Protolytic Equilibria Acid-Base Reactions

The equilibrium constant of this reversible proton transfer reaction is 

The acidity (as defined by the pK NB a strong acid has a low p.K) of a molecule is strongly influenced by its electrostatic charge distribution, and this is not surprising since the partial negative charge on the O atom in 

The Forster cycle therefore permits the calculation of the excited state pA (from AH ) simply from spectroscopic data (assuming of course that the ground state pK is known, from usual titration measurements). 

The pA value derived from the Forster cycle is however a theoretical5 value, two important assumptions being involved in its derivation  

The deprotonation time is therefore often so long that protolytic equilibrium cannot become established during the lifetime of singlet excited states (say 

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If it is accepted that reactions catalyzed by acids or bases involve at some stage a slow acid-base reaction, then the Bronsted relation (cf. Sec. II.4) assumes a reasonable aspect. The constants used to express the strengths of the catalyzing species are usually defined with reference to an equilibrium with some standard acid-base system such as the solvent, but they could in principle be defined in terms of the (hypothetical) protolytic equilibrium between the catalyst and the substrate. The Bronsted relation then amounts to a parallelism between the rates and equilibrium constants of a series of similar reactions. The general form of the relation can in fact be inferred without any reference to a molecular interpretation. Suppose that we have any acid-base equilibrium... 

This question of equilibration of the protonation and deprotonation processes leads to another fundamental problem in the case of excited state reactions between which states can a protolytic equilibrium be at all established A molecule has only one ground state, so there can be no ambiguity about the thermal protolytic equilibrium which connects of course the ground states of the acid and base forms. However, there are many excited states of both these forms, excited states which can differ greatly in electron distribution (e.g. mr and 7T7T states) or even in multiplicity (e.g. singlet and triplet states). 

Acid-base indicators for titrations in nonaqueous solvents are normally weak protolytes. For dissociation model I as given in Table 1, the acid-base equilibrium of a weak acid type of indicator in the pure solvent can be represented in water by the following reaction ... 

Polarographic data yield ki2 = 1.3 X lO W" sec, which agrees well with specific rates of similar reactions shown in Table II. The specific rate kn of the much slower dehydration reaction has been determined by both the temperature and pressure jump methods to be about 0.5 sec at pH 3 and 25 °C with some general acid-base catalysis. While the hydration-dehydration equilibrium itself involves no conductivity change, it is coupled to a protolytic reaction that does, and a pressure jump determination of 32 is therefore possible. In this particular case the measured relaxation time is about 1 sec. The pressure jump technique permits the measurement of chemical relaxation times in the range 50 sec to 50 tisec, and thus complements the temperature jump method on the long end of the relaxation time scale. 

Reaction (II) could be the neutralization of acetic acid by potassium hydroxide, yielding potassium acetate which can be isolated in the crystalline state. On dissolution in water the K+ cation is only hydrated in solution but does not participate in a protolytic reaction. In this way, the weak base CH3COO is quantitatively introduced into solution in the absence of an equilibrium amount of the conjugate weak acid CH3COOH. Thus... 

These equations represent a transfer of a proton from A, (Acid,) to B2 (Base2). Reactions between acids and bases are hence termed protolytic reactions. All these reactions lead to equilibrium, in some cases the equilibrium may be shifted almost completely in one or another direction. The overall direction of these reactions depends on the relative strengths of acids and bases involved in these systems. 

Reactions involving the transfer of a proton are known as protolytic reactions. The equilibrium constants of protolyses allow a comparison to be made of acid and base strengths. The conjugate pair, H3O+, H2O, is used as a standard,... 

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