New results of the reactions with hexafluoroacetone and related compounds

The reaction of hexafluoroacetone (HFA) with CN- and SCN-compounds of main group and transition elements is described. The cyclic addition reactions to the CN-moiety is catalyzed by base, preferentially triethylamine. Of particular interest are the adducts of HFA with (SCN)₂ and Hg(SCN)₂. They are precursors for various new compounds containing six- membered rings. The elimination of oxygen by the reaction of HFA with N=WCl₃ may be considered as a new Wittig type reaction. The dimer of hexafluorothioacetone inserts into its SC bond the carbon atom of an isonitrile group to yield five-membered rings. PCl₅ adds to (CF₃)₂CC(CN)₂, prepared from HFA and CH₂(CN)₂, to form a six-membered phosphorus containing ring.Hexafluoroacetone (HFA) was first prepared in 1941 [1]. It is easily available either by the reaction of hexachloroacetone and anhydrous hydrogen fluoride in the presence of a catalyst [2] or by the isomerisation of hexafluoropropenoxide in the presence of a Lewis acid [3,4]. We have prepared HFA using hexachloroacetone, anhydrous HF and a CrO_x catalyst. The reaction proceeds well at 350°C. By-products, which are formed in small amounts, are the very toxic chloropentafluoroacetone and perfluoropropionic acid fluoride. The chloropentafluoroacetone is easily separated by distillation and the perfluoropropionic acid fluoride is removed by treating the crude product with water. Very pure hexafluoroacetone is obtained by distillation of the HFA · H₂O adduct from concentrated sulfuric acid.     

Characterization of the Electrical Conductivity of Concentrated Suspensions of Silica in Aqueous Solutions of Sulfuric Acid

The electrical conductivity of stable concentrated suspensions of SiO₂ in aqueous solutions of sulfuric acid was studied as influenced by the concentration and size of SiO₂ particles.     

Microwave-assisted expeditious hydrolysis of isobenzofuranone derivatives using silica supported acid under organic solvent-free conditions

Silica sulfuric acid was found to be an efficient, reusable and environment-friendly catalyst for fast hydrolysis of various isobenzofuranone to corresponding 2-ketomethylquinoline derivatives in a high yield under solvent-free using microwave irradiation. As the activator of silica sulfuric acid the wet SiO₂ was chosen. The reactions in conventional conditions were compared with the microwave assisted reactions. This approach can prove beneficial since the recovery of solvents from conventional reaction systems always results in some losses.     

Synthesis of 6-amino-4-aryl-3-methyl-1,4-dihydropyrano[2,3-c]pyrazole-5-carbonitriles by heterogeneous reusable catalysts

A new, efficient and environmentally benign protocol for the one-pot, four-component synthesis of 1,4-dihydropyrano[2,3-c]pyrazoles by reaction of hydrazine monohydrate, ethyl acetoacetate, arylaldehydes and malononitrile in the presence of a green catalytic amount of P₂O₅/SiO₂, H₃PO₄/Al₂O₃, cellulose sulfuric acid and starch sulfuric acid is described. The use of non-toxic and inexpensive materials, simple and clean work-up, short reaction times and good yields of the products are the advantages of this method.     

Acid-catalyzed isomerization of caryophyllene in the presence of SiO<Subscript>2</Subscript> and Al<Subscript>2</Subscript>O<Subscript>3</Subscript> impregnated with sulfuric acid

The conversion of caryophyllene upon contact with Al₂O₃ and SiO₂ impregnated with sulfuric acid was carried out, and the components of the resulting mixtures were identified. Having in hands such “standard” mixtures greatly facilitates identification of components of sesquiterpene fractions of essential oils and other mixtures of natural origin. The catalytic activity of silica gel impregnated with sulfuric acid (H⁺-SiO₂) in the acid-catalyzed isomerization of caryophyllene is significantly higher than that for H⁺-Al₂O₃ and is comparable with the activity of concentrated sulfuric acid.     

Investigation of the electrochemical behaviour of lead dioxide in aqueous sulfuric acid solutions by using the in situ EQCM technique

The charge-discharge characteristics and the aging mechanism of PbO₂ layers in contact with sulfuric acid solutions of different concentrations (1.5–5.0 M) were studied by using combined cyclic voltammetry and electrochemical quartz crystal microbalance (EQCM) techniques. For this purpose, thick lead dioxide layers were electrodeposited on gold substrate from aqueous solutions of Pb(NO₃)₂ dissolved in nitric acid. Based on the electrochemical and the mass change responses, it was found that in more concentrated solutions of H₂SO₄, the main reduction reaction was the transformation of lead dioxide to lead sulfate. However, in less concentrated sulfuric acid media, the transformation of lead dioxide to lead(II) ion became the main reaction. These Pb²⁺ ions transformed into lead sulfate crystals later by a chemical reaction. Because the electrochemical oxidation of lead sulfate is less favourable in sulfuric acid medium of higher concentrations, thus, PbO₂ layers cannot be tested by continuous cyclization, which is necessary to study their aging parameters. Therefore, a delay step before each cyclic voltammogram was applied while the non-conductive lead sulfate dissolves or alternatively, by applying a pre-oxidation step prior to each cyclic voltammetry experiment to produce electrochemically significant amount of lead dioxide which can be reduced during the following negative potential sweep.

     

Kinetics of sulfuric acid breakdown of beryllium raw material activated by fusion with sodium carbonate

The kinetics of sulfuric acid breakdown of a mixture of beryl and bertrandite–phenakite–fluorite concentrates, activated by fusion with sodium carbonate, was studied. The apparent activation energy of the reaction of the granulated fusion cake with sulfuric acid was determined (E _app = 9.1 kJ mol^–1). This value of E _app is typical of diffusion-controlled processes and suggests that the process is controlled by the rate of diffusion of acid molecules to the reaction surface through the layer of the forming reaction products.     

Studies of the reaction of crystalline calcium carbonate with aqueous solutions of NH4F,KF and NaF

The reaction between solid calcium carbonate and the aqueous fluorides NH₄F, KF, and NaF has been completely investigated. The reaction of CaCO₃ (solid) is completely independent of the dimensions of its polycrystalline particles and gives calcium fluoride. The calcium fluoride is formed in the same form and size as the grains of the original calcium carbonate. A course crystalline fluorite is formed at a satisfactory rate and with a sufficiently high mechanical strength to be of industrial interest.The course of the reaction appears to involve penetration of the fluoride solution into the body of a grain through voids which develop in the solid material owing to the formation of polycrystalline CaF₂ with a different molar volume as compared with CaCO₃. Data were obtained on the rate of formation and nature of the fluoride formed.The fluorite which is formed around the dissolving calcite was shown by X-ray diffraction and electron microscopy to have a polycrystalline aggregated structure and an estimate is made of crystallite size.The fluorite grains are pseudomorphs of the calcite crystals and there is crystallographic orientation of the product with respect to the parent phase.     

Metal-free catalytic oxidation of urazoles under mild and heterogeneous conditions via combination of ammonium nitrate and catalytic amounts of silica sulfuric acid

Highly efficient production of 4-substituted-1,2,4-triazole-3,5-diones from urazole derivatives using ammonium nitrate and silica sulfuric acid (SiO₂-OSO₃H), as a cheap and nontoxic catalyst, in the presence of wet SiO₂ (50% w/w) under mild and heterogeneous conditions is reported. The process presented here is operationally simple, environmentally benign and produces high yield.     

Carbon-13 kinetic isotope effects in the decarbonylations of lactic acid containing13C at the natural abundance level

The¹³C kinetic isotope fractionation in the decarbonylation of lactic acid of natural isotopic composition by sulfuric acid has been studied in the temperature range of 20–80°C. The¹³C₍₁₎ isotope separation in the decarbonylation of lactic acid by concentrated sulfuric acid depends strongly on the temperature above 40°C. Below this temperature the¹³C isotope effect in the decarbonylation of lactic acid by concentrated sulfuric acid is normal similarly as has been found inthe decarbonylation of lactic [1-¹⁴C] acid. The experimental values of k(¹²C)/k(¹³C) ratios of isotopic rate constants for¹²C and¹³C are close to, but slightly higher than theoretical¹³C-kinetic isotope effects calculated (neglecting tunneling) under the asumption that the C₍₁₎-OH bond is broken in the rate-controlling step of the dehydration reaction. Dilution of concentrated sulfuric acid with water up to 1.4 molar (H₂O)/(H₂SO₄) ratio caused the increase of the¹³C isotope fractionation from 1.0273 found in concentrated sulfuric acid at 80.5°C to 1.0536±0.0008 (at 80.6°C). A discussion of the abnormally high temperature dependence of¹⁴C and¹³C isotope fractionation in this reaction and the discussion of the problem of relative¹⁴C/¹³C kinetic isotope effects is given.     

Preparation of super-hydrophobic white carbon black from nano-rice husk ash

Nano-rice husk ashes were prepared by burning rice husk with a self-propagating method. The white carbon black with high purity was prepared by an alkali dissolving–acid reaction method from nano-husk ash. The super-hydrophobic SiO₂ films were prepared by the sol–gel method using hexamethyldisilazane as a modifier. The effects of the pH and reaction time in the acid reaction process on the purity of the white carbon black, and the effect of the modifier on the hydrophobic property of SiO₂ films were studied. The performances were characterized by XRD, BET, SEM, IR, and contact angle analyzer. The results showed that the purity of white carbon black reached 98.48 % when the NaOH solution with the rice husk ash was heated for 2 h at 90 °C, then the pH of the solution was adjusted by sulfuric acid to 3, and the acid reaction time was 2 h. The contact angle of SiO₂ films was more than 160° when volume ratio of the modifier to silica–sodium hydroxide mixed solution was 0.15. The mechanism of the modifier on SiO₂ surfaces is a graft copolymerization. The hydrophobic groups in the modifier replace the hydroxy groups on SiO₂ surfaces and make SiO₂ surfaces present super-hydrophobicity.     

Preparation of cellulose I nanowhiskers with a mildly acidic aqueous ionic liquid: reaction efficiency and whiskers attributes

Cellulose I nanowhiskers were prepared in relatively high yield (48 ± 2 %) by single-stage hydrolysis of microcrystalline cellulose with an aqueous solution of 1-butyl-3-methylimidazolium hydrogen sulfate ([Bmim]HSO₄). This reaction occurred under mildly acidic reaction conditions with an [H⁺]/[AGU] ratio of 0.24 mol/mol, i.e., 2 orders of magnitude lower than with concentrated sulfuric acid. The nanowhiskers exhibited small width and width distribution and also smaller length than nanowhiskers obtained with concentrated acid. With a relatively low content of sulfur they also exhibited higher thermal stability than whiskers obtained with concentrated sulfuric acid. The lower solvating power of the aqueous ionic liquid compared to that of concentrated sulfuric acid likely contributes to the greater hydrolysis efficiency in the present system.     

Inter- und intramolekulare Umlagerung der Sulfogruppe in 2-Naphtol-1-sulfonsäure

The isomerisation of 2-naphthol-1-sulfonic acid (potassium salt) into 2-naphthol-6-sulfonic acid has been studied using labelled sulfuric acid (H₂³⁵SO₄). In 40 to 50% aqueous sulfric acid the reaction takes place exclusively by an intermolecular mechanism (protio-desulfonation and resulfonation). In glacial acetic acid, in the presence of an excess of sulfuric acid, the rearrangement is partly intramolecular. With an equimolar amount of sulfuric acid the rearrangement is completely intramolecular. This reaction is first order with respect to 2-naphthol-1-sulfonic acid and zeroth order with respect to excess of sulfuric acid. A mechanism for the reaction is proposed.     

Inhibition by F- ions of the low-temperature chemiluminescence oxidation of U4+ by xenon difluoride in frozen aqueous solutions of H2SO4

An increase by several orders of magnitude in the intensity of chemiluminescence detected upon the reaction of U^IV and XeF₂ in aqueous solutions of H₂SO₄ at 200 K was interpreted in the framework of concepts on catalysis of the reaction by the juvenile surface of sulfuric acid crystalline hydrates. This is indirectly confirmed by the observed effect of inhibition of the lowtemperature reaction by fluoride ions.     

Photocatalytic Activity and Photo-Induced Wettability Conversion of TiO2 Thin Film Prepared by Sol-Gel Process on a Soda-Lime Glass

We have developed the transparent photoactive TiO₂ thin film coated on soda lime glass (SLG) by sol-gel process. Titanium dioxide thin films coated on SLG exhibit lower photocatalytic activity due to the thermal diffusion of Na ion from the SLG substrate. Thin SiO₂ film precoating is very effective to prevent the thermal diffusion of Na ion. We have evaluated the photocatalytic decomposition of gaseous acetaldehyde and the photo-induced surface wettability of TiO₂ films with and without SiO₂ precoating layer. As expected, the TiO₂ film on SiO₂/SLG is more photoactive to decompose acetaldehyde than that on SLG. However, as for wettability conversion, there was little difference in the conversion rate between TiO₂ film without SiO₂, and TiO₂ film with SiO₂. Different dependence of Na ion diffusion on two kinds of photo-induced reaction on TiO₂ is discussed based on the difference of the photo-induced reaction mechanism.     

Experimental Study of Zirconium(IV) Extraction from Fluoride-Containing Acid Solutions

Zirconium(IV) extraction from acid solutions was studied, and the optimal parameters of the process were found. Extractants for zirconium(IV) recovery from nitric and sulfuric acid solutions in the presence of fluoride ions were selected. The distribution coefficients of zirconium(IV) and fluoride ion were determined.     

A Novel Catalytic Method for the Oxidation of Sulfides to Sulfoxides with Silica Sulfuric Acid and Sodium Nitrite in the Presence of KBr and/or NaBr as Catalyst

Highly efficient selective oxidation of sulfides to sulfoxides by NaNO₂ and silica sulfuric acid catalyzed with KBr or NaBr has been reported. This oxidation was carried out in the presence of wet SiO₂ (50% w/w) in acetonitrile at room temperature with good to excellent yields.     

On the Beckmann rearrangement of ketoximes: A kinetic study in trifluoromethanesulfonic acid

The formation and the destruction of an intermediate involved in the Beckmann rearrangement of 2,4,6‐trimethylacetophenone oxime have been studied in concentrated trifluoromethanesulfonic acid by kinetic and spectroscopic measurements. Observed (k_obs) and thermodynamic rate constants (k^o) have been estimated and the values compared with the ones obtained in perchloric, sulfuric, and methanesulfonic acids. In the range 80–100 wt% of sulfuric acid, combined analysis of k_obs and k^o rates shows a specific catalysis due to [H₂SO₄] species. In trifluoromethanesulfonic acid, lower rate constants, compared to the values in sulfuric acid, have been observed which differ at 99 wt% by a factor of 10³ ca. The catalytic effect of different strong acids, the structure of the intermediate inferred from Raman and NMR spectra, and the role of the ion‐pairs involved in the reaction are discussed. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36: 417–426, 2004     

Solvent‐free Oxidation of Alcohols by Silica Sulfuric Acid/Sodium Dichromate Dihydrate or Potassium Permanganate/Wet SiO2 System

A combination of silica sulfuric acid and sodium dichromate dihydrate or potassium permanganate in the presence of wet SiO₂ was used as an effective oxidizing agent for oxidation of alcohols to their corresponding aldehydes or ketones in solvent free conditions.     

The gas density balance in the Quantitative analysis of gas mixtures,without prior seperation of components : The determination of silica

When a gas mixture with a major component A which has a molecular weight considerably different from those of all other components (these being close together about a molecular weight of m) is introduced into the gas density balance with a carrier gas also of molecular weight m, the integral response of the gas density balance is essentially proportional to the amount of A in the sample and independent of the concentration ratios between all other components. The applicability of this principle is demonstrated in the determination of silicon tetrafluoride in mixtures with hydrogen fluoride and water, which were formed by reactions between silica, ammonium hydrogen fluoride and sulfuric acid. A direct determination of silica in solid samples containing 10–40 mg SiO₂ is possible with a relative standard deviation of 2.5%.