Hammett plots downward deviations

Paradoxically, Hammett plots are usually most informative at the very point at which they depart from linearity, but the major inference that can be drawn from this departure is found to differ depending on whether the deviation is concave upwards or concave downwards . 

There are, however, also examples of deviations from simple Hammett plots in which the curvature is in the opposite direction, concave downwards, and these deviations have a rather different significance. 

This happens in Fig. 13.8, as we have seen, with the compound (38 X, Z = Me). On each side of this maximum the, different, rate-limiting step will be slowing down progressively, and so therefore will the overall reaction rate. Shifts in rate-limiting step, within the same overall reaction pathway, are thus distinguished by concave downwards deviations in Hammett plots this in contrast to the concave upwards deviations which, as we have already seen (p. 364), are characteristic of a change in overall reaction pathway. 

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