Rate-determining step change

A bell-shaped pH-rate profile can also be produced in a two-step reaction involving a single ionizable group if the rate-determining step changes when the pH is altered. An example, the oximation of acetone, is shown in Fig. 5-12. 

With the buffer ratio held constant ([B-]/[BH] = 0.5), and the ionic strength maintained at 1 mol dm"3 with potassium chloride, initial rates show saturation kinetics over the range 0.01 < [B ] < 0.25 mol dm 3. This behaviour is compatible with the intermediacy of the carbanion 13, and the rate-determining step changing from its rate-limiting formation at low buffer concentrations to rate-limiting expulsion of quinuclidine from the carbanion, formed rapidly and reversibly, at high buffer concentrations [21]. 

The aminolysis of a series of. S -2,4-dintirophcnyl 4-Y-benzoates (60 Y = Me, H, Cl, NO2) with a series of substituted pyridines at 25 °C in 44% ethanol-water proceeded stepwise through a zwitterionic tetrahedral intermediate, the rate-determining step changing from its breakdown to its formation as the basicity of the amine increased.49 However, the aminolysis of the same series of benzoates (60) with secondary alicyclic amines in the same solvent was judged to be concerted, and the reasons for the proposed change in mechanism were discussed.50... 

One of the most important reactions in materials chemistry is that of the dissolution of iron. However, the mechanism of this is rather complex because the rate-determining step changes with conditions, even, e.g., with stress (which may be very localized in real-life specimens). On the other hand, the dissolution mechanisms often involve an intermediate such as FeOH+, which allows some guidance in the following complex traffic. 

One of the main questions in the cobalt(III)-promoted hydrolysis of activated amino acid esters is whether the ratedetermining step is addition of hydroxide to the carbonyl carbon, or loss of the alkoxide from the intermediate. Work with /3-alanine ester showed that below pH 8.5 the ratedetermining step was the elimination of alkoxide. At pH 10 and above, the rate-determining step changes and the addition of hydroxide to the activated ester becomes the rate-controlling step. This is due to the fact that above pH 10 the hydroxyl group of the intermediate becomes deprotonated (equation 7). The deprotonation of the hydroxyl group accelerates the loss of alkoxide by 10 times. ... 

The break-point temperature in dehydration (above which the rate was temperature insensitive) matched the maximum temperature for dehydrogenation, suggesting that a common intermediate exists for each reaction, and that the product selectivity is determined by interactions with other molecules and the surface. Above 650 K, the catalytic dehydration channel dominates, but the rate-determining step changes above 700 K. Below 700 K, the reaction rate is nearly independent of the partial pressure of formic acid (ca. 0.2 order). Above 700 K, the rate of the reaction is essentially independent of temperature, implying that reaction is limited by formic acid adsorption and dissociation thus, above 700 K, the rate becomes first-order with respect to the partial pressure of formic acid. Higher pressures of formic acid over the crystal surface should therefore increase the transition temperature - this behavior was observed by Iwasawa and coworkers, and the turnover frequency for catalytic dehydration approached the collision frequency of formic acid at high... 

Evidently, although the system tolerates both aromatic and aliphatic aldehydes, the introduction of an electron-withdrawing substituent on the aromatic substrate results in a decreased yield. To gain information about the mechanism of the overall tandem reaction, kinetic studies were carried out to identify the rate-determining step. Changing the catalyst concentration in the reaction between 3-methylbutyraldehyde, nitromethane and dimethyl malonate revealed that the reaction is first order in nickel catalyst, indicating that the Michael addition of dimethyl malonate to the nitroalkene is the ratedetermining step. 

Solvent effects are essentially free-energy correlations [57] and we shall omit in this chapter those of strongly acidic media as these are clearly not pertinent to biochemistry. The effect of variation of solvent on a rate or equilibrium constant may be treated in the same way as a substituent effect. We may apply the same criteria to elucidate mechanistic or rate-determining step changes to free-energy relationships with solvent effects. Let us consider the hydrolysis of 5-phenyl-oxazoline-2-ones... 

The difference now is that at low pH, the rate-determining step changes from being the dehydration step (which can then go very fast because of the high concentration of acid) to being the addition step, which is slowed down by protonation of the amine. Whereas a reaction will always go by the fastest of the available mechanisms, it is also bound to go at the rate... 

As with simple imines, the identity of the rate-determining step changes with solution pH. As pH decreases, the rate of the addition step decreases because protonation of the amino compound reduces the concentration of the nucleophilic unprotonated form. Thus, while the dehydration step is normally rate-determining in neutral and basic solution, addition becomes rate-determining in acidic solutions. 

Use of a rate-determining step changed a complex rate law, for which an order could not be defined, into a relatively simple law of order three. It is for this reason that rate laws which can be assigned integral orders are so often encountered. 

Figure 10 Theoretical variations at a fixed potential of the HER current, the surface coverage 6jj, and the bulk fractional concentration with the H adsorption free energy AGadg for two HER mechanisms (a) electroadsorption step in quasi-equilibrium (a case where the rate-determining step changes from electrocombination to chemical combination) (b) electroadsorption coupled with chemical combination. The symmetry factors are all taken equal to 1/2. The represented case is for Langmuir type H adsorption.

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