Figure 14. Geometries of intermediates and transition states in elementary reactions 1–3 in the gas phase.
In elementary reaction 3, H+ ions are also needed to form the five-membered ring; otherwise, a stable intermediate cannot be formed. For the two-step elementary reaction, the ΔE a values in the gas- and liquid-phase environments are shown in Table 13. When the reaction is carried out in a gas-phase environment, the reactant molecules can collide directly without resistance. In the triethylamine solvent, the reactant molecules are solvated, which hinders the effective collision of the molecules. It can be inferred from the discussion in section 3.1 that the system is less affected by the polar triethylamine solvent. On the other hand, in elementary reaction 3, triethylamine has an attractive effect on the H+ ions in solution. According to a comprehensive comparison, for elementary reactions 2 and 3, the ΔE a values in the solvent phase are slightly higher than those in the gas phase. However, in elementary reaction 1, there is no triethylamine solvent, and the ΔE afor forming intermediates 1a’ and 2a’ from reactants1a and 2a , respectively, is higher.
Table 3. Activation energies (ΔE a) of each reaction step in the liquid and gas phases.