Autoprotolysis equilibrium

Ammonolysis of 2-chlorobenzothiazole in liquid ammonia was studied by Lemons et al. and found to be approximately first-order with respect to this substrate at the fairly high concentrations used. The actual nucleophilic reagent was, as expected, the neutral species NH3, and reaction via the amide ion NH2 arising from the autoprotolysis equilibrium [Eq. (5)] was excluded on the grounds that addition of ammonium chloride did not depress the reaction rate. In accordance with this interpretation and in connection with the existence of aromatic substitutions other than normal it is of interest that 2-chlorobenzothiazole was found to react difiFerently with sodamide, although the products were unidentified in this case. 

Now we come to a very important point that will be the basis of much of the discussion in this chapter and the next. Because Kw is an equilibrium constant. the product of the concentrations ofHjO+ and OH ions is always equal to Kw. We can increase the concentration of H30+ ions by adding acid, and the concentration of OH ions will immediately respond by decreasing to preserve the value of Kk. Alternatively, we can increase the concentration of OH ions by adding base, and the concentration of H30 ions will decrease correspondingly. The autoprotolysis equilibrium links the concentrations of H30+ and OH" ions rather like a seesaw when one goes up, the other must go down (Fig. 10.10). 

In aqueous solutions, the concentrations ofH O and OH ions are related by the autoprotolysis equilibrium if one concentration is increased, then the other must decrease to maintain the value of KIV. 

A very, very small concentration of OH ions, which maintain the autoprotolysis equilibrium. 

Apart from water, there are four species in a solution of a weak acid HA, namely, HA, A-, H30+, and OH- so we need four equations to find their concentrations. Two relations that we can use are the autoprotolysis equilibrium of the solvent and the proton transfer equilibrium of the acid HA ... 

Moderately activated aromatic compounds can be nitrated only with concentrated nitric add. Under these conditions NC)3 and H2N03 are produced in the autoprotolysis equilibrium (Figure 5.17). The N02 cation (nitronium ion), which is a considerably better electrophile than an HN03 molecule, is formed by eliminating a molecule of H20 from H2N03. Such a nitronium ion initiates, for example, the following reaction ... 

The second difficulty stems from the autoprotolysis equilibrium of the water or, in other words, from the fact that dihydrogen may form at the interface between the electrode and the electrolyte due to the reduction of and/or of H2O (see section 2.1.2.3). In some operating conditions, a priori this leads to non-linear concentration profiles for the H" and OH" ions throughout the diffusion layer, unlike the typical situation found for unidirectional-diffusion steady states. 

The following autoprotolysis equilibrium has been assumed for fluorosulphuric... 

In these equilibria, A is the conjugate base of the acid HA, and BH+ is the conjugate acid of the base B. Even in the absence of added acids and bases, proton transfer occurs between water molecules, and the autoprotolysis equilibrium ... 

The calculation of the pH of a solution of a base involves an additional step. The first step is to calculate the concentration of OH ions in the solution from the value of by using the equilibrium-table technique and to express it as the pOH of the solution. The additional step is to convert that pOH into a pH by using the water autoprotolysis equilibrium, eqn 4.27, in the form pH = - pOH, with... 

Acceptor molecules or Lewis acids are, nevertheless, important in a discussion of Bronsted acids. Indeed, in some cases the addition of a Lewis acid to a solvent is the only known way in which the concentration of the solvated proton can be increased, i.e., in which an acid solution in the Bronsted sense can be obtained. The Lewis acid disturbs the autoprotolysis equilibrium of the protonic solvent by combining with the characteristic anion, thus diminishing its concentration and accordingly raising the concentration of the characteristic cation for instance ... 

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