New aspects of electrophylic aromatic substitution mechanism: Computational model of nitration reaction

Mechanism and Kinetics of the Reaction

Classic mechanism of nitration assumes that the active nitration particle is nitronium NO2+:
(1) HNO3 + HA ⇔ H2NO3+ + A−
(2) H2NO3+ ⇔ NO2+ + H2O
(3) ArH + NO2+ ⇔ [ArH…NO2]+
(4) [ArH...NO2]+ ⇒ ArNO2 + H+
where HA is some proton acid (H2SO4 or HNO3 itself). Nevertheless experimental regularities of nitration reaction is not unambiguous: the rate of reaction is usually proportional to concentration of aromatic substrate ArH (first order reaction), though in some cases zero order was reported (i.e. independence of substrate's concentration). These special cases are called “encounter controlled reaction” which means that all molecular encounters of substrate and some active participle are successful, so no substrate selectivity may exist. Nevertheless the ratio of isomeric products of often does not correspond to mere statistic one. This is the problem of regioselectivity (inner selectivity), and it gives life to hypotheses of intermediates: π-complex [4] or complex with electron transer [5]. Influence of concentration of the second reagent, HNO3, was hardly studied. When measured, effective kinetic order was found to be in a wide range from 2 to 10 and strongly depended on conditions of the reaction^*. There was also the experimental evidence indicating that addition of water to reaction mixture caused changes in dependency of reaction rate toward ArH from zero to first order (e.g., Ref. [6]). All these can be possible only if there are alternatives to (3) stage of reaction mechanism. We may assume that such changes are due to changes of nitration agent. In non-hard media, such an agent could be nitracidium HNO3+.

^* This special question is considered in another report: Sokolov, A. V. Influence of Solvent on Kinetics of Nitration: Model of Associates.