred to as H" ). The proton exchange between HCl and water is virtually quantitative in water, HCl behaves as a very strong acid with a negative pl

In a previous publication ( ), results were presented on the micellar properties of binary mixtures of surfactant solutions consisting of alkyldimethylamine oxide (C12 to Cig alkyl chains) and sodium dodecyl sulfate. It was reported that upon mixing, striking alteration in physical properties was observed, most notably in the viscosity, surface tension, and bulk pH values. These changes were attributed to 1) formation of elongated structures, 2) protonation of amine oxide molecules, and 3) adsorption of hydronium ions on the mixed micelle surface. In addition, possible solubilisation of a less soluble 1 1 complex, form between the protonated amine oxide and the long chain sulfate was also considered. 

At about the same time that the work described above was being carried out at Glasgow, Kresge and McClelland and their coworkers in Toronto observed that whereas the rate constants (kH+) for the hydronium-ion catalysed formation of 2-hydroxyethyl benzoate from a series of 2-alkoxy-2-phenyl-l,3-dioxolans [69] varied with the alkoxy group at pH4-7.5, they were independent of the alkoxy-group in 5 x 10 2 M hydrochloric acid. 

There are at least three possible mechanisms for the spontaneous breakdown of hemiorthoesters, hemiacetals, and related species. Firstly, there may be a rapid and reversible ionization equilibrium followed by hydronium-ion catalysed breakdown of the anion (9) (Gravitz and Jencks, 1974). A necessary condition for this mechanism to be valid is that k2 calculated from kHi0 and Ka should fall below the diffusion controlled limit of c. 10loM 1s 1. The second mechanism (10) is similar to this but involves formation of the anion and hydronium ion in an encounter pair which react to give products faster than the diffuse apart (Capon and Ghosh, 1981). With this mechanism therefore the ionization equilibrium is not established and the rate constant for... 

Whether the formation of the zwitterionic intermediate occurs by way of carbonic acid ( 2 ) or directly by the reaction of the hydronium ion with HCO3 still remains uncertain from the kinetic considerations. 

A third, the hydronium ion OH2 + H+ -> OH, will be discussed later. Complex formation is quite a general phenomenon, and even mixed complex ions such as (P03F3) 2 and (P02F2)- can be formed on the other hand, not every coupling of two fluorides or oxides will form a complex, and we will have to investigate what the conditions are for stability of a complex. 

Mechanisms involving general species catalysis of the formation of a tetrahedral intermediate seem unlikely. General acid catalysis of the addition of water by the hydronium ion, viz. 

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