Eigen mechanism plots

Another approach used to interpret curvature of Br0nsted plots has been given by Murdoch (1972). This model, which incorporates Marcus theory, shows that the diffusive steps (10a, c) of the three-stage Eigen mechanism can also influence curvature. It is shown mathematically that increased difficulty of diffusion has the same... 

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... 

Bronsted plots with some scatter should always be examined carefully because they might conform with the Eigen equation. A classic case is the acid-catalysed reaction of aniline with HNCO which can be fitted to a shallow Bronsted line (slope = —0.19)[9] however, the data are also consistent with an Eigen dependence (Fig. 11.5B) arising from a mechanism involving trapping of an addition intermediate by proton transfer (Scheme 11.11). 

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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