Principles Governing the Reaction Pathway

In the preceding section, the interaction energy between two reacting molecules has been discussed with the assumption of no nuclear configuration change. In the donor-acceptor interaction the delocalization stabilization is dominant. Eq. (3.25) indicates the importance of HO and LU in the donor-acceptor interaction. But the expression of Eq. (3.21) shows that in general cases the contribution of HO and LU to the quantity D is not so discriminative as those of the other MO s. 

However, there exists a reason which makes the role of the frontier orbitals in the process of chemical reactions more essential than expected from the expression of D. This can be understood if the change in nuclear configuration along the reaction path is taken into consideration. The discussion of this point will be made with the aid of three principles governing the reaction pathway. 

The molecular orbital has, in general, its own nodal planes. The only MO which lacks nodal planes is the lowest-energy MO all the other MO s must have at least one nodal plane in order to be orthogonal to the lowest-energy MO. 

In common molecules, an atom is as a rule bonding with neighboring atoms in each occupied MO, and antibonding in each unoccupied MO. This circumstance is seen in every example illustrated in Fig. 4.1. Also 

Qualitatively, similar relationships are ascertained in hderoaromatic systems where the same conclusion is derived by a nmnerical calculation. In more elaborate calculations than the Hiickel method, such as the Pariser-Parr-Pople approximation 21,23) similar distinct parallelisms are recognized (Table 4.1). Essentially the same drcumstances exist also 

---

Principles Governing the Reaction Pathway axe obtained. Since the usual hydrocarbons possess occupied MO s lower... 

Contents Molecular Orbitals. - Chemical Reactivity Theory. - Interaction of Two Reacting Species. - Principles Governing the Reaction Pathway. - General Orientation Rule. - Reactivity Indices. - Various Examples. -Singlet-Triplet Selectivity. - Pseudoexcitation. -Three-species Interaction. - Orbital Catalysis. -Thermolytic Generation of Excited States. - Reaction Coordinate Formalism. - Correlation Diagram Approach. 

The infancy of these first-principles methods as applied to periodic zeolite lattices means that further detailed work is necessary, particularly in the area of verification of the ability of the pseudopotential to reproduce dynamic as well as static structural properties. However, the results found with these methods demonstrate that the debate concerning the modeling of the activation of methanol within a zeolite is far from concluded. The proton transfer to methanol as a reaction in its own right is, however, of relatively little interest. It does not govern the pathway or energetics of reactions such as dehydration to give dimethyl ether (DME). These are governed instead by the individual transition states that lead to the products, as we discuss in the next section. 

The kinetic partitioning of enzyme intermediates is an important principle, and the rules governing kinetic partitioning are quite simple. The fractional yield of a given reaction is given simply as the rate of the desired reaction divided by the sum of the rates of all reactions involving the intermediate. For example, consider the forked reaction pathway in Scheme XXI. 

The foregoing discussion amplifies the importance of reversibility in the chemistry of aldehydes and ketones. Thus, in the aldol condensation, in addition to competing keto-enol tautomerizations and solvent-to-carbonyl additions (Section 18-2), the reaction itself can, in principle, lead to different products. When these alternatives are all reversible, the final outcome is governed by thermodynamics. In accord with Le Chatelier s principle (Section 17-5), as the species that converts to product by the thermodynamically most favored pathway is depleted, the equilibria of all alternative processes will shift to replenish it. Ideally, the result will be only one end molecnle otherwise the method would be of limited use. We shall see that this scenario is common to other transformations involving carbonyl compounds. 

---