Reactions of Aromatic Compounds - Ar-SE - Basics and Theory
Regioselectivity in Second ArSE
If aromatic compounds that already contain a first substituent are applied to electrophilic aromatic substitutions, contrary to unsubstituted benzene, the reaction rate, as well as the regioselectivity of the second substitution, are influenced by the nature of the first substituent. In the case of an activating first substituent, the second substitution mainly occurs in ortho and para positions regarding the first substituent and takes place, if at all, predominantly in meta position if the first substituent is deactivating. That is, the reaction rates of second electrophilic aromatic substitution in ortho, meta and para positions are different. They depend on the nature of the first substituent.
The different reaction rates may be explained by the variable stabilization of the σ complexes of the second aromatic electrophilic substitution, which can serve as a model for the corresponding transition states (Hammond postulate).
In contrast to meta substitution, the σ complexes of ortho and para substitutions enable onein that the positive charge is directly located at the carbon that carries the first substituent ("Y"). Therefore, the positive charge's stabilization by an electron-donating first substituent in these σ complexes is obviously larger than in the σ complex of meta substitution. As a result of the more stable σ complexes and, thus, more stable transition states, the reaction rates of ortho and para substitutions are higher than that of meta substitution. The meta substitution is not noticeably speeded up by an electron-donating first substituent.
On the other hand, ortho and para substitution are considerably retarded by an electron-withdrawing first substituent for the same reasons, while the meta substitution's reaction rate is not particularly influenced. Therefore, meta substitution predominates in this case, though the reaction rate of meta substitution is lower than that of unsubstituted benzene.
First substituents with +M effect enable an additional resonance structure of the σ complexes in ortho and para substitution, though not in meta substitution.
Due to the localization of the positive charge at the heteroatom, the additional resonance structure represents a relatively stable species. Thus, it considerably contributes to the stabilization of the σ complex. As a result, a first substituent with a +M effect leads to almost exclusive ortho and para substitution, while meta substitution virtually never occurs.