Hydrogen bonds between hydrogen halides and unsaturated hydrocarbons

Ab initio molecular orbital theory is used to study geometrics and energies of hydrogen-bonded complexes between hydrogen fluoride, hydrogen chloride (as proton donors) and acetylene, ethylene (as proton acceptors). Symmetrical T-shaped structures are found to be equilibrium structures for all four complexes. The strengths of the hydrogen bonds are found to be less than for conventional hydrogen bonds involving lone pairs of electrons.     

N,N-Dimethylbenzenocarbothioamide Complexes of IIB metal halides

The complexes Zn(DMBCTA)X₂ (X = Cl, Br or I), Cd₇(DMBCTA)₄Cl₁₄, Cd(DMBCTA)X₂ (X = Br or I) and Hg(DMBCTA)X₂ (X = Cl, Br or I; DMBCTA = N,N-Dimethylbenzenocarbothioamide) were isolated and characterized by elemental analysis, conductance and molecular weight measurements, and IR, Raman, and ¹H NMR spectroscopy.     

Charge-transfer complexes of nitric oxide,tri or tetravalent halides and aromatic hydrocarbons

Complexes of nitric oxide, some aromatic hydrocarbons and the halides BF₃, BCl₃, AlCl₃, FeCl₃, SiCl₄, GeF₄, SnCl₄ and TiCl₄ have been identified and characterized by spectroscopic measurements.     

Oxygenation of aromatic hydrocarbons with hydrogen peroxide catalyzed by rhodium carbonyl complexes

Hexanuclear rhodium carbonyl cluster, Rh₆(CO)₁₆, catalyzes benzene hydroxylation with hydrogen peroxide in acetonitrile solution. Phenol and (at lower concentration) quinone are formed with the maximum attained total yield and turnover number 17% and 683, respectively. Certain other rhodium carbonyl complexes, containing cyclopentadienyl ligands, Rh₂Cp₂(CO)₃ and Rh₃(CpMe)₃(CO)₃, are less efficient catalysts. Cyclopentadienyl derivatives of rhodium which do not contain the carbonyl ligands, Rh(CpMe₅)(CH₂?CH₂)₂, RhCp(cyclooctatetraene) and Rh₂Cp₂(cyclooctatetraene) turned out to be absolutely inactive in the benzene hydroxylation. Styrene is transformed into benzaldehyde and (at lower concentration) acetophenone and 1‐phenylethanol. Copyright © 2008 John Wiley & Sons, Ltd.     

Transition metal ion-assisted photochemical generation of alkyl halides and hydrocarbons from carboxylic acids

Near-UV photolysis of aqueous solutions of propionic acid and aqueous Fe(3+) in the absence of oxygen generates a mixture of hydrocarbons (ethane, ethylene and butane), carbon dioxide, and Fe(2+). The reaction becomes mildly catalytic (about five turnovers) in the presence of oxygen which converts a portion of alkyl radicals to oxidizing intermediates that reoxidize Fe(2+). The photochemistry in the presence of halide ions (X(-) = Cl(-), Br(-)) generates ethyl halides via halogen atom abstraction from FeX(n)(3-n) by ethyl radicals. Near-quantitative yields of C(2)H(5)X are obtained at ≥0.05 M X(-). Competition experiments with Co(NH(3))(5)Br(2+) provided kinetic data for the reaction of ethyl radicals with FeCl(2+) (k = (4.0 ± 0.5) × 10(6) M(-1) s(-1)) and with FeBr(2+) (k = (3.0 ± 0.5) × 10(7) M(-1) s(-1)). Photochemical decarboxylation of propionic acid in the presence of Cu(2+) generates ethylene and Cu(+). Longer-chain acids also yield alpha olefins as exclusive products. These reactions become catalytic under constant purge with oxygen which plays a dual role. It reoxidizes Cu(+) to Cu(2+), and removes gaseous olefins to prevent accumulation of Cu(+)(olefin) complexes and depletion of Cu(2+). The results underscore the profound effect that the choice of metal ions, the medium, and reaction conditions exert on the photochemistry of carboxylic acids.     

Porous metal membranes for solid-phase extraction of polycyclic aromatic hydrocarbons

Solid-phase extraction (SPE) is one of the most important techniques for sample preparation, purification, concentration and cleanup. Membranes made from synthetic organic polymers, cellulose, or glass fibers are used for sample pretreatment. In this work, we report that a porous metal membrane, the metal filter in HPLC, was used as a novel kind of solid-phase extraction adsorbent material. To evaluate the performance of the porous metal membrane for the SPE, naphthalene, fluorene, anthracene, phenanthrene, fluoranthene, pyrene, chrysene, perylene and benzo(a)pyrene were selected as analytes. Several parameters that affected the extraction efficiency such as the extraction time, the concentration of NaCl, the extraction temperature and the agitation speed were optimized. The experimental result indicates that the porous metal membrane possesses high adsorption ability to the tested polycyclic aromatic hydrocarbons (PAHs). Under the optimum conditions, the detection limits of the developed method were in the range of 0.03-0.082 μg L(-1) (S/N = 5), and excellent linear correlations between peak area and concentration of PAHs were found over the range of 0.1-60 μg L(-1). The precisions (RSD) for five replicate extractions of the PAHs from sample solutions were in the range of 2.6-5.0%. The recoveries of the PAHs from tap water and river water samples spiked with 9 PAHs (20 μg L(-1) of each individual PAH) ranged from 83.0% to 112.5%. The porous metal membrane is durable, simple, inexpensive, reproducible and has a high adsorption ability for use in SPE of PAHs.     

Liquid-phase synthesis of cyclic diene diepoxides using metal halides and hydrogen peroxide

Optimal conditions were found for induced hydroxyhalogenation of cyclic dienes (tetrahydroindene, 4-vinylcyclohexene and 5-vinyl- and 5-cyclohexenylbicyclo[2.2.1]hept-2-enes) in the system [MHlg-HA or HHlg]-H₂O₂ (or NaClO). Dehydrohalogenation of the chloro- and bromohydrins thus obtained with powdered potassium carbonate gave the corresponding diepoxy derivatives, and their hydrolysis led to mixtures of stereoisomeric tetrahydric alcohols.     

Enhancement of the solubilities of polycyclic aromatic hydrocarbons by weak hydrogen bonds with water

Summary The thermodynamics of mobile order is applied to predict the aqueous solubility of liquid and solid aliphatic and polycyclic aromatic hydrocarbons. The solubility values are mainly determined by the magnitude of the hydrophobic effect. However, contrary to the solubilities of the alkanes, the solubilities of polycyclic aromatic hydrocarbons in water predicted in absence of solute-solvent hydrogen (H) bonds are systematically too low. This shows the contribution of weak specific interactions between the OH groups and the electrons of the aromatic substances. According to the theory, these interactions are characterized by a stability bility constant K_o which can be derived from solubility data. At 25°C, this constant amounts to 80 cm³/mol, the order of magnitude of which can be explained by the competition of these intermolecular bonds with the rather weak self-association bonds in the secondary chains of water.     

Direct synthesis of phenols by iron-catalyzed biphasic oxidation of aromatic hydrocarbons with hydrogen peroxide

The hydroxylation of a series of aromatic hydrocarbons with hydrogen peroxide, catalyzed by iron complexes with pyrazine-3-carboxylic acid N-oxide, was investigated, operating in a biphasic reaction medium.

The new catalyst showed a high selectivity to the corresponding phenols, minimizing the over-oxidation reactions to polyoxygenated derivatives and tars which, along with dimers formation, are the major limitations of the classical Fenton’s reagent for a practical synthetic application.

In the case of alkylbenzenes, the competitive side chain oxidation at the benzylic positions also occurred. Electron rich substrates, such as anisole, were oxidized with very poor selectivity.

The reactions were carried out in a biphasic system that allows a convenient recovery and recycling of the catalyst by phase separation techniques. The catalyst showed a complete retention of activity after six consecutive reaction cycles.

The new catalyst appears as a promising tool for the direct synthesis of phenols, in alternative to the conventional multi-step methods.     

Induced bromination of aromatic hydrocarbons with alkali metal bromides in the presence of oxidants

The features of induced bromination of aromatic hydrocarbons in the NaBr(KBr)-HX-H₂O₂(NaOCl) system were studied.     

Complexes of osmium halides with monodentate and bidentate ligands

Complexes of osmium halides with donor ligands L (L = PPh₃, AsPh₃, SbPh₃, PMePh₂, C₂H₄(AsPh₂)₂, PPr₂ⁿPh, CH₂(AsPh₂)₂) have been prepared. The IR and NMR spectra of complexes of the type trans-[OsCl₄L₂], fac-[OsX₃L₃] and trans-[OsX₂L₄] are recorded and discussed. For the trans complexes the halogen sensitive modes occur near those of the related [OsX₄]²⁻ species. Tentative assignments for ν(OsY) (Y = P, As, Sb) are discussed on the basis of the experimental results. Raman and ESR spectra of selected complexes have been recorded and are discussed. NMR data indicates the presence of monodentate diarsine ligands in trans- (OsCl₄(CH₂(AsPh₂)₂)₂)·C₂H₅OH.     

Interaction of N-vinylbenztriazole with halogens and hydrogen halides

Russian Chemical Bulletin -     

Polymerization of aromatic hydrocarbons with sulfur

The polymerization reactions of elemental sulfur with the polynuclear aromatic hydrocarbons pyrene and chrysene were studied. Heating the hydrocarbons with sulfur produces a series of sulfur-containing oligomers in which the aromatic ring systems remain intact. Polymerization is effected through the dehydrogenative action of sulfur and leads to thermally stable five- or six-membered heterocyclic ring systems. The major loss of sulfur during subsequent heat treatment occurs only at carbonization temperatures above 1000°C.     

Thermochemistry and kinetics of hydrogen abstraction by methyl radical from polycyclic aromatic hydrocarbons

Thermodynamic and kinetic properties relating to hydrogen abstraction by methyl radical from various sites in polycyclic aromatic hydrocarbons (PAHs) have been investigated. The reaction enthalpies (298 K), barriers (0 K), and activation energies and pre-exponential factors (700-1100 K), have been calculated by means of density functional theory, specifically with B3-LYP/6-311G(d,p) geometries, followed by BMK/6-311+G(3df,2p) single-point energy calculations. For uncongested sites in the PAHs, a reasonable correlation is obtained between reactivities (as characterized by the reaction barriers) and reaction enthalpies. This is reflected in a Bell-Evans-Polanyi (BEP) relationship. However, for congested sites, abstraction is accompanied both by lower reaction enthalpies (due to relief of steric strain) and also by reduced reactivities (due to significantly increased steric hindrance effects in the transition structures), so that the BEP relationship does not hold. In addition, the reaction enthalpies and kinetic parameters for the series of linear acenes indicate that abstraction is more difficult from the central rings.