Mechanistic and energetic aspects of aromatic nitration with NO +2 in sulfolane

Abstract

The electrooxidation of several commonly used aromatics has been studied in sulfolane at 303 K by means of the cyclic voltammetric (CV) technique (at a Pt electrode). It has been shown that the anodic peak potentials ( E a p) of the irreversible cyclic voltammograms obtained for these aromatics correlate well with the ionization potential ( I p), and that the CV peak shift ( E a p - E ° (ArH .+/ArH)) stems from the electrochemical kinetics for the EC mechanism for arene oxidation. Taking into account previous work on the theoretical analysis of the electrochemical kinetics, the peak potential E a p can be related to the standard oxidation potential of the ArH .+/ArH couple. Furthermore, we also examined the interactions of these aromatics with the electron acceptor NO + 2 using the potentiometric technique (employing a silver/silver chloride indicator electrode) as follows: NO 2Cl + ArH + AgClO 4 → ArNO 2 + HClO 4 + AgCl(s) (I) Analysis of the potentiometric curves using Nernst's law allowed us to determine the standard potential for reaction (I) and the Gibbs energy differences Δ G(II) and Δ G(III) corresponding to the nitration processes (NO + 2, ClO − 4) + ArH ⇌ ArNO 2 + HClO 4 (II) and NO + 2 +ArH ⇌ ArNO 2 + H + (III) The feasibility of the outer-sphere electron transfer NO + 2 + ArH ⇌ NO . 2 + ArH .+ (IV) has been investigated by application of the Marcus theory in order to gain further insight into the energetics of this ET process and to characterize the intermediate stages through which the nitration reaction may proceed in sulfolane.