Demonstration of Intermediates

The build-up of measurable concentrations of an intermediate in a stepwise reaction does not occur in the majority of cases and the concentration can be very small compared with that of reactant and product. Under these conditions, the Bodenstein steady-state assumption is valid and Equation (2), the mechanism involving an intermediate, can be solved to give Equation (3). 

In this case we employ Hammett terminology but any free energy relationship such as a Bronsted or a Swain -Scott equation could be used. 

Similar equations govern a plot where the individual steps of Equation (2) obey Bronsted equations (see Equations 11 and 12). logA = P a -I- C (n = 1,-1 or 2 corresponding to A , and Ztj)- 

Alternative definitions are an intermediate is diagnosed if the observed rate constant on one side of the break-point in a non-linear free energy relationship is less than that calculated from the correlation on the other side. Another definition is that a stepwise mechanism gives a non-linear correlation with a convex upwards curvature to the free energy line in this case the sense of convexness must be defined. 

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Hoefler H, Denk H, Lackinger E, et al. Immunocytochemical demonstration of intermediate filament cytoskeleton proteins in 78. human endocrine tissues and (neuro-) endocrine tumors. Virch... 

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