493. Kinetics and mechanism of aromatic nitration. Part II. Nitration by the nitronium ion, NO2 +, derived from nitric acid

Abstract

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...

Rates and activation energies of nitronium ion formation and reaction in the nitration of toluene in ∼78% sulphuric acid

“…It is highly likely that with toluene at these high acid concentrations k3' >> k-,' and formation of the encounter complex (reaction 2) is rate controlling when the nitration rate is first order in toluene (13 ' (15). With these it is likely that k-,' >> k,' and [3] I is rate controlling, so that k2 = (kl'aH+/k-laH20) (k,'/k-,')k,' and E, = A H , + AH, + E,', AH, ' being the heat of reaction 2, the formation of the encounter complex. One expects AH, to be small and , E,' to be relatively independent of the medium.…”

“…where k, = k,' aH+ and When k2[ArH] >> k , , [4] simplifies to which accounts for the rate being zero order with respect to the aromatic substrate under certain conditions such as those that often occur in organic solvents (2)(3)(4)(5). O n the other hand whenk, >> k, [ArH], [4] simplifies to which accounts for the rate being first order with respect to the aromatic substrate under other conditions such as those which normally obtain in aqueous sulphuric acid.…”

Gas-Phase Reactions of Nitronium Ions with Acetylene and Ethylene: An Experimental and Theoretical Study

“…This work aims to bring about a detailed understanding of the two gasphase reactions in Equations (1) and (2) through the combined application of experimental techniques and a detailed ab initio study at the MP2/6 ± 31 G* level of theory. In particular, we report new theoretical and experimental results for the reaction in Equation (1), whereas for the reaction in Equation (2) we use both earlier experimental and theoretical results [7,8] and new results that allow us to perform a more comprehensive comparative analysis of the two reactions. We have found that the two reactions are mechanistically very similar and that they proceed mainly according to a 1,3-dipolar-type addition.…”

“…We have found that the two reactions are mechanistically very similar and that they proceed mainly according to a 1,3-dipolar-type addition. This reaction leads to the formation of two structurally related end products, nitrosated ketene from the reaction in Equation (1) and nitrosated acetaldehyde from the reaction in Equation (2).…”

Continuous adiabatic industrial benzene nitration with mixed acid at a pilot plant scale

“…The organic reactant (i.e., benzene or toluene) is transferred into the aqueous phase, also called mixed acid, where it reacts with the nitronium ion formed from the nitric acid. The sulphuric acid acts as a catalyst [1,2]. The strength of the sulphuric acid used determines the type of reaction developed, enabling operation in the kinetic or the fast reaction regimes, if the remaining parameters are kept constant [3].…”