Reactions Proceed via Transition States

 The concept of the transition state is fundamental to under standing the chemical and thermodynamic basis of catalysis. Equation (7) depicts a group transfer reaction in which an entering group E displaces a leaving group L, attached initially to R:

The net result of this process is to transfer group R from L to E. Midway through the displacement, the bond between R and L has weakened but has not yet been completely severed, and the new bond between E and R is yet incompletely formed. This transient intermediate—in which neither free substrate nor product exists—is termed the transition state, E…R…L. Dotted lines represent the “partial” bonds that are undergoing formation and rupture. Figure 1 provides a more detailed illustration of the transition state intermediate formed during the transfer of a phosphoryl group.

Fig1. Formation of a transition state intermediate during a simple chemical reaction, A + B → P + Q. Shown are three stages of a chemical reaction in which a phosphoryl group is transferred from leaving group L (green) to entering group E (blue). Top: Entering group E (bracket A) approaches the other reactant, L-phosphate (bracket B). Notice how the three oxygen atoms linked by the triangular lines and the phosphorus atom of the phosphoryl group form a pyramid. Center: As E approaches L-phosphate, the new bond between E and the phosphoryl group begins to form (dotted line) as that linking L to the phosphoryl group weakens. These partially formed bonds are indicated by dotted lines. Bottom: Formation of the new product, E-phosphate (bracket P), is now complete as the leaving group L (bracket Q) exits. Notice how the geometry of the phosphoryl group differs between the transition state and the substrate or product. The phosphorus and three oxygen atoms that occupy the four corners of a pyramid in the substrate and product become coplanar, as emphasized by the triangle, in the transition state.

Reaction (7) can be thought of as consisting of two “partial reactions,” the first corresponding to the formation (F) and the second to the subsequent decay (D) of the transition state intermediate. As for all reactions, characteristic changes in free energy, ΔGF and ΔGD are associated with each partial reaction:

For the overall reaction (10), ΔG is the numeric sum of ΔGF and ΔGD. As for any equation of two terms, it is not possible to deduce from their resultant ΔG, either the sign or the magnitude of ΔGF or ΔGD.

Many reactions involve several successive transition states, each with an associated change in free energy. For these reactions, the overall ΔG represents the sum of all of the free energy changes associated with the formation and decay of all of the transition states. It therefore is not possible to infer from the overall ΔG the number or type of transition states through which the reaction proceeds. Stated another way, overall reaction thermodynamics tells us nothing about mechanism or kinetics.

ΔGF Defines the Activation Energy

 Regardless of the sign or magnitude of ΔG, ΔGF for the over whelming majority of chemical reactions has a positive sign, which indicates that formation of the transition state requires surmounting one or more energy barriers. For this reason, ΔGF for reaching a transition state is often termed the activation energy, E_act . The ease—and hence the frequency—with which this barrier is overcome is inversely related to E_act. The thermodynamic parameters that determine how fast a reaction proceeds thus are the ΔGF values for formation of the transition state(s) through which the reaction proceeds. For a simple reaction, where ∝ means “proportionate to,”

The activation energy for the reaction proceeding in the opposite direction to that drawn is equal to –ΔGD .

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