A method for quantifying how the energy of specific intermediates or transition states dictate overall reaction rate in multistep reactions has been developed
Microkinetic models describe multistep reactions using all of their component elementary processes and respective rate constants to provide information about the reaction's macroscopic behaviour. Such analysis can be used to find optimal reaction conditions efficiently. These complex systems often have elementary steps that are more important than others in defining overall reaction rate. A method to measure this — by quantifying how changing the rate constant of one step influences the overall reaction rate — has already been developed by Charles Campbell of the University of Washington, Seattle.
Now, Campbell and colleagues from Denmark have extended1 this method so it can be used to identify the transition states and intermediates that control the overall rate. The method defines a parameter that conveys how the overall reaction rate changes when the free energy of a specific transition state or intermediate is changed — stabilizing or destabilizing the state — and all other reaction free energies are kept constant.
This creates the possibility of adjusting the overall reaction rate by stabilizing the important transition states or destabilizing the key intermediates. Though such changes are difficult to achieve in practice, Campbell suggests that relative stabilities could be altered by changing the solvent or the reactant's molecular structure.
References
Stegelmann, C., Andreasen, A. & Campbell, C. T. Degree of rate control: how much the energies of intermediates and transition states control rates. J. Am. Chem. Soc. 10.1021/ja9000097 (2009).
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Armstrong, G. Locating key states. Nature Chem (2009). https://doi.org/10.1038/nchem.233
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DOI: https://doi.org/10.1038/nchem.233