Acid-base equilibria proton-transfer reactions

C17-0128. Pure sulfuric acid (H2 SO4) is a viscous liquid that causes severe bums when it contacts the skin. Like water, sulfuric acid is amphiprotic, so a proton transfer equilibrium exists in pure sulfuric acid, (a) Write this proton transfer equilibrium reaction, (b) Construct the Lewis stmcture of sulfuric acid and identify the features that allow this compound to function as a base, (c) Perchloric acid (HCIO4) is a stronger acid than sulfuric acid. Write the proton transfer reaction that takes place when perchloric acid dissolves in pure sulfiaric acid. 

Note that A is called the conjugate base of HA and BH+ the conjugate acid of B. Proton transfer reactions as described by Eq. 8-1 are usually very fast and reversible. It makes sense then that we treat such reactions as equilibrium processes, and that we are interested in the equilibrium distribution of the species involved in the reaction. In this chapter we confine our discussion to proton transfer reactions in aqueous solution, although in some cases, such reactions may also be important in nonaqueous media. Our major concern will be the speciation of an organic acid or base (neutral versus ionic species) in water under given conditions. Before we get to that, however, we have to recall some basic thermodynamic aspects that we need to describe acid-base reactions in aqueous solution. 

Most proton transfer reactions are fast they have been carefully studied by relaxation methods. A system consisting of a conjugate acid-base pair in water is a three-state cyclic equilibrium as shown in Scheme IV. [The symbolism is that used by Bemasconi. ... 

What Are the Key Ideas Bronsted acids are proton donors Bronsted bases are proton acceptors. The composition of a solution of an acid or base immediately adjusts to satisfy the values of the equilibrium constants for all the proton transfer reactions taking place. 

For cryptands in which the molecular cavity is larger than in the case of the [l.l.l]-species [78], proton transfer in and out of the cavity can be observed more conveniently. Proton transfer from the inside-monoprotonated cryptands [2.1.1] [79], [2.2.1] [80], and [2.2.2] [81 ] to hydroxide ion in aqueous solution has been studied by the pressure-jump technique, using the conductance change accompanying the shift in equilibrium position after a pressure jump to follow the reaction (Cox et al., 1978). The temperature-jump technique has also been used to study the reactions. If an equilibrium, such as that given in equation (80), can be coupled with the faster acid-base equilibrium of an indicator, then proton transfer from the proton cryptate to hydroxide ion... 

When the reference acid, HA, is water, we can set up a scale of base strengths from the equilibrium constant, Kb, measured for the proton-transfer reaction shown in Equation 23-3 ... 

Proton transfers in solution reach equilibrium very rapidly and for all weak acids and bases, we have to consider the reverse proton transfer reaction as well as the forward reaction. For example, the CN ion produced when HCN loses a proton to water can accept a proton from a water molecule and form HCN again. Therefore, according to the Bronsted definition, CN- is a base it is called the conjugate base of HCN. In general, a conjugate base is the species left when an acid donates a proton ... 

The key to solving acid-base equilibrium problems is to think about the chemistry—that is, to consider the possible proton-transfer reactions that can take place between Bronsted-Lowry acids and bases. 

The absence of scatter in a Bronsted plot for a general base-catalysed reaction can imply that the reaction mechanism involves a rate-limiting proton transfer step. This is because proton transfer to the base in the reaction is closely similar to the equilibrium proton transfer to the base in the reaction which defines the p Ka of the conjugate acid of that base. The observation of scatter, especially for sterically hindered bases (such as 2,6-dimethylpyridine), is evidence that nucleophilic catalysis is operating as opposed to general base catalysis. 

The high pA"a for HNO would normally not be expected to entirely preclude reactivity of NO- at neutral pH. However, the HNO/NO pair is unique in that proton transfer requires a spin change and that both species are consumed by rapid self-dimerization [(168) 8 x 106M 1 s 1 for Eq. 3 (106)]. The intersystem crossing barrier slows proton transfer by as much as seven orders of magnitude (169) thus allowing dimerization (and other reactions) to not only become competitive with, but to exceed, the rate of proton transfer. Thus for the HNO/ NO pair, an acid-base equilibrium has little relevance the chemistry is instead dependent on the forward and reverse rate constants for proton transfer relative to consumption pathways. 

On excitation the phenolic group becomes more strongly acidic while the carboxylic acid group becomes a stronger base and proton exchange then occurs between the two. In confirmation of this explanation, methyl salicylate was found to behave similarly, but methyl 2-methoxybenzoate, with no transferable proton, showed a normal Stokes shift. Quenching experiments demonstrated that at room temperature the proton transfer reaction reached equilibrium within the lifetime of the excited state. 

Acid-base equilibrium — Using the Bronsted-Lowry definition (see -> acid-base theories), an acid-base reaction involves a -> proton transfer from an acid to a base. Removal of a proton from an acid forms its conjugate base, while addition of a proton to a base forms its conjugate acid. Acid-base equilibrium is achieved when the -> activity (or -> concentration) of each conjugate... 

Sections 3.3.1 and 4.2.1 dealt with Bronsted acid/base equilibria in which the solvent itself is involved in the chemical reaction as either an acid or a base. This Section describes some examples of solvent effects on proton-transfer (PT) reactions in which the solvent does not intervene directly as a reaction partner. New interest in the investigation of such acid/base equilibria in non-aqueous solvents has been generated by the pioneering work of Barrow et al. [164]. He studied the acid/base reactions between carboxylic acids and amines in tetra- and trichloromethane. A more recent compilation of Bronsted acid/base equilibrium constants, determined in up to twelve dipolar aprotic solvents, demonstrates the appreciable solvent influence on acid ionization constants [264]. For example, the p.Ka value of benzoic acid varies from 4.2 in water, 11.0 in dimethyl sulfoxide, 12.3 in A,A-dimethylformamide, up to 20.7 in acetonitrile, that is by about 16 powers of ten [264]. 

When a base catalyzed reaction with proton transfer in the first step is carried out in D20 solution, the substrate exchanges its acidic hydrogen with the deuterium of the solvent before the reaction takes place if the mechanism is fast pre-equilibrium proton transfer with subsequent slow step. If, on the other hand, hydrogen exchange does not occur prior to the reaction, it may be concluded that proton transfer is the rate-determining step. 

A proton transfer reaction represents an equilibrium. Because an acid donates a proton to a base, thus forming a conjugate acid and conjugate base, there are always two acids and two bases in the reaction mixture. Which pair of acids and bases is favored at equilibrium The position of the equilibrium depends on the relative strengths of the acids and bases. 

In proton transfer reactions, equilibrium favors the weaker acid and weaker base (2.4). 

A few points should be noted. (1) Bases may be anionic or neutral, and acids may be neutral or cationic. (2) The acid-base reaction is an equilibrium. The equilibrium may lie far to one side or the other, but it is still an equilibrium. (3) There is both an acid and a base on both sides of the equilibrium. (4) This equilibrium is not to be confused with resonance. (5) Proton transfer reactions are usually very fast, especially when the proton is transferred from one heteroatom to another. 

Calculation of the equilibrium constant for the proton transfer reaction is easy. Proton transfers go toward the formation of the weaker base. Let s use the reaction of hydroxide with hydrochloric acid as an example. The reaction goes from the stronger base, hydroxide, p/fabH 15 7, to the weaker base, chloride, pifabH 7. 

In 1923, Broensted was the first to develop an acid-base concept that was no longer related to substances, but rather to the function of particles. Acids are proton donors and are capable, with suitable reaction partners, to donate protons to base particles or proton acceptors, i.e. protolysis or proton transfer reaction. For example, HC1 molecules, as acid particles, transfer protons when colliding with H20 molecules (see Fig. 7.3). In this sense, the proton donors of pure sulfuric acid are H2S04 molecules, the acid particles of the sulfuric acid solution are the hydronium ions (or also the hydrogen sulfate ions in concentrated solutions). In weak acids, the protolysis equilibrium is to be considered, equilibria and their constants are well defined. 

If excited-state proton transfer is much slower from fluorescence, the fluorescence intensity-versus-pH curve will be exactly like the absorbance-versus-pH curve (i.e., no excited-state proton transfer occurs). If excited-state proton transfer is much faster than fluorescence, the fluorescence intensity-versus-pH curve will reflect the acid-base equilibrium in the lowest excited singlet state and the dissociation constant taken from the fluorescence intensity-versus-pH curve will be that of the excited-state reaction. Equilibrium in the excited state is a rare phenomenon and will not be dealt with further here. 

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