Structure of the Activated Complex

| Home | | Organic Chemistry |

Chapter: Organic Chemistry : Mechanisms of Organic Reactions

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 from reactant to product. 


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 negative charge on each iodine. As shown below, this activated complex is a simplified but reasonable depiction of the structure of the activated complex.




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.

Contact Us, Privacy Policy, Terms and Compliant, DMCA Policy and Compliant

TH 2019 - 2022 pharmacy180.com; Developed by Therithal info.