Theoretical study on the nitration of pyridine: comparison with electrophilic substitution of dithieno[3,4- b:3',4'- d]pyridine

Abstract

The nitration of pyridine was investigated by means of theoretical calculations. The structure and electronic properties of the Wheland intermediates (σ complexes) and the transition states (TSs) were calculated at the ab initio 3-21G level. Harmonic frequencies were calculated to characterize the stationary points. The TS geometry of the ring system shows greater similarity to the Wheland intermediate than to the parent structure. Orientation effects could be explained by the electronic distribution of the TS structures. The reaction profiles, TS structures and electrostatic potential maps obtained are compared with those obtained for the nitration of the more reactive dithieno [3,4- b:3',4'- d] pyridine ring system. The shape of the reaction profiles obeys the Hammond postulate and analysis of the electrostatic potential maps explains the manifestation of the reactivity-selectivity principle (RSP). In the TS, the π character of the C-NO 2 bond is found to be larger for the more reactive species.