Reactions of Aromatic Compounds (overall)
Energetics of Electrophilic Aromatic Substitution
When benzene is treated with deuterated sulfuric acid, deuterium may be used in order to observe the reaction course by NMR spectroscopy. If non-deuterated sulfuric acid is applied, the course of reaction could not be investigated, as the non-specific exchange of hydrogen for hydrogen does not amount to any modifications of the starting product.
The treatment of benzene with deuterated sulfuric acid results in an electrophilic aromatic substitution of hydrogen by deuterium. The question is whether or not is added to the partially positively charged ortho or para position of the σ complex or whether a proton is eliminated, which wolud result in the aromatic π system's recovery. Actually, the addition of , which is usually observed in combination with alkenes, does not appear in the case of benzene or any other aromatic compounds.
This result may be appropiately grounded with the help of a reaction coordinate/energy diagram.
First of all, the attack of the aromatic π electrons on the electrophile yields the σ complex. As this requires that the aromatic system is broken, the σ complex formation is an endothermic process, though the positive charge of the σ complex is delocalized and the formation of the new σ bond would alone be exothermic. Practically speaking, electrophilic aromatic substitution often proceeds rapidly even at mere room temperature if an appropiate electrophile is applied.
Why does the σ complex stabilize in the second step by deprotonation and not by the addition of an anion?
The addition of an anion (red curve in the energy diagram) to the σ complex yields a non-aromatic cyclohexadiene derivative, while the elimination of a proton (green curve) enables the molecule to recover the aromatic π system. Therefore, the elimination of a proton leads to the energetically favored product, as the aromatic resonance stabilization accounts for a decrease in energy. In addition, the transition state of the more stable product's formation is relatively lower in energy than the transition state that is passed through when the cyclohexadiene derivative is produced. As a result, the more stable product is more rapidly formed and its chemical yield is therefore higher ("product-development control").
Of course, the σ complex may expel either a proton or a deuterium cation. However, in the latter case only the starting product is recovered. This does not influence the chemical yield, nor the product structure.