Even multistep processes can be simplified to a consideration of the energy and structure of the activated complex of the rate-determining step. One very common and successful approach for describing the structure of the activated complex is based on the notion that it must have structural features of both the reactant and product.
STRUCTURE OF THE ACTIVATED
COMPLEX
Even
multistep processes can be simplified to a consideration of the energy and
structure of the activated complex of the rate-determining step. One very
common and successful approach for describing the structure of the activated
complex is based on the notion that it must have structural features of both
the reactant and product. Moreover it is generally assumed that the energy
curve in the vicinity of the transition state is reasonably symmetrical; thus
the relative energies of the reac-tants and products give some indication of
the structure of the activated complex. This is a very important concept since
it relates energy terms to structural charac-teristics. Free-energy changes
associated with a chemical reaction can be measured fairly easily, whereas
structural information about the activated complex cannot be directly measured
at all. Thus the connection between energy and structure is unique in providing
insight into the structure of the activated complex.
In
a typical reaction coordinate diagram which describes energy changes as the
reactants progress to products, the ordinate is calibrated in energy units and
the abscissa (reaction coordinate) describes structural changes that occur on
going
The reaction coordinate (r ) is an arbitrary axis which cannot easily be defined in simple
units but corresponds to structural changes in many dimensions. These
structural changes must, however, all be related to the differences in structure between reactants and products. Recalling
the earlier example of the ionization of a tertiary bromide to a carbocation,
it is seen that the reaction coordinate corresponds to several distinct types
of changes — breaking of the C–Br bond, flattening of the carbocationic carbon,
charge development on car-bon and bromine, and change in the solvent shell
around the bromide (Figure 5.5). Thus the reaction coordinate cannot represent
a single type of change, except by extreme oversimplification.
The connection between the energy and structure of the activated complex can be illustrated in the following way. If the energy barrier connecting reactants and products is symmetric and the reactants and products are of equal energy, then it is easy to see that the transition state will lie halfway along the reaction coordinate, and thus the structure of the activated complex is “halfway” between reactants and products. To illustrate this situation, consider the substitution of one iodide for another in the reaction of methyl iodide with iodide ion. (We can be sure this reaction occurs by using radioactive iodide.)
*I−
+ CH3–I →
*I–CH3 +
I−
The
products are identical to the reactants and thus have the same energies (Figure
5.6). Consequently this substitution reaction has a transition state which
falls midway between the reactants and products along the reaction coordinate.
Thus the activated complex has a structure midway between that of reactants and
products (Figure 5.7). This corresponds to a collection of atoms having a
pen-tavalent carbon with trigonal bipyramidal geometry (the carbon is half
inverted in geometry) with a half bond between each iodine and carbon and half
of a full
If
the products are lower in energy than the reactants in a reaction step, the
symmetry of the activation barrier causes the transition to lie less than
halfway along the reaction coordinate (Figure 5.8). The structure of the activated
complex is more closely related to the structure of the reactants than to the
structure of the products.
For
example, the reaction of methyl triflate with cyanide ion is also a
bimolec-ular substitution reaction, and it is a very exothermic process. Thus
the transition state lies more toward the reactants (Figure 5.9), and the
structure of the activated complex will be more like the structure of the
reactant than the structure of the products. Thus in the activated complex the
cyanide – methyl bond will be little formed (< 1/2 ) and the carbon – trifloxy bond will be largely intact (>1/2 ). Most of the negative charge
will remain on the cyanide nucleophile and little will have developed on the
triflate leaving group. The geometry at the carbon will still be
tetrahedral-like, although some flattening will have occurred.
For
a reaction step in which the products are less stable than the reactants
(endergic), the transition state will lie farther along the reaction coordinate
toward the products and the activated complex will have a structure more
similar to the products than the reactants (Figure 5.10).
If we consider reaction of mesylate ion with methyl bromide, we find that this is an endergic reaction; thus the transition state lies along the reaction coordi-nate farther toward the products than the reactants (Figure 5.11). The activated complex will therefore have a structure more resembling the products. There will be significant carbon – oxygen bond formation between the mesylate group and carbon and only a weak residual bond between carbon and bromine. The bromine will have acquired significant negative charge and the carbon will be partially inverted in geometry.
It is possible, therefore, to gain significant insight into the structural character-istics of the activated complex from the structures of the reactants and products and their relative energies.
For an exothermic reaction step, the activated complex
more resembles the reactants and is described as early. For an endothermic
reac-tion step the activated complex more resembles the products and is
described as late. The more exothermic is a process, the earlier is the
transition state, while the more endothermic is a process, the later is the
transition state.
These
connections between energy and structure provide a powerful method for
characterizing the activated complex. Furthermore changes made in reactants and
products produce energy changes which can be translated into changes in
structure of the activated complex; hence it is possible to predict how changes
in structure will influence changes in energy and rates of reaction. These considerations
are central to understanding the mechanisms of chemical reactions, and they
permit us to make the best structural and reagent choices for a particular
conversion.
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