Abstract
The substituent effect of the amino and nitro groups on the electronic system of benzene has been investigated quantum chemically using quantitative Kohn-Sham molecular orbital theory and a corresponding energy decomposition analysis (EDA). The directionality of electrophilic substitution in aniline can accurately be explained with the amount of contribution of the 2pz orbitals on the unsubstituted carbon atoms to the highest occupied π orbital. For nitrobenzene, the molecular π orbitals cannot explain the regioselectivity of electrophilic substitution as there are two almost degenerate π orbitals with nearly the same 2pz contributions on the unsubstituted carbon atoms. The Voronoi deformation density analysis has been applied to aniline and nitrobenzene to obtain an insight into the charge rearrangements due to the substituent. This analysis method identified the orbitals involved in the C-N bond formation of the π system as the cause for the π charge accumulation at the ortho and para positions in the case of the NH2 group and the largest charge depletion at these same positions for the NO2 substituent. Furthermore, we showed that it is the repulsive interaction between the πHOMO of the phenyl radical and the πHOMO of the NH2 radical that is responsible for pushing up the πHOMO of aniline and therefore activating this π orbital of the phenyl ring towards electrophilic substitution.
Highlights
In this work, we would like to study the substituent effects of NH2 and NO2 groups on benzene using Kohn–Sham density functional theory
Complementary to the Kohn–Sham Molecular Orbital (KS-MO) analysis and the energy decomposition analysis (EDA), we have investigated the Voronoi Deformation Density (VDD) atomic charges (eqn (3)) and charge rearrangements (eqn (4)) on the phenyl ring due to the substituent effects to predict the regioselectivity of electrophilic substitution and explain the reactivity of the aromatic ring
We studied using density functional theory the substituent effect of NH2 and NO2 groups on the electronic system of benzene caused by the formation of the C–N bond
Summary
We would like to study the substituent effects of NH2 and NO2 groups on benzene using Kohn–Sham density functional theory. Completely opposite electronic properties and opposite effects on the electrophilic substitution reactions of benzene.[1] The amino group is activating and ortho- and para-directing, whereas the nitro substituent is known to be deactivating and meta-directing. This difference in reactivity can be made plausible by the Lewis structures of aniline and nitrobenzene (Scheme 1), but it does not give an explanation of the influence of the substituent on the molecular s and p orbitals located on the phenyl ring. Based on frontier molecular orbital theory to explain chemical reactivity, our quantitative KS-MO results together with the VDD charges clarify the orientation of the electrophilic aromatic substitution and the change in reactivity of the benzene ring caused by the NH2 or NO2 substituents
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