Figure 15. Intrinsic reaction coordinate (IRC) curves for the different substituted reactants.
As can be seen in Table 14, for reactant 1a substituted with phenyl, thiophene, and alkyl groups, the required ΔE a, i.e., degree of difficulty, for elementary reaction 2 increases in the order Ph < thiophene < n-Pr. Thus, a conjugated ring system is more favorable for the reaction. The C atoms on the benzene ring are hybridized to provide 2p orbitals in the sp2 mode, while the S atom in thiophene is conjugated with the 3p orbital electrons. The overlap of orbitals is slightly worse because the radius of a 3p orbital is larger than that of a 2p orbital. Consequently, the conjugation effect of thiophene is not as good as that of benzene.
For substituted benzene, electron-withdrawing groups, such as F, Cl, and Br, are more conducive to the reaction because they lower the ΔE a. Among the substituted reactants, the ΔE a for the F-substituted reactant is slightly higher than those of the Cl- and Br-substituted reactants because the induced electron-withdrawing effect of F, which is a conjugate electron donor, is weaker. Substitution with CH3, OCH3, or other electron-donating groups lead to a higher ΔE a, that is, an electron-donating group is not conducive to the reaction.
Table 4. Activation energies (ΔE a) for different substituted compounds.