Principle of Microscopic ReversibilityMicroscopic Reversibility
The Principle of Microscopic Reversibility
General Information
The Principle of Microscopic Reversibility can be stated in several different ways:
If a certain series of steps constitutes the mechanism of a forward reaction, the mechanism of the reverse reaction (under the same conditions) is given by the same steps traversed backwards.
(Note: The phrase "under the same conditions" means that this applies only to thermal reactions, not photochemical ones.)
The sequence of transition states and reactive intermediates in the mechanism of a reversible reaction must be the same, but in reverse order, for the backward reaction as for the forward reaction.
If the mechanism in one direction is known, then the mechanism in the opposite direction is known.
The lowest-energy pathway in the forward direction will be the lowest-energy pathway in the reverse direction.
Let's illustrate this with an example of a generic organometallic complex, LnM(A)(B) undergoing reductive elimination of AB:
In the example shown above, the activation energy, f in the forward direction is 20 kcal/mol. If we wish to do the reverse reaction, oxidative addition of AB to LnM, then we have to surmount a higher barrier with a r of 30 kcal/mol. This does not mean that the reverse reaction does not or can not occur, only that it is thermodynamically disfavored.
In other cases, the relative energies of the products and reactants might make the forward activation barrier higher in energy or even equal to that of the reverse reaction:
Reaction Mechanisms of Inorganic and Organometallic Systems, 3rd Edition
Robert B. Jordan / Hardcover / Published 2007 / 521 pages
Easily understandable advanced undergraduate to graduate-level text dealing with kinetics, ligand substitution, fluxional processes, mechanisms (and skepticism!), electron transfer, photochemistry, experimental methods, solvent exchange, orbital symmetry rules, C-H activation and more. Comes with 900+ references and sample problems.
Self-Test
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f in the forward direction is 20 kcal/mol. If we wish to do the reverse reaction, oxidative addition of AB to LnM, then we have to surmount a higher barrier with a 