Proton transfer to bases

Rate coefficients fej and k x for reactions such as (43), (44) and (45) where B represents an oxygen or nitrogen base, including solvent species and buffer components, will be discussed here. When equilibrium constants are available, values for rate coefficients in both directions can be obtained. 

In 1924 Bronsted and Pedersen [67] found that base catalytic coefficients (feB) which they had measured for a large number of bases in the decomposition of nitramide were related to the base strength of the catalysts by eqn. (46) 

Since 1924, the Bronsted relation has been applied to many general acid and base catalysed reactions, such as those discussed in Sect. 2.2, as well as to proton transfer equilibria like (43)—(45). Over limited ranges of acid strength and for variation within a similar catalyst type, G and a or j3 are constant and the relation holds well. Different catalyst types in a reaction often do not fit on a single Bronsted plot, but give different Bronsted lines. This was observed for the decomposition of nitramide [68]. It has also been observed in proton transfer from l,4-dicyano-2-butene(51) 

It has been observed that over wide pK ranges the Brdnsted exponents a and (3 vary and Brdnsted plots are curved. Brdnsted and Pedersen expected this, since it is obvious that the rate cannot increase indefinitely with the strength of the base catalyst. They also expected that a and (3 

The Bronsted relation has proved to be a useful equation for correlating rate and equilibrium results for proton transfer reactions. However, following the analysis by Leffler and Grunwald [73] in 1963 considerable effort has been made to go further than this and understand why the relation should hold, and also to attach some significance to the values of a and 3 in terms of the structure of the transition state for proton transfer. An alternative approach from that to be discussed here interprets the Bronsted relation from molecular potential energy diagrams [74]. 

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Proton transfer studies, however, show that as with normal protonic acids, direct proton transfer to base B... 

This is an A2 mechanism with proton transfer to base in the second step. A typical example is the acid catalyzed enolization of ketones [17, 26). 

The main reaction in MeCN occurs through a base-catalyzed pathway involving formation of a zwitterionic intermediate, equilibrium formation of an anionic intermediate and a rate-limiting proton transfer to base, rate constant kc, followed by a fast leaving-group expulsion. The corresponding reactions in DMSO, however, are found to proceed by both uncatalyzed (via kh) and catalyzed pathways. Similar reactions in benzene show that the Hammett coefficient determined with substituted anilines for the catalyzed path is extremely large (p = —7.7) relative to that for the uncatalyzed path (p = —4.7). The Bronsted fix value (which may, however, be unreliable since the p fa(H20) values are... 

The mechanism of this reaction is outlined m Figure 17 8 It is analogous to the mech anism of base catalyzed hydration m that the nucleophile (cyanide ion) attacks the car bonyl carbon m the first step of the reaction followed by proton transfer to the carbonyl oxygen in the second step... 

As pointed out in Chapter 4, the first step in the reaction is proton transfer to the alcohol from the hydrogen halide to yield an alkyloxonium ion. This is an acid-base reaction. 

FIGURE 16.12 Catalysis of nitrophenylacetate hydrolysis by imidazole—an example of general base catalysis. Proton transfer to imidazole in the transition state facilitates hydroxyl attack on the substrate carbonyl carbon. 

We see that an acid and a base react, through proton transfer, to form another acid and another base ... 

However, the aminoazo product is formed via two pathways. The first is through the 1 1 addition complex (HAArNj )n as side-equilibrium and an intermolecular rearrangement involving redissociation of this complex into the reagents followed by formation of another 1 1 addition complex (HAArNJ )c and the classical C-o-complex (oc in Scheme 13-13). The second pathway starts from the first mentioned 1 1 complex (HAArNJ )N to which a second molecule of amine is added. This complex forms the aminoazo product by proton transfer to a base. The base may be the second amine molecule of the 1 2 complex. 

STRATEGY Because NH4+ is a weak acid and Cl- is neutral, we expect pH < 7. We treat the solution as that of a weak acid, using an equilibrium table as in Toolbox 10.1 to calculate the composition and hence the pH. First, write the chemical equation for proton transfer to water and the expression for Ca. Obtain the value of Ka from Kh for the conjugate base by using K, = KxJKh (Eq. 11a). The initial concentration of the acidic cation is equal to the concentration of the cation that the salt would produce if the salt were fully dissociated and the cation retained all its acidic protons. The initial concentrations of its conjugate base and H30+ are assumed to be zero. 

Besides these generalities, little is known about proton transfer towards an electrode surface. Based on classical molecular dynamics, it has been suggested that the ratedetermining step is the orientation of the HsO with one proton towards the surface [Pecina and Schmickler, 1998] this would be in line with proton transport in bulk water, where the proton transfer itself occurs without a barrier, once the participating molecules have a suitable orientation. This is also supported by a recent quantum chemical study of hydrogen evolution on a Pt(lll) surface [Skulason et al., 2007], in which the barrier for proton transfer to the surface was found to be lower than 0.15 eV. This extensive study used a highly idealized model for the solution—a bilayer of water with a few protons added—and it is not clear how this simplification affects the result. However, a fully quantum chemical model must necessarily limit the number of particles, and this study is probably among the best that one can do at present. 

Numbers used in this cycle AG° for addition of water to give a monoanionic adduct, Table 1.7 AG° for proton transfer reactions, based on pAT, values estimated by the method of Branch and Calvin.) The dissociative energy is taken from Table 1.7. 

Simple electron transfer at electrodes Proton transfer to cyanocarbon bases... 

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