Page 304 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

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Substituting for PhCOCH from the equilibrium expression for Step 1 gives 285
2
K K PhCHO SECTION 3.3
3
2
−

I = K PhCOCH OEt = K PhCHO PhCOCH
3
3
1
OEt General Relationships
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between Thermodynamic
Stability and Reaction
This provides the final rate expression:
Rates
–
Rate = k obs [ OC H ][PhCHO][PhCOCH ]
2 5
3
Note that the form of this third-order kinetic expression is identical to that in the case
where the second step was rate determining and would not distinguish between the
two possibilities.
Experimental studies of this base-catalyzed condensation have revealed that it is
third order, indicating that either the second or fourth step must be rate determining.
Studies on the intermediate I obtained by an alternative synthesis have shown that k 4
is about four times as large as k so that about 80% of I goes on to product. These
3
reactions are faster than the overall reaction under the same conditions, so the second
step must be rate controlling. 53
These examples illustrate the relationship between kinetic results and the deter-
mination of the reaction mechanism. Kinetic results can exclude from consideration
all mechanisms that require a rate law different from the observed one. It is often true,
however, that related mechanisms give rise to identical predicted rate expressions. In
that case, the mechanisms are kinetically equivalent and a choice between them is not
possible solely on the basis of kinetic data. A further limitation on the information
that kinetic studies provide should also be recognized. Although the data can give the
composition of the activated complex for the rate-determining and preceding steps, it
provides no information about the structure of the activated complex or any interme-
diates. Sometimes the structure can be inferred from related chemical experience, but
it is never established solely by kinetic data.

3.3. General Relationships between Thermodynamic Stability
and Reaction Rates

We have considered the fundamental principles of thermodynamics and reaction
kinetics. Now we would like to search for relationships between stability, as defined
by thermodynamics, and reactivity, which is described by kinetics. Reaction potential
energy diagrams are key tools in making these connections.

3.3.1. Kinetic versus Thermodynamic Control of Product Composition
Product composition at the end of the reaction may be governed by the equilibrium
thermodynamics of the system. When this is true, the product composition is governed
by thermodynamic control and the stability difference between the competing products,
as given by the free-energy difference, determines the product composition. Alter-
natively, product composition may be governed by competing rates of formation of

53
E. Coombs and D. P. Evans, J. Chem. Soc., 1295 (1940); D. S. Noyce, W. A. Pryor, and A. H. Bottini,
J. Am. Chem. Soc., 77, 1402 (1955).