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Related Concept Videos

Reaction Mechanisms03:06

Reaction Mechanisms

Chemical reactions often occur in a stepwise fashion, involving two or more distinct reactions taking place in a sequence. A balanced equation indicates the reacting species and the product species, but it reveals no details about how the reaction occurs at the molecular level. The reaction mechanism (or reaction path) provides details regarding the precise, step-by-step process by which a reaction occurs.
For instance, the decomposition of ozone appears to follow a mechanism with two steps:
Rate-Determining Steps03:08

Rate-Determining Steps

Relating Reaction Mechanisms
In a multistep reaction mechanism, one of the elementary steps progresses significantly slower than the others. This slowest step is called the rate-limiting step (or rate-determining step). A reaction cannot proceed faster than its slowest step, and hence, the rate-determining step limits the overall reaction rate.
The concept of rate-determining step can be understood from the analogy of a 4-lane freeway with a short-stretch of traffic-bottleneck caused due to...
Multi-Step Reactions02:31

Multi-Step Reactions

Chemical reactions often occur in a stepwise fashion involving two or more distinct reactions taking place in a sequence. A balanced equation indicates the reacting species and the product species, but it reveals no details about how the reaction occurs at the molecular level. The reaction mechanism (or reaction path) provides details regarding the precise, step-by-step process by which a reaction occurs. Each of the steps in a reaction mechanism is called an elementary reaction. These...
Reaction Mechanisms: Rate-limiting Step Approximation01:29

Reaction Mechanisms: Rate-limiting Step Approximation

The rate-determining step, or RDS, in a chemical reaction is the slowest step that determines the overall reaction rate. It is identified by using the observed rate law and typically involves approximation methods like the RDS approximation or the steady-state approximation.In the RDS approximation, also known as the rate-limiting-step or equilibrium approximation, the reaction mechanism consists of one or more reversible reactions near equilibrium, followed by a slower RDS, and then one or...
Reaction Mechanisms: The Steady-State Approximation01:26

Reaction Mechanisms: The Steady-State Approximation

The steady-state approximation, also referred to as the quasi-steady-state approximation to differentiate it from a true steady state, is a widely used method for simplifying calculations in complex reaction mechanisms. This approach is particularly useful when dealing with multi-step reactions that involve reverse reactions or several steps, which can significantly increase mathematical complexity and make the reactions nearly unsolvable analytically.The steady-state approximation operates on...
Consecutive Reactions01:22

Consecutive Reactions

Consecutive reactions involve a sequence where the product of a preceding reaction becomes the reactant for the subsequent one. In a simple scheme, A transforms into B, which further reacts to form C, with rate constants k1 and k2, respectively. This concept is evident in the radioactive decay series. Assuming an initial state with only A present, the conservation of matter leads to three coupled differential equations, determining the concentrations of A, B, and C over time.The rate of change...

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Related Experiment Video

Updated: Jul 8, 2026

Spin Saturation Transfer Difference NMR (SSTD NMR): A New Tool to Obtain Kinetic Parameters of Chemical Exchange Processes
11:44

Spin Saturation Transfer Difference NMR (SSTD NMR): A New Tool to Obtain Kinetic Parameters of Chemical Exchange Processes

Published on: November 12, 2016

Apparent Kinetic Isotope Effects for Multi-Step Steady-State Reactions.

Ian H Williams1

  • 1Department of Chemistry, University of Bath, Bath BA2 7AY, U.K.

The Journal of Physical Chemistry. B
|March 27, 2025
PubMed
Summary
This summary is machine-generated.

A new method simplifies calculating the apparent kinetic isotope effect (KIE) for multistep reactions. This approach uses transition states and their kinetic significance, offering a clearer alternative to enzyme reaction analysis.

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Last Updated: Jul 8, 2026

Spin Saturation Transfer Difference NMR (SSTD NMR): A New Tool to Obtain Kinetic Parameters of Chemical Exchange Processes
11:44

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Isotopic Effect in Double Proton Transfer Process of Porphycene Investigated by Enhanced QM/MM Method

Published on: July 19, 2019

Area of Science:

  • Chemical kinetics
  • Reaction mechanisms
  • Computational chemistry

Background:

  • Kinetic isotope effects (KIEs) are crucial for elucidating reaction mechanisms.
  • Analyzing KIEs in multistep reactions, especially enzyme-catalyzed ones, can be complex.
  • Conventional methods often rely on intrinsic KIEs, which may be difficult to determine accurately.

Purpose of the Study:

  • To develop a simplified method for expressing and analyzing apparent KIEs in multistep steady-state reactions.
  • To provide a more direct comparison between computed and experimental KIEs.
  • To illustrate the application of the method using computational chemistry.

Main Methods:

  • Formulating the apparent KIE as a sum of terms, each representing a transition state (TS).
  • Each term is a product of the individual TS's KIE and its kinetic significance (weighting factor).
  • Utilizing density functional theory (DFT) calculations for an SN1 nucleophilic displacement reaction.

Main Results:

  • The apparent KIE can be expressed using relative Gibbs energies of sequential TSs, avoiding intermediate information.
  • A single apparent KIE value can arise from various combinations of individual KIEs and weighting factors.
  • DFT calculations show the apparent KIE varies with nucleophilic species concentration, reflecting rate-limiting steps.

Conclusions:

  • The proposed method offers a simpler and potentially more reliable approach to KIE analysis compared to conventional methods.
  • Direct comparison of computed apparent KIEs with experimental data is recommended over using derived intrinsic KIEs.
  • The study demonstrates the utility of the method for understanding rate-limiting steps in complex reactions.