Reaction mechanisms microscopic reversibility

When the addition and elimination reactions are mechanically reversible, they proceed by identical mechanistic paths but in opposite directions. In these circumstances, mechanistic conclusions about the addition reaction are applicable to the elimination reaction and vice versa. The principle of microscopic reversibility states that the mechanism (pathway) traversed in a reversible reaction is the same in the reverse as in the forward direction. Thus, if an addition-elimination system proceeds by a reversible mechanism, the intermediates and transition states involved in the addition process are the same as... 

We now introduce the principle of microscopic reversibility. This states that the transition states for any pathway for an elementary reaction in forward and reverse directions are related as mirror images. The atoms are in the same places but the momentum vectors are, of course, reversed since in general the transition state is proceeding in one direction only. In other words, the forward and reverse mechanisms are identical, according to this principle. 

Application of the principle of microscopic reversibility can be used to eliminate a mechanism suggested at one time for the nucleophilic substitution reactions of square-planar platinum(II) complexes. For the sake of specificity, we take PtCl - as a typical... 

When the reaction proceeds by this pathway, 29 and similar intermediates are not involved and the mechanism is exactly (by the principle of microscopic reversibility)... 

This reaction is reversible and suitable p-hydroxy alkenes can be cleaved by heat (17-34). There is evidence that the cleavage reaction occurs by a cyclic mechanism (p. 1351), and, by the principle of microscopic reversibility, the addition mechanism should be cyclic too. Note that this reaction is an oxygen analog of the ene... 

There is an extra complication for the associative limit of this reaction. Addition of water to the monoanion would give a species with very acidic hydrogens, so that dissociation must be expected to be concerted. By microscopic reversibility, the very similar leaving group ethanol must depart by an analogous path. Thus the mechanism becomes ... 

An expression for the equilibrium occupancy of pARt can again be obtained using the methods outlined in Chapter 1. A potential complication is that this mechanism contains a cycle, so the product of the reaction rates in both clockwise and counterclockwise directions should be equal in order to ensure the principle of microscopic reversibility is maintained. In this case, microscopic reversibility is maintained. Thus,... 

Guideline 7. A postulated mechanism for a reaction in the forward direction must also hold for the reverse reaction. This guideline is a consequence of the principle of microscopic reversibility. (See Section 4.1.5.4.) Three corollaries of this guideline should also be kept in mind when postulating a reaction mechanism. First, the rate limiting step for the reverse reaction must... 

The "principle of microscopic reversibility", which indicates that the forward and the reverse reactions must proceed through the same pathway, assures us that we can use the same reaction mechanism for generating the intermediate precursors of the "synthesis tree", that we use for the synthesis in the laboratory. In other words, according to the "principle of microscopic reversibility", [26] two reciprocal reactions from the point of view of stoichiometry are also such from the point of view of their mechanism, provided that the reaction conditions are the same or at least very similar. A corollary is that the knowledge of synthetic methods and reaction mechanisms itself -according to the electronic theory of valence and the theory of frontier molecular orbitals- must be applied in order to generate the intermediate precursors of the "synthesis tree" and which will determine the correctness of a synthesis design and, ultimately, the success of it. 

An accurate knowledge of the thermochemical properties of species, i.e., AHf(To), S Tq), and c T), is essential for the development of detailed chemical kinetic models. For example, the determination of heat release and removal rates by chemical reaction and the resulting changes in temperature in the mixture requires an accurate knowledge of AH and Cp for each species. In addition, reverse rates of elementary reactions are frequently determined by the application of the principle of microscopic reversibility, i.e., through the use of equilibrium constants, Clearly, to determine the knowledge of AH[ and S for all the species appearing in the reaction mechanism would be necessary. 

The mechanism of such a reaction, represented in Scheme 3.9, comes directly from the reverse mechanism of ethane metathesis, each step being considered as microscopically reversible. In a clockwise catalytic cycle ethane metathesis occurs, whereas in the counter-clockwise catalytic cycle propane methane-ol-ysis occurs the case with heavier alkanes is of course more complicated. 

The Principle of Microscopic Reversibility and its large-scale consequence, known as the Principle of Detailed Balancing enable investigators to understand the mechanism of the reverse reaction to the same level of accuracy as that achieved for the forward reaction. 

In the course of a reaction the nuclei and electrons assume positions that at each point correspond to the lowest free energies possible. If the reaction is reversible, these positions must be the same in the reverse process, too. This means that the forward and reverse reactions (run under the same conditions) must proceed by the same mechanism. This is called the principle of microscopic reversibility. For example, if in a reaction A — B there is an intermediate C, then C must also be an intermediate in the reaction B — A, This is a useful principle since it enables us to know the mechanism of reactions in which the equilibrium lies far over to one side. Reversible photochemical reactions are an exception, since a molecule that has been excited photochemically does not have to lose its energy in the same way (Chapter 7). 

The important statistical mechanical principle of microscopic reversibility asserts that the mechanism of any chemical reaction considered in the reverse direction must be exactly the inverse of the mechanism of the forward reaction. A consequence of this principle is that if the mechanism of a reaction is known, that of the reverse reaction is also known. Furthermore, it follows that the forward and reverse reactions catalyzed by an enzyme must occur at the same active site on the enzyme and the transition state must be the same in both directions. The principle of microscopic reversibility is often useful when the likelihood of a given mechanism is being considered. If a mechanism is proposed for a reversible reaction in one direction the principle of microscopic reversibility will give an unambiguous mechanism for the reverse reaction. Sometimes this reverse mechanism will be chemically untenable and, recognizing this, the enzymologist can search for a better one. 

The rate-determining step, rds, for a given mechanism may change with potential. However, there is no violation of the principle of microscopic reversibility inspection of Fig. 9 shows that, at any potential, the transition state is always the same for the forward and backward reactions in eqn. (112). Transition states at different electrode potentials, on the other hand, need not be the same (see Fig. 8). Furthermore, there is no reason why the potential E, determined by kinetic factors, at which the transition of rds occurs, should be the same as the thermodynamic equilibrium potential Ee. 

Care must always be taken to verify that a proposed reaction mechanism satisfies the principle of microscopic reversibility. Periodically, someone publishes a mechanism that contravenes the principle because the reverse reaction uses a different pathway from the forward reaction, under the same set of reaction conditions at equilibrium or in the steady state. 

Hence, by the principle of microscopic reversibility, the reverse reaction (the hydrolysis of peptides by the acylenzyme mechanism) must also occur. The question is whether or not this reaction is rapid enough to account for the observed hydrolysis rate. This can be answered by measuring (kcJKM)s for the synthesis of a peptide by the acylenzyme route, and for the hydrolysis of the peptide (kcJKM)u for the hydrolytic reaction can then be calculated from the Haldane equation,... 

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