Reaction regioselectivity doesn’t require a transition state

Toluene nitration by nitronium (NO₂⁺) salts yields curious regioselectivity: Although nitration should occur equally at all positions of the ring because the reaction is highly exothermic and the energy barrier is near zero, only 2% of the products are substituted at the meta position. Of various mechanisms proposed to explain this selectivity, none has proven to be satisfactory. That is because the reaction involves no intermediates and no transition states beyond the initial encounter of toluene and nitronium, according to computational work by Yexenia Nieves-Quinones and Daniel A. Singleton of Texas A&M University (J. Am. Chem. Soc. 2016, DOI: 10.1021/jacs.6b07328). The researchers find that after the toluene and nitronium encounter each other, they don’t immediately react. Instead, the nitronium wanders around the area above the aromatic carbons until random fluctuations reorient the counterion and solvent molecules, from stabilizing nitronium to stabilizing the product cation. Once that reorganization occurs, the nitronium faces downhill paths for reacting with any of the carbons—but the paths for ortho or para substitution are steeper and easier to access than for meta or ipso substitution.