Self-ionizing reactions

Thus the compound HC03 is a conjugate base in the first reaction and an acid in the second reaction. A compound that can act either as an acid or a base is called amphiprotic. In the self-ionization reaction... 

Autoprotolysis constant — The ion-product calculated from the ion activities of the conjugate acidic and basic species of an -> amphiprotic solvent (SH). The chemical equation of such self-ionization reactions can be schematized as 2HS H2S+ + S , where H2S+ is the conjugate cation, S the conjugate anion. The autoprotolysis constant can be formulated as JCauto = [H2S+] ... 

If the solvent is water, these two ions are always H+(aqj and OH (aqj, but in the case of liquid ammonia, which is also a good solvent, the corresponding ions would be NH4 and NH2. That the solvent does play some special role is implied by the self-ionization reactions... 

Now consider the concentration of H30 ion produced by the self-ionization of water. In pure water, the concentration of H30 produced is 1.0 X 10 M in an acid solution, the contribution of HsO from water will be even smaller. You can see this by applying Le Chatelier s principle to the self-ionization reaction. When you increase the concentration of H30 in water by adding an acid, the self-ionization of water reverses until a new equilibrium is obtained. 

Even the purest water has some ability to conduct an electric current. This conductivity is attributed to the presence of and OH ions produced by the self-ionization reaction (page 248)... 

Halide donor-acceptor reactions (of XX ) are generally those in which X is donated to or accepted from an interhalogen. They include self-ionization reactions such as that of BrF3 shown in Equation (18.53). This property makes bromine trifluoride a common aprotic (without protons) self-ionizing solvent. In addition to its self-ionization, BrF3 readily accepts fluoride ions from other sources, such as alkali-metal fluorides, to produce salts containing the bromine tetrafluoride ion, as shown in Equation (18.54). Conversely, it can donate fluoride ions to produce salts containing the bromine difluoride cation, as shown in Equation (18.55). 

Many of the best and most widely used solvents for acids and bases, such as water, have both acidic and basic properties. They are described as amphoteric or amphiprotic solvents, and they undergo an autoprotolysis or self-ionization reaction in which one molecule acts as a base and another as an acid, for instance ... 

In Section 16-1, we learned that the H2O molecule can act as either an acid (reaction 16.2) or a base (reaction 16.1) it is amphiprotic. It should come as no surprise that amongst themselves water molecules can produce HgO" " and OH ions via the following self-ionization reaction or autoionization reaction ... 

Notice that the solvent, H2O, can act as either an acid or a base it is an acid in Equation 3.7, but a base in Equations 3.4 and 3.5. This dual possibility is most apparent in the familiar self-ionization reaction of water. In Arrhenius notation this is written... 

Reaction (5.N) describes the nylon salt nylon equilibrium. Reactions (5.0) and (5.P) show proton transfer with water between carboxyl and amine groups. Since proton transfer equilibria are involved, the self-ionization of water, reaction (5.Q), must also be included. Especially in the presence of acidic catalysts, reactions (5.R) and (5.S) are the equilibria of the acid-catalyzed intermediate described in general in reaction (5.G). The main point in including all of these equilibria is to indicate that the precise concentration of A and B... 

Acid-Base Reactions. Anhydrous hydrazine undergoes self-ionization to a slight extent, forming the hydrazinium, N2H5, and the hydrazide, N H7ions ... 

Tire self-chemical ionization reaction of CS2 under chemical ionization conditions (approx. 1 Torr) generated 83, which sulfurized pyridine (97MI1) and nitriles (97JPC6970) to give the corresponding cation radicals 61 and 62, respectively. Ab initio calculations on 83 at the G2 (MP2, 8VP) level revealed that the ylide radical cation form 63 is more stable than the dithiiranethione radical cation form (64) by 42 kJ/mol (97JPC6970). 

It is very instructive to compare the kinetics and plausible mechanisms of reactions catalyzed by the same or related catalyst(s) in aqueous and non-aqueous systems. A catalyst which is sufficiently soluble both in aqueous and in organic solvents (a rather rare situation) can be used in both environments without chemical modifications which could alter its catalytic properties. Even then there may be important differences in the rate and selectivity of a catalytic reaction on going from an organic to an aqueous phase. TTie most important characteristics of water in this context are the following polarity, capability of hydrogen bonding, and self-ionization (amphoteric acid-base nature). 

El an elimination reaction mechanism in which the slow step is a self-ionization of the molecule to form a carbocation. Thus, the ratecontrolling step is unimolecular. 

It was found that this heterogeneous reaction can be stopped by the addition of small amounts of lithium. It is likely that lithium with its higher electrode potential is able to prevent the self-ionization of liquid ammonia, and the heterogeneous reaction between ammonium in solution and liquid ammonia is inhibited. When this heterogeneous reaction is prevented, the decomposition follows a perfectly satisfactory second-order reaction, and the temperature must be raised up to the range from zero to 20° in order to obtain a measurable reaction rate. The reaction is satisfactorily explained on the assumption that ammonium dissociates in mercury giving ammonium ions and free electrons NH4—>NH4+ -be . The older view that the ammonium exists as a free radical, NH4, seems less likely. Such a radical would be unstable and it would not be expected to have such a long life. Ammonium ions, however, are more stable. 

Some ionizing solvents are of major importance in analytical chemistry whilst others are of peripheral interest. A useful subdivision is into protonic solvents such as water and the common acids, or non-protonic solvents which do not have protons available. Typical of the latter subgroup would be sulphur dioxide and bromine trifluoride. Non-protonic ionizing solvents have little application in chemical analysis and subsequent discussions will be restricted to protonic solvents. Ionizing solvents have one property in common, self-ionization, which reflects their ability to produce ionization of a solute some typical examples are given in table 3.2. Equilibrium constants for these reactions are known as self-ionization constants. 

Spontaneous ionization requires both good leaving groups and that the resulting carbenium ions are sufficiently stable. For example, although primary triflates are very stable covalent species which do not self-ionize, secondary triflates with phenyl substitents are very reactive and spontaneously ionize. The ionization equilibrium of styryl triflate could not be established because of side reactions such as Friedel-Crafts alkylation [56], On the other hand, methoxymethylium triflate is partially ionized with equilibrium constants Kj = 5-10 4 at 10° C and Kt = 210 4 at -70° C in S02 [57]. In this system, ionization is endothermic. Secondary triflates with alkoxy substituents, such as those in polymerizations of vinyl ethers, are apparently more strongly ionized than their primary counterparts [58,59],... 

Olah (33) has shown that alkyl fluorides undergo a self-condensation reaction (eq. 10) similar to that previously described for the ionization and condensation of methane so that this additional pathway is not an unexpected one. 

The properties of all other hydrogen hahdes are far removed from those of AHF and, therefore, their use as nonaqueous solvents is rather limited. Nevertheless, they are used for physical studies of solutions and for some synthetic purposes, such as in the formation of salts with HCl2 or BCU and related anions. Anhydrous HX can be considered to undergo self-ionization (equation 90) and can therefore be used to perform reactions of an acid-base nature, and solvolytic and redox reactions (equations 91-93). 

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