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.