The cationic oligomerization of p-aminostyrene 2. Dimerization in the intial stage

Summary It was found that p-aminostyrene (PAS) could be oligomerized by using several aminium perchlorates as catalysts in acetonitrile to give a dimer D in the initial stage of the reaction and finally the isomerization oligomer 1 at the end, in quite similar manner to the preceding paper (1), where dimerization (hydroamination) is the main reaction in the initial stage. The pK_a values for the amines were evaluated by the indicator method, and the [H⁺]₀ and [MH⁺]₀ values were calculated from equilibrium equations. As a result, the rate increased with decreasing pK_a of the catalytic amine used and reached an overall constant value. From the relationships between the rate of the reaction and the [H⁺]₀ or [MH⁺]₀ values calculated numerically, however, it was determined that the rate was more affected by the [H⁺]₀ or [M⁺]₀ values rather than the pK_a of the catalyst. It was assumed that the reaction would be initiated by the electrophilic attack of MH⁺ on the double bond of the monomer. Three successive reactions were proposed for the dimerization step: slow protonation followed by subsequent fast reactions; amine addition to the resultant styryl cation and proton elimination. It was assumed that oligomerization might proceed via the same three successive reactions as proposed for the dimerization step. Received: 24 July 2000/Revised version: 8 September 2000/Accepted: 18 September 2000     

Synthesis and characterization of some new chelating amphoteric ion-exchange resins

Some new chelating amphoteric ion exchange resins have been synthesized by condensing catechol, 8-hydroxyquinoline, hydroquinone, salicyclic acid, sulfosalicyclic acid, 3-hydroxy-2-naphthoic acid, p-hydroxybenzoic acid, β-resorcylic acid, and anthranilic acid with epichlorohydrin employing diethylenetriamine as a cross linking agent in a nonaqueous medium, toluene. These resins are characterized by their physicochemical properties such as moisture retention ability, true density, apparent density, void fraction, concentration of ionogenic groups, volume capacity, total ion exchange capacity, copper exchange capacity, rate of ion exchange, pH titration curves, apparent pK_a and pK_b values, isoionic point values, oxidative degradation, thermal stability, the effect of temperature of equilibration on the capacity of the resin, swelling behavior in various solvents, and absorption specificity towards certain bivalent metal cations in ammonium acetate–dimethylformamide media.     

Titration studies of phytic acid

The 12 pK_a values for phytic acid have been obtained from pH titration data and nonlinear regression analysis. The pK_a values ranged from 1.9 to 9.5. The results are discussed in terms of previously determined pK_a values. Possible reasons are given for the differences between previously determined values and present values.     

UPTAKE OF METAL IONS BY A NEW CHELATING ION EXCHANGE RESIN. PART 3: PROTONATION CONSTANTS VIA POTENTIOMETRIC TITRATION AND SOLID STATE 31P NMR SPECTROSCOPY

Abstract

A new chelating ion exchange resin which incorporates methylenediphosphonate, carboxylate, and sulfonate functional groups in a polystyrene-divinylbenzene matrix has been prepared. This resin exhibits exceptionally high affinity for polyvalent cations even from moderately acidic aqueous media. Metal ion coordination occurs primarily at the diphosphonate group with the secondary binding sites contributing to charge neutralization when necessary and possible, and to increasing hydrophilicity of the resin pores. In the present investigation, the protonation equilibria of the phosphonate groups in the resin are investigated via potentiometric titration and solid-state ³¹P NMR spectroscopy of the resin. Intrinsic equilibrium constants for the first two diphosphonate protonation reactions are pk₄7equal; 10.47 and pK₃ = 7.24. The last two protons added to the diphosphonate group are acidic having pK_a values less than 2.5. These protonation constants are consistent with those reported previously for monomer analog 1,1-diphosphonic acids. This result implies that thermodynamic data available in the literature can be used to predict the relative affinity of the resin for polyvalent cations.     

Effect of the carboxylate group distribution on potentiometric titration of acrylamide-acrylic acid copolymers

Summary the potentiometric behavior of acrylamide-acrylic acid copolymers is discussed as a function of their method of preparation: base hydrolysis of polyacrylamide or radical copolymerization. The values of pK₀ (the intrinsic pK at zero degree of ionization) and the variations of pK_a versus the ionization degree are found to depend on the origin of the polymers and such differences are qualitatively attributed to significant discrepancies in the carboxylate group distribution.     

The electrochemical oxidation of phenylenediamines

Five p-phenylenediamines were oxidized electrochemically at a glassy carbon rotating ring-disk-electrode assembly. The overall reaction comprises the transfer of two electrons per molecule. At low to intermediate potentials the rate determining step for all the five compounds is the acid dissociation of the semiquinonediimine radical cation formed in a pre-equilibrium involving the transfer of one electron. Only at higher potentials can the radical cation be oxidized in parallel to the neutral semiquinone radical to form the quinonediimines. In contrast, the neutral semiquinonediimine radicals are oxidized at the same or even a lower potential than the parent phenylenediamines. The dissociation rate constants of the radical cations are in the range of 10-300 s⁻¹. The effect of the substituents on the oxidation potentials is as expected by their property to donate electrons to the mesomeric system. Their effect on the dissociation rates, however, seems to rather correlate with the inductive effect and might influence not only the electronic structure of the molecule itself but also the structure of the ions and the solvent around them. The semiquinonediimine radical cations are found to be slightly more acidic than their protonated parent phenylenediamine cations with pK_a values somewhat below 6.     

Study of Aromatic Compounds Derived from Sugarcane Bagasse: II. Effect of Concentration

The results of a set of experiments that were designed to integrate current knowledge on the major constituents of sugarcane bagasse lignin, specifically p‐coumaric, ferulic, syringic acids, and vanillin are presented. These aromatic compounds have well‐known antioxidant power and they are very important for the food, pharmaceutical, and nutraceutical industries. Following a study on the effect of pH upon the spectra of these monomers and the determination of their pK_a values, their self‐association properties in ethanol as a solvent, particularly at concentrations < 10^–4 M were investigated using UV spectrophotometry and their K_ass values were determined. The results collected allowed to better understand the chemistry of sugarcane lignin and provide a contribution to the possible exploitation of this abundant straw material for the recovery of fine chemicals.     

Studies of thermal,mechanical properties,and kinetic cure reaction of carboxyl-terminated polybutadiene acrylonitrile liquid rubber with diepoxy octane

Epoxy resins, despite their unique properties, have limitations in many applications due to their low fracture toughness. One of the most effective methods to overcome this limitation is to use toughening agents, such as carboxyl terminated poly butadiene-acrylonitrile (CTBN) in the epoxy matrix. CTBN can react with various compounds, such as epoxies. In this study, we investigated the severity of CTBN sedimentation with di epoxy octane (DEO) in the presence basis catalysts. The studied of the physical properties of the synthesized copolymer in the presence of pyridine compared to other catalysts increases mechanical properties (248.43% elongation, 0.63 MPa strength, and 32 hardness with Shore A) and decreases the glass transition temperature (−45.1) of the copolymer. Investigated the cure kinetics of the CTBN-DEO reaction was in the presence of pyridine using a nonisothermal technique of differential scanning calorimetry, and the curing kinetic parameters, such as activation energy (E_a), pre-exponential factor (A), and rate constant (k), were calculated by different kinetic methods. The obtained curing kinetic values with different kinetic methods are well-matched, the E_a values are in the range of 91.3–97.1 kJ.mol⁻¹ and the A values are in the range of 0.48 × 10¹¹–1.51 × 10¹¹ S⁻¹.     

Struktur und Reaktivität heterosubstituierter Nitrile. XVII [1]. Kinetik und Mechanismus der Phenol-Addition an Arylcyanate in Gegenwart tertiärer Amine

Structure and Reactivity of Heterosubstituted Nitriles. XVII. Kinetics and Mechanism of the Addition of Phenols to Aryl Cyanates in Presence of Tertiary Amines The rate of carbonic acid diaryl ester imides formation by the addition of phenols to aryl cyanates in the presence of tertiary amines was determined by IR-spektroscopy. The rate constants are first order in aryl cyanates and correlate linear with [ArOH] [ArO⁻ ∥ R₃NH⁺]. The third order rate constants depend linear on the (σ + σ⁻)-values of the phenol substituents, the σ⁰-values of the aryl cyanate substituents, and the pK_a of the tertiary ammonium salts. These results support a transition state formed by the nucleophilic attack of a phenoxide trialkyl ammonium ion pair to an aryl cyanate phenol associate.     

Kinetics of CO2 absorption into a novel 1‐diethylamino‐2‐propanol solvent using stopped‐flow technique

A stopped‐flow apparatus was used to measure the kinetics of carbon dioxide (CO₂) absorption into aqueous solution of 1‐diethylamino‐2‐propanol (1DEA2P) in terms of observed pseudo‐first‐order rate constant (k_o) and second‐order reaction rate constant (k₂), in this work. The experiments were conducted over a 1DEA2P concentration range of 120–751 mol/m³, and a temperature range of 298–313 K. As 1DEA2P is a tertiary amine, the base‐catalyzed hydration mechanism was, then, applied to correlate the experimental CO₂ absorption rate constants obtained from stopped‐flow apparatus. In addition, the pK_a of 1DEA2P was experimentally measured over a temperature range of 278–333 K. The Brønsted relationship between reaction rate constant (obtained from stopped‐flow apparatus) and pK_a was, then, studied. The results showed that the correlation based on the Brønsted relationship performed very well for predicting the absorption rate constant with an absolute average deviation of 5.2%, which is in an acceptable range of less than 10%. © 2014 American Institute of Chemical Engineers AIChE J, 60: 3502–3510, 2014     

Proton-Ionizable Crown Compounds: Transport of Alkali and Alkaline Earth Cations Using Proton-Ionizable Triazolo Macrocycles

Abstract

The macrocycle-mediated flaxes of the alkali and alkaline earth metal cations have been determined in a H₂O-CH₂Cl₂-H₂O bulk liquid membrane system. Water-insoluble proton-ionizable macrocycles of the triazolo type were used. The proton-ionizable feature allows the coupling of cation transport to reverse H⁺ transport. This feature offers promise for the effective separation and/or concentration of alkali metal ions with the metal transport being driven by a pH gradient. A counter anion in the source phase is not co-transported. Transport of the alkali cations only occurred when the source phase pH was greater than the aqueous pK_a value for the carriers. Transport increased regularly with increasing source phase pH. Transport of alkaline earth cations from neutral pH source phases was minimal. The alkali cation selectivity order was K⁺ > Rb⁺ > Cs⁺ > Na⁺ > Li⁺ for the l8-crown-6 sized macrocycles, while little selectivity was observed with the 15-crown-5 sized macro-cycle.     

The calculation of sensitometric properties of 1,2,4-triazolo[1,5-a]pyrimidines

Quantitative structure–property relationships were proposed by using artificial neural networks and information received from ¹³C NMR spectra. The suitability of 1,2,4-triazolo[1,5-a]pyrimidines as stabilizers in photographic silver halide materials was determined from their chemical structures. For the numeric coding of the chemical structures of differently substituted 1,2,4-triazolo[1,5-a]pyrimidines 1 – 44 only information available from their ¹³C NMR spectra was used. Even an assignment of the ¹³C NMR chemical shift values to the carbons was not necessary. The best results were achieved by combination of the ¹³C NMR chemical shifts of carbons of the basic heterocycle and the relative fog D_rel using a feed-forward two-layer neural network. For some compounds with a good stabilizing effect the calculated results strongly differ from experimental values giving indication of a mechanism which is not covered by the ¹³C NMR chemical shifts.     

pKa-Values of Guaiacyl and Syringyl Phenols Related to Lignin

Abstract

The literature relating to the pK_a -values of guaiacyl- and syringyl-derived phenols has been thoroughly surveyed and summarized. In addition, the pK_a -values of a number of guaiacyl, syringyl and other phenols related to lignin have been determined using a spectrophotometric method combined with multivariate evaluation. Differences and similarities between the acidities of a number of substances are extensively discussed. The pK_a -value strongly affects the delignification during pulping, bleaching and leaching of lignin during pulp washing.     

Reaction of cysteine and methionine with malonaldehyde

Malonaldehyde (M), a product of polyunsaturated fatty acid oxidation, reacted with the sulfhydryl as well as with the amino groups of cysteine (cys). The cys-M product had an absorption maximum at 310 mμ, and the extinction coefficient at pH 7.0 was 2.3 × 10⁴. Elementary analysis of the cys-M product agreed with a structure in which 2 moles of cysteine had reacted with 3 moles of malonaldehyde. The molecular weight of cys-M preparations increased on storage and the UV absorption changed from 310 to 315–320 mμ, with a consequent shift to longer wavelength in the visible. Methionine (meth) reacted with malonaldehyde under the same reaction conditions only at the α-amino group, similar to glycine (gly). The apparent pK_a of the carboxyl group of gly-M increased to 3.36 and that of meth-M to 3.19, representing an increase of about one pK_a unit over the natural amino acids. For gly-M and meth-M the respective absorption maxima were 272 and 282 mμ. The spectral shifts from 267 to 315 mμ of the amino acid-M products with respect to β-oxyacrolein were explained in terms of increasing substitution at the β-carbon of the α,β-unsaturated carbonyl system. When the α-amino-malonaldehyde condensation products of methionine and glycine reacted with semicarbazide the original amino acids and disemicarbazone of malonaldehyde were formed.     

Imidocarbocyanines—spectral sensitizers of photographic silver halide emulsions

The review deals with the main results of several investigations of the relationship between the chemical structure of imidocarbocyanines and their basicity, redox potentials and photographic action in silver halide emulsions. A number of quantitative and qualitative relationships connecting p K_a and E_1,2^Ox(Rcd) of imidocarbocyanines and the Hammett σ_p-constants of different electronic substituents ranging from CH₃O (σ_p = ?0·27) to CF₃SO₂ (σ_p = 0·96) and the Taft σ^*-constants of groupings attached to the nitrogen atom (alkyl, aryl, ω-trifluoromethylalkyl or sulphoalkyl), are discussed. The main factors contributing to the total effect of spectral sensitization of silver halide emulsions by imidocarbocyanines are analyzed and a relationship between the degree of suppression of their sensitizing action by colour components and the tendency to aggregate on the surface of silver halide crystals is established.     

Oxidation of Esculetin,a Model Pollutant Present in Cork Processing Wastewaters,by Chemical Methods

The ozonation of esculetin (6,7-dihydroxycoumarin), a major pollutant present in the wastewater generated in the cork industry, was accelerated at high pH, with apparent second-order rate constants in the range from 3.3 × 10⁴ L/(mol·s) at pH=2 to 8.4 × 10⁷ L/(mol·s) at pH=9. The acid-base equilibrium of esculetin was studied, resulting in a pK_a value of 7.37. Taking into account this pK_a, the rate constants for the reaction between ozone and the un dissociated and dissociated forms of esculetin were 3.0 × 10⁴ and L/(mol·s) 6.67 × 10⁸ L/(mol·s), respectively. Apparent first-order rate constants for the photolysis by UV irradiation were also evaluated, with values between 0.12 × 10^?2 min^?1 at pH=2 and 1.15 × 10^?2 min^?1 at pH=9, while the quantum yields for this photo-degradation reaction varied from 0.99 × 10^?2 mol/Eins to 11.1 × 10^?2 mol/Eins at these pHs. The Fenton's reagent system was used for the generation of hydroxyl radicals, and the rate constant for the reaction between esculetin and these radicals was determined to be 1.06 × 10¹⁰ L/(mol·s). Finally, several chemical oxidation systems were used in the degradation of this pollutant: single oxidants (ozone, UV irradiation) and advanced oxidation processes (Fenton's reagent, UV/H₂O₂, O₃/H₂O₂, O₃/UV, O₃/H₂O₂ /UV, and photo-Fenton system). The results revealed that the most efficient methods in terms of esculetin removal were ozonation among the single oxidants, and the photo-Fenton system among the combined processes.     

Determination of Structure and Properties of Acetylacetone Dioxime,Benzoylacetone Dioxime and Bis-Benzoylacetone Dioximato-Copper (II): An Ultraviolet Study

UV spectra of acetylacetone dioxime (2,4-pentanedione dioxime = AADO), benzoylacetone dioxime (BADO) and bis-benzoylacetone dioximato-Cu(II) [Cu- (BADO*)₂; BADO* = oxidized BADO] are recorded for aqueous and CHCl₃ solutions at different pH values and interpreted. pK_a values (1st dissociation constant) for AADO and BADO at constant ionic strenths (μ = 0.12) are derived: pK_a (AADO) = 10.50; pK_a (BADO) = 9.50, and the influence of molecular structure on acid-base properties is discussed. Diagrams of possible molecular orbital energy-level transitions for the oxime and the nitroso chromophores are given. The marked influence of acids (HC1) and bases (triethylamine, TEA) on the shape of the UV spectra of Cu- (BADO*)₂ is discussed and explained in terms of the acid-base properties of the nitroso and the oxime groups. Additional evidence has been adduced for the presence of nitroso group and of hydrogen bonds in these complexes.     

Isothermal Cure of an Epoxy System Containing Tetraglycidyl-4,4′-diaminodiphenylmethane (TGDDM), a Multifunctional Novolac Glycidyl Ether and 4,4′-Diaminodiphenylsulphone (DDS): Vitrification and Gelation

Times to gelation and vitrification have been determined at different isothermal curing temperatures between 200 and 240°C for an epoxy/amine system containing both tetraglycidyl-4,4′-diaminodiphenylmethane (TGDDM) and a multifunctional Novolac glycidyl ether with 4,4′-diaminodiphenylsulphone (DDS). The mixture was rich in epoxy, with an amine/epoxide ratio of 0·64. Gelation occurred around 44% conversion. Vitrification was determined from data curves of glass transition temperature, T_g, versus curing time obtained from differential scanning calorimetry experiments. The minimum and maximum values T_g determined for this epoxy system were T_g0=12°C and T_gmax=242°C. Values of activation energy for the cure reaction were obtained from T_g versus time shift factors, a_T, and gel time measurements. These values were, respectively, 76·2kJmol^-1 and 61·0kJmol^-1. The isothermal time–temperature–transformation (TTT) diagram for this system has been established. Vitrification and gelation curves cross at a cure temperature of 102°C, which corresponds to glass transition temperature of the gel. © of SCI.     

Crystallization kinetics of γ phase poly(vinylidene fluoride)(PVDF) induecd by tetrabutylammonium bisulfate

The phase transition behavior of poly(2-ethyl-2-oxazoline) (PEtOx) under complexation with star-shaped poly(acrylic acid) (PAA) having various arm numbers (two, three, four, and six) has been studied by turbidity and laser light scattering measurements. The change in cloud point temperature (T _cp) of PEtOx was monitored as a function of pH, ionic strength, and arm number of the star polyelectrolyte. The shift in T _cp to a lower value than that of pure PEtOx was more pronounced at pH 4.2 (pH?a), when the carboxylic acid groups are protonated as compared to pH 7.0 (pH?>?pK_a ), when the acid moieties are partially ionized. Dynamic light scattering showed that these complexes may have micellar core-shell type structure with a mean hydrodynamic radius (R _h) ranging from 12 nm to ～200 nm depending upon the temperature. Significant shift in T _cp was observed for six-arm star poly(acrylic acid) complexes at both pH values. This change in the T _cp is accredited to the differences in the driving forces of phase transition, including hydrogen bonding between carboxylic acid groups of PAA and the carbonyl moiety of PEtOx as well as the hydrophobic interactions.     

Comprehensive study of the structural and electronic properties of complexes formed by Mz+ (Li+, Na+, K+, Be2+, Mg2+, Ca2+) cation and thiophene and its derivatives

The interactions of metal cations (Li⁺, Na⁺, K⁺, Be²⁺, Mg²⁺, Ca²⁺) with thiophene and its derivatives have been investigated using MP2/6-311++G** and B3LYP/6-311++G** methods in the gas phase and the water solution. The following substituents are taken into considerations: F, Cl, Br, CH₃, OH and C₂H₃, which cover electron-withdrawing and electron-donating effects. The substituent, the metal cations and the solvent effects on the cation–π interactions are investigated. The physical properties such as dipole moment, chemical potential and chemical hardness of studied complexes have been systematically explored. Natural population analysis data, the electron density and Laplacian properties have been used to evaluate the cation–π interaction. The electron density (ρ) and Laplacian (?²ρ) properties, estimated by atoms-in-molecules calculations, indicate that the cation–π interactions possess low ρ and positive ?²ρ, which are in agreement with electrostatic character of the cation–π interactions. Charge transfer values are calculated using Mulliken charges and reveal that the electron charge transfer takes place from thiophene and its derivatives to the metal cation in the complexes under the study. To evaluate the aromaticity of the rings upon complexation, several well-established indices of aromaticity such as the nucleus-independent chemical shift and the harmonic oscillator model of aromaticity and the aromatic fluctuation index have been used. The molecular electrostatic potential is given the visual representation of the chemically active sites and comparative reactivity of atoms. Furthermore, the effects of interactions on NMR data have been used for more investigation of the analyzed complexes.