The hydroxylation of alkanes and alkyl chains

A solution of 30% aqueous H₂O₂ in trifluoroacetic acid converts linear alkanes and cycloalkanes to an equilibrium mixture of secondary alcohols and their trifluoroacetates. There is no trace of further oxidation to ketones. Yields and conversions can be 70–90%, but are limited by elimination and epoxidation to form vicinal glycols. Linear alkyl chains are also hydroxylated in good yields with selectivity for introducing the alcohol group remote from electronegative substituents. The active reagent is trifluoroperoxyacetic acid, and it is effective in the presence of 4–10% water. When tertiary hydrogens are present, the oxidations are more complex.     

Primary radical formation in the vapor-phase nitration of alkanes with nitrogen dioxide

Summary It is shown with the aid of thermodynamic calculations that it is very unlikely that free radicals will be formed by reaction of nitrogen dioxide with alkanes in accordance with the equation RH +NO₂ R + HNO₂.     

Orientation effects in the nitration of the dithieno[b,d]pyridine N-oxides. Its dependence on acidity of the reaction medium

The nitration of three dithienopyridine-N-oxides was investigated. The regiochemistry of the reaction was dependent on the reaction conditions used. Under strongly acidic conditions the positional preference is similar for the N-oxides and free bases. However, under mildly acidic or neutral conditions a completely different substitution pattern was obtained. In the latter case those ring positions were substituted which are expected to be unfavored or forbidden in electrophilic substitution of the free bases. The structures of the nitro derivatives were proven by extensive use of ¹H and ¹³C nmr spectroscopy.     

NO-Mediated aromatic nitration during decomposition of phenolic S-nitrosothiols in non-aqueous aerobic medium

Five novel S-nitrosothiol compounds (6-10) derived from L-cysteine were generated in solution and their decomposition rate was followed by UV spectroscopy. In acetonitrile, compounds 9 and 10 were the most stable of this series with a half-life of 24 h. The final organic decomposition products of the five S-nitrosothiols were also analysed. Derivatives 8, 9, and 10, possessing a phenolic hydroxyl group, afforded an unexpected decomposition pathway, with nitration of aromatic ring occurring in non-aqueous media. A mechanism involving a phenoxy radical seems to be implicated.     

Study of nitration equilibrium in the glycerin-aqueous nitric acid system. 1. Dependence of the equilibrium constants of nitration reactions on the temperature,acidity of the medium,and structure of the nitrated compound

The equilibrium constants of seven sequential-parallel reactions of conversion of glycerin into glycerin trinitrate in aqueous HNO₃ were measured. The effect of the acidity of the medium on the equilibrium nitration constants is correlated with processes of protonation of glycerin and its nitrates. The equilibrium nitration constants are higher for primary hydroxides than for secondary hydroxides, and they decrease in both series in going from glycerin to its dinitrates.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 8, pp. 1723–1729, August, 1990.     

Thermal reactions of selected solids including reactants that melt during chemical change

Selected kinetic and mechanistic studies of thermal reactions of initially solid substances are reviewed with emphasis on the evidence that some of these chemical changes proceed with the essential participation of melting. The reactions considered are classified on the extent and the role of such melting and the various types of behaviour observed are discussed with reference to solid state rate processes in crystals. It is stressed that melting is an important feature in theoretical considerations of crystal reactivity because chemical changes often proceed more rapidly in a melt than in the solid state. However, literature reports concerned with reactions of solids do not always explicitly mention the possibility of melting during discussions of reactions mechanisms. The present paper comments on methods capable of detecting liquefaction during reaction, a feature of behaviour that is not always easily identified experimentally. Also considered here is the recognition of reaction intermediates, which provide important evidence concerning the course of the chemical changes through which the reactant is transformed into product. This short review draws attention to the considerable value of chemical evidence in elucidating mechanisms of reactions of solids including the necessity for identifying intermediates and the role of any melt or liquid participating.     

Effect of acidity of the medium and polarization of the surface on albumin adsorption by carbon fibers

Adsorption of bovine serum albumin (BSA) was studied at different acidities of the medium on the non-charged and titanium hydroxide-modified surfaces of the carbon fibers (ACF and ACF-B (Ti)). The fiber covered with titanium hydroxide was shown to have the highest adsorption capacity at pH = 4.7, which is close to the isoelectric point of BSA. The cathodic polarization has the greatest effect on the BSA desorption ability, which enabled the determination of conditions for recovering the adsorption capacity of the fibrous sorbent.     

Effect of medium acidity on the efficiency of oxidation of 2,4,6-trinitrotoluene to 2,4,6-trinitrobenzoic acid

An effect of boric acid additives on oxidation of 2,4,6-trinitrotoluene (TNT) to 2,4,6-trinitrobenzoic acid (TNBA) with chromic anhydride in concentrated (96—100%) H₂SO₄ has been studied. In the presence of tetrahydrosulfatoboric acid HB(HSO₄)₄ formed in situ (up to 5 mol.%) or added as a preliminary prepared solution (up to 1 mol. %), TNT is selectively oxidized to TNBA in the yields up to 95—99%. The mechanism including formation of TNT dication as a key step of its oxidation at the methyl group has been suggested.     

Evaluating the contributions of desolvation and base-stacking during translesion DNA synthesis

DNA polymerases catalyze the insertion of a nucleoside triphosphate into the growing polymer chain using the template strand as a guide. Numerous factors such as hydrogen bonding interactions, base-stacking contributions, and desolvation play important roles in controlling the efficiency and fidelity of this process. We previously demonstrated that 5-nitro-indolyl-2'-deoxyriboside triphosphate, a non-natural nucleobase with enhanced base-stacking properties, was more efficiently inserted opposite a non-templating DNA lesion compared to natural templating nucleobases (E. Z. Reineks and A. J. Berdis, Biochemistry, 2004, 43, 393-404). The catalytic enhancement was proposed to reflect increased base-stacking interactions of the non-natural nucleobase with the polymerase and DNA. However, the effects of desolvation could not be unambiguously refuted. To further address the contributions of base stacking and desolvation during translesion DNA replication, we synthesized indolyl-2'-deoxyriboside triphosphate, a nucleobase devoid of nitro groups, and measured its efficiency of enzymatic insertion into modified and unmodified DNA. Removal of the nitro group reduces the catalytic efficiency for insertion opposite an abasic site by 3600-fold. This results from a large decrease in the rate of polymerization (similar 450-fold) coupled with a modest decrease in binding affinity (similar 8-fold). Since both non-natural nucleobases show the same degree of hydrophobicity, we attribute this reduction to the loss of base-stacking contributions rather than desolvation capabilities. Indolyl-2'-deoxyriboside triphosphate can also be inserted opposite natural nucleobases. Surprisingly, the catalytic efficiency for insertion is nearly identical to that measured for insertion opposite an abasic site. These data are discussed within the context of pi-electron interactions of the incoming nucleobase with the polymerase:DNA complex. Despite this lack of insertion selectivity, the polymerase is unable to extend beyond the non-natural nucleobase. This result indicates that indolyl-2'-deoxyriboside triphosphate acts as an indiscriminate chain terminator of DNA synthesis that may have unique therapeutic applications.     

Unified mechanistic concept of electrophilic aromatic nitration: convergence of computational results and experimental data

The mechanism of electrophilic aromatic nitration was revisited. Based on the available experimental data and new high-level quantum chemical calculations, a modification of the previous reaction mechanism is proposed involving three separate intermediates on the potential energy diagram of the reaction. The first, originally considered an unoriented pi-complex or electron donor acceptor complex (EDA), involves high electrostatic and charge-transfer interactions between the nitronium ion and the pi-aromatics. It explains the observed low substrate selectivity in nitration with nitronium salts while maintaining high positional selectivity, as well as observed oxygen transfer reactions in the gas phase. The subsequent second intermediate originally considered an oriented "pi-complex" is now best represented by an intimate radical cation-molecule pair, C(6)H(6)(+)(*)()/NO(2), that is, a SET complex, indicative of single-electron transfer from the aromatic pi-system to NO(2)(+). Subsequently, it collapses to afford the final sigma-complex intermediate, that is, an arenium ion. The proposed three discrete intermediates in electrophilic aromatic nitration unify previous mechanistic proposals and also contribute to a better understanding of this fundamentally important reaction. The previously obtained ICR data of oxygen transfer from NO(2)(+) to the aromatic ring are also accommodated by the proposed mechanism. The most stable intermediate of this reaction on its potential energy surface is a complex between phenol and NO(+). The phenol.NO(+) complex decomposes affording C(6)H(6)O(+)(*)/PhOH(+) and NO, in agreement with the ICR results.     

Effect op protonation of 2,2′-dithienyl carbonyl compounds on the direction of substitution during nitration

The nitration of 5-formyl- and 5-acetyl-2,2-dithienyls by the action of potassium nitrate in 60–95% sulfuric acid solutions was studied. An increase in the acidity of the medium and a decrease in the reaction temperature from +30°C to –30°C lead to an increase in the percentage of the 5-nitro isomer in the mixture.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 8, pp. 1047–1050, August, 1977.     

Adsorption of alkanes on a bismuth electrode from an alcoholic medium

The adsorption of higher alkanes (from C₁₀H₂₂ to C₂₁H₄₄) on a polarized bismuth electrode was studied from methanolic, ethanolic and n-butanolic solutions of LiClO₄ by differential capacity measurements. Alkane molecules, which do not contain either polar groups or double bonds, are compounds of low chemical activity. The adsorption of this group of compounds is purely physical. Nevertheless, a considerable difference in their adsorption activity on the various areas of the electrode surface caused by the two types of bond (metallic and covalent) between the atoms in the bismuth crystal lattice was discovered. Such a large difference indicates the different wettabilities of the separate single-crystal faces by alkanes. A weak dependence of the interaction parameter in the Frumkin isotherm on the length of the alkane molecule as well as on the length of the hydrocarbon chain of the solvent molecule was established. Various adsorption effects at high bulk concentrations of adsorbate (desolvation of the cations of the supporting electrolyte, thickening of the adsorption layer, maximum in the isotherm) were analysed.     

A simple nitration of electrophilic alkenes with tetranitromethane in the presence of triethylamine. Synthesis of functionalized β-nitroalkenes

A convenient method for the preparation of functionalized β-nitroalkenes based on the nitration of α,α-di- and α,α,β-trisubstituted unsaturated aldehydes, ketones and esters with tetranitromethane in the presence of triethylamine is described. Series of substituted β-nitroalcohols and β-nitroalkenes are obtained in good yields under mild conditions.