Handbook of Solvents - George Wypych - ChemTech - Ventech!

2.1 Solvent effects on chemical systems 31
Figure 2.1.13. Representation of the stationary points (reactants, reactant complex, transition states, and products)
for the molecular mechanism of compound I. For the TS´s the components of the transition vectors are depicted by
arrows.
ergy surface. In this path the height of the barrier that exists between the reactant and TS is
correlated to the rate of each different pathway (kinetic control), while the relative energy of
reactants and products is correlated to equilibrium parameters (thermodynamic control).
The second aim is how the solute-solvent interactions affect the different barrier heights and
relative energies of products, mainly when charged or highly polar structures appear along
the reaction path. In fact, the differential stabilization of the different stationary points in the
reaction paths can treat one of them favorably, sometimes altering the relative energy order
found in vacuo and, consequently, possibly changing the ratio of products of the reaction.
An analysis of the potential energy surface for the molecular model I led to the location of
the stationary points showed in Figure 2.1.13.
The results obtained for the addition of azide anion to tetrafuranosides with different
molecular models and in different solvents can be summarized as follows: 65
• For compound I, in vacuo, P1 corresponds to the path with the minimum activation
energy, while P2 is the more stable product (Figures 2.1.14 and 2.1.15). When the
solvent effect is included, P1 corresponds to the path with the minimum activation
energy and it is also the more stable product. For compound II, in vacuo, P2 is the
more stable product and also presents the smallest activation energy. The inclusion
of the solvent effect in this case changes the order of products and transition states
stability. For compound III, P1 is the more stable product and presents the smallest
activation energy both in vacuo and in solution.
• A common solvent effect for the three reactions is obtained: as far as the dielectric
constant of the solvent is augmented, the energy difference between P1 and P2