The plan underlying the investigation as developed in this collection of papers, which includes papers of several concurrent series, is explained. The series to which this paper belongs is concerned with the kinetics and products of aromatic nitration. The other series deal with various kinds of physical and synthetical evidence, all bearing directly or indirectly on the subject of nitration. In this paper the kinetics of aromatic nitration by nitric acid are considered. The treatment is divided into Sections, and the following summary of the paper is intended to render any of its Sections intelligible without the necessity of reading the others.(1) Kinetic Orders of Aromatic Nitration : The Nitronium Ion (p. 2402).-Aromatic nitration displays second-order kinetics in sulphuric acid (Martinsen), and first-order kinetics in nitric acid ; and in either nitromethane or acetic acid with nitric acid in constant excess, it exhibits zeroth-order kinetics for sufficiently reactive aromatic compounds, and first-order kinetics for sufficiently unreactive compounds. Nothing of special interest concerning mechanism follows from these kinetic results in sulphuric acid, and in nitric acid, though they are consistent with what is deduced; but the zeroth-order kinetics in organic solvents, and its transition to firstorder kinetics with diminishing aromatic reactivity, proves without alternative the formation and effectiveness of the nitronium ion, NO,+. The kinetically fundamental process is shown to be localised in nitric acid, t o be activated, to be heterolytic, and to produce electron-deficient nitrogen. Therefore it is the bond-fission HO-;NO, + HO-+KO,+ I disregarding the associated, instantaneous proton-transfers, which are treated under (2).(2) Solute Effects on Nitration Kinetics : Mechanism of Formation of the Nitronium Ion (p. 2412) .-Aromatic nitration in nitric acid, or in nitromethane, or acetic acid, with nitric acid in constant excess, is accelerated by sulphuric acid, and retarded by ionising nitrates, the latter effect being specific, and not a general salt effect. The effect of sulphuric acid on the rate, and that of nitrate ion on the reciprocal of the rate, obey linear limiting laws. The effects occur alike with zeroth-and first-order reactions, always without disturbance to the reaction order.These results show that one proton is employed, and one nitrate ion eliminated, in the conversion of nitric acid into a nitronium ion; and that these changes are reversed. It also follows that they take place in an earlier reaction-step than that which produces nitronium ion, because a zeroth-order nitration only maintains its kinetic order when the latter step is not reversed. Thus the following two-stage mechanism is unequivocally established : 2HK0, H,N03+ + NO3-(fast) H,N03+ + NO,+ + H,O (slow)Consistently, the kinetic effect of water in zeroth-order nitration is negligible in comparison with that of nitrate ion. Relatively large amounts of water will retard the nitration by reversing the second of the above reacti...
