Change in the rate-determining step

If the reaction is complex, a change in the rate-determining step may take place along the pK scale. Most acyl transfers are thought to take place via a tetrahedral intermediate, as in Scheme II. 

It has been shown that the rate of formation of oximes is at a maximum at a pH that depends on the substrate but is usually 4, and that the rate decreases as the pH is either raised or lowered from this point. We have previously seen (p. 425) that bell-shaped curves like this are often caused by changes in the rate-determining step. In this case, at low pH values step 2 is rapid (because it is acid catalyzed), and step 1... 

Examination of a series of imines of differing electronic properties showed that a change in the rate-determining step of this stoichiometric C=N hydrogenation occurs as the imine becomes more electron-rich. Hydrogenation of N-iso-propyl-(4-methyl)benzilidene amine led to an amine complex of ruthenium. In addition, the C=N hydrogenation was accompanied by isomerization of the imine to a ketimine (Eq. (47)). 

Hydrolyses of Schiff Bases. The behavior of Schiflf bases and their complexes has been sufficiently studied so that the similarities of the kinetic paths of their hydrolyses can be clearly delineated. The reaction is interpreted in terms of a change in the rate determining step as the pH is changed. In neutral solutions the rate determining step is considered (7) the hydration step ... 

The orders in hydroxide anion and hydronium ion are generally found to be equal to 1 [36-39, 41-43]. However, Swain etal. reported that hydroxide anion catalyzed hydrolysis of triphenylmethoxysilane in 40% aqueous acetonitrile is second order in hydroxide anion below 3 x 10 3M [44]. It becomes first order above that concentration. They attributed the change in order to a change in the rate-determining step of the reaction. Eaborn has suggested that the apparent change in order of the hydroxide anion is due to acidic impurities in the acetonitrile, and that the reactions are first order in hydroxide [33]. 

Xie M, Vander Velde D, Morton M, Borchardt RT, Schowen RL. pH-induced change in the rate-determining step for the hydrolysis of the Asp/Asn-derived-cyclic-imide intermediate in protein degradation J Am Chem Soc 1996 ... 

On the other hand, when present in high concentration, H202 can compete with dioxygen in the re-oxidation of the dicopper(I) complex to the dicopper(II) state therefore, the increase in its concentration results in its progressive involvement as a copper(I) oxidant. The increase of the reaction rates in this case can be explained by a change in the rate-determining step of the reaction, as was previously proposed by Casella and co-workers [38]. 

Fig. 7.15. Schematic representation of reaction showing proposed co-ordination number changes in the rate-determining steps for (a) La3+ (b) Gd3+ (c) Yb3+ [22].

When the first step is rate determining, 3 c = Pi when the second step is rate determining, p m = p2. If a change in the rate-determining step occurs over the p range studied, a curved Br nsted-type plot will be observed (unless pi = P2), Eq. (7-66) describing the curve quantitatively. In the special case A i = kz, we have /tohs = kill, letting k = 1 and K = for this case, we get... 

ElcB)i mechanism was the observation of a change in the rate-determining step in the elimination reaction of Al-(2-cyanoethyl)pyridinium... 

---