Eigen plot

It is of interest to consider the form of the Bronsted plot or Eigen plot to be expected for reaction of a series of related intramolecularly hydrogen-bonded acids with hydroxide ion by the mechanisms in Schemes 5 and 6. The effect of a substituent on the value of the dissociation constant of an intramolecularly hydrogen-bonded acid (8 log K) will be two-fold. The stability of the undissociated acid will be modified because of a substituent effect on the... 

The first evidence that two step proton transfer from a hydrogen-bonded acid could occur consisted of Eigen plots for proton removal by buffer bases. The demonstration of a change in the rate-limiting step as in (30) and (31) provides even more clear evidence and permits the calculation of the rate coefficients and equilibrium constant for opening of the hydrogen bond. 

A. C. Amyes, T. L. Formation and stability of enolates of acetamide and acetate anion an Eigen plot for proton transfer at y-carbonyl carbon./. Am. Chem. Soc. 2002, 124, 2957— 2968. 

Figure 13. Eigen plot for proton transfer rates o, (including and 9 ) ,...

The rate-limiting step in proton transfer between electronegative atoms is either /c, the diffusion-controlled encounter of the acid-base pair for thermodynamically favourable transfers or the diffusion-controlled dissociation of the acid-base pair, for thermodynamically unfavourable transfers (Figure 1). The rate-limiting step is rarely the proton transfer within the encounter complex. Eigen plots are predominantly observed in reactions in which proton transfer occurs to or from heteroatoms in reactive intermediates. 

Construct an Eigen plot (see Figure 2, Chapter 6) using Equation (31) and the data of Table 5 for the formation of carbinolamine from 4-methoxybenzaldehyde and methoxyamine catalysed by general acids (AH). ... 

Some carbanions are known in which the negative charge appears to be mostly localized on the carbon atom, and these are protonated at rates approaching the diffusion-controlled limit. Results for the disulphones [26, 27] and several cyano-compounds [28] are shown as an Eigen plot in Figure 2. Values for the disulphone and most of... 

The Bronsted equation can be seen to fit a region of the Eigen plot. If ApK is varied over too wide a region, a is no longer a constant, as Bronsted himself originally cautioned. 

Figure 13.15 Eigen plots for the acid-base catalysis of strong acids following mechanism (13.XXIII). In acid catalysis in the forward direction, HA is the catalyst and varies along the reaction series, while the substrate B is kept constant. In the forward direction, the change in HA gives the Bronsted coefficient for acid catalysis, and for the reverse direction the change in A" gives...

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