Kinetics and mechanism of the nitrosation of 2-nitropropane, 1-nitropropane, and nitroethane

Abstract

The kinetics of the nitrosation of the nitronic acids derived from 2-nitropropane, 1-nitropropane, and nitroethane, in aqueous perchloric acid using nitrous acid, show a first-order dependence upon the [nitronic acid] and a curved acidity dependence. The results are readily explained by a mechanism in which nitrosation (by H₂NO₂⁺ or NO⁺) occurs initially at an oxygen atom, and releasing a proton. The O-nitroso intermediate, which is formed reversibly, then undergoes an internal O- to C-nitroso group rearrangement to give the nitroso nitro product (pseudonitrole) from the secondary nitro compound, and (after tautomerisation) the nitro oxime product (nitrolic acid) from the primary nitro compounds. Neither the first nor the second step is fully rate-limiting, under the conditions used, but the first step is probably close to the diffusion-controlled limit. The reactions are strongly catalysed by Cl^–, Br^–, and SCN^–, but the measured rate constants show a curved dependence on [Cl^–] and [Br^–], suggesting that the intermediates are formed reversibly. However, the reactions are first-order in [H⁺], so that reaction now occurs via ClNO, BrNO, and ONSCN at the carbon atom of the nitronic acid. The results are discussed, together with those in the literature which describe nitrosation reaction where different nitrosating agents attack the same molecule at different sites. In all three cases studied a further, much slower reaction occurs, which results in the formation of more of the same products (when [nitronic acid]₀[HNO₂]₀). This reaction which is first-order in [H⁺] and [nitronic acid], but is not chloride or bromide ion-catalysed, also occurs when no nitrous acid is added. The product is formed by the slow hydrolysis of the protonated form of the nitronic acid, releasing nitrous acid, which reacts further with the unchanged nitronic acid.