Abstract
It has been discovered that dilute nitric acid in reversed micelle systems can oxidize the Br- ion to Br2 and we have proposed that the nitryl (or nitronium) ion NO2+ should be the active species in the oxidation process. Nitration of phenol in reversed micelle systems with dilute nitric acid, CHCl3/CTAC/H2O (2.0 mol dm-3 HNO3 in the 1.0% (v/v) H2O phase), has been performed at 35 oC to obtain 2- and 4-nitrophenols, where CTAC represents cetyltrimethylammonium chloride. In aqueous 2.0 mol dm-3 HNO3 solution accompanied by 4.0 mol dm-3 LiCl (and a small amount of LiBr as the bromide resource), trans-1,4-dibromo-2-butene was successfully brominated to 1,2,3,4-tetrabromobutane. This result is good evidence that the Br- ion can be oxidized to Br2 in dilute nitric acid (2.0 mol dm-3) providing it contains concentrated salts. For chloride salts, the cation effects increased as Et4N+ << Na+ < Li+ < Ca2+ < Mg2+. Even the evolution of Cl2 has been demonstrated from < 2.0 mol dm-3 HNO3 solution containing concentrated LiCl, MgCl2, and CaCl2 as well as AlCl3. The dissolution of precious metals (Au, Pt, and Pd), especially, of gold has been demonstrated in 0.1 - 2 mol dm-3 HNO3 accompanied by alkali metal, alkaline earth metal, and aluminum chlorides. The complete dissolution time of pure gold plate (20±2 mg, 0.1 mm thickness) in 2.0 mol dm-3 HNO3 accompanied by 1.0 mol dm-3 AlCl3 has been shortened remarkably with temperature increase from 15 to 80 oC. The dissolution rate constants, log (k /s-1), of a piece of gold wire (19.7±0.5 mg) in 20 mL of 2.0 mol dm-3 HNO3 accompanied by the metal chlorides, in general, increase with increasing salt concentrations at 40 and 60 oC. The gold can be dissolved in the solution of <1.0 mol dm-3 HNO3 and <1.0 mol dm-3 HCl, i.e. a “dilute aqua regia. We have achieved a total dissolution of five pieces of the gold wire (totally 0.10 g) in 100 mL of the 1:1 mixture between seawater and 2.0 mol dm-3 HNO3 at ca. 100 oC.
Highlights
Nitric acid has two different functions, a strong acid and a strong oxidizing agent in diluted and in concentrate conditions, respectively, and is one of the best known and most widely applied oxidizing agents in chemistry
The δ(1H) value of the CTAB micelle system decreased after two hours with changing of the color from colorless to yellow, Mechanism of enhanced oxidation ability of dilute nitric acid and dissolution of pure gold
We have suggested that the oxidizing species should be the NO2+ ion which is formed from the dilute nitric acid in the micelle system
Summary
Nitric acid has two different functions, a strong acid and a strong oxidizing agent in diluted and in concentrate conditions, respectively, and is one of the best known and most widely applied oxidizing agents in chemistry. Water can lose its property as bulk water (H-O-H) to get that of a non-aqueous solvent, such as an alcohol (R-O-H) or even an ether (R-O-R) if the highly “self-assembled structure” of bulk water is destroyed in the following cases: (a) the residual water (c(H2O) ~10-3 mol dm-3) in organic solvents; (b) aqueous solutions or organic solvent-water mixtures containing highly concentrated salts; (c) nanoscale water droplets in nano-tubes or reversed micelle systems; (d) water on metal electrodes, ion-exchange resins, proteins, organic solvents as the solvent extraction, and glass vessels; (e) water under supercritical conditions Such waters can be “reduced” to the authentic singular H2O molecules ( called “dihydrogen ether”, (H)-O-(H)) [8]. We examine the oxidation of Br- to Br2 by dilute nitric acid in aqueous bulk solution containing concentrated salts in order to recognize that the NO2+ ion can be produced within nanoscale water droplets in reversed micelle systems. In Part 2, pure gold dissolution in seawater mixed with aqueous nitric acid is demonstrated
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