South African Journal of Chemistry - Volume 55, Issue 1, 2002
Volumes & issues
Volume 55, Issue 1, 2002
Source: South African Journal of Chemistry 55, pp 1 –12 (2002)More Less
The copolymerization of difurylmethane with maleic anhydride in the presence of azobisisobutyronitrile as an initiator has been investigated at 65 °C, 72 °C and 85 °C in different solvents. Reaction feeds containing 85 mol % and higher of difurylmethane yielded a soluble alternating copolymer in which the furan residue entrained in the polymer chain formed 2, 5 linkages and had 3, 4 unsaturation. The 1:1 difurylmethane-maleic anhydride adduct was readily prepared, and at the above temperatures and in the presence of the radical initiator partially underwent retro-Diels-Alder reaction. Copolymerization of the difurylmethane and maleic anhydride thus formed yielded the unsaturated alternating copolymer. Copolymers prepared from reaction feeds with proportions of difurylmethane lower than 85 mol % had higher cross-link density and exhibited higher thermodecomposition temperatures.
Source: South African Journal of Chemistry 55, pp 13 –33 (2002)More Less
1-Alkyl-1, 4-dihydro-4-iminoquinoline-3-carboxylates undergo neutral hydrolysis (in H2O or H2O-EtOH mixtures) to yield water-soluble 4-iminoquinoline-3-carboxylic acids and the corresponding 4-oxo esters. Such 4-imino acids are also accessed by treating an appropriate 1-alkyl-1, 4-dihydro-4-oxoquinoline-3-carboxylic acid successively with thionyl chloride and an amine-H2O mixture, or from treatment of a 4-imino ester salt with aqueous amine. In the latter procedures 7-fluoro substituted substrates gave rise to 7-alkylamino derivatives even at room temperature. The title compounds are inferred to have an intramolecularly H-bonded charge transfer structure, and some of their chemical reactions and spectral (HRMS, 1H NMR) properties are described.
Source: South African Journal of Chemistry 55, pp 34 –42 (2002)More Less
Thermal properties of cross-linked and uncross-linked LDPE/wax blends were investigated. The blends were prepared by thoroughly mixing the powdery ingredients, followed by pressing at 180 °C for ten minutes. The extent of cross-linking was determined by means of gravimetric analysis of the gel content of the samples. The thermal properties were determined by differential scanning calorimetry (DSC). The analyses of cross-link density of the samples indicated that increased amounts of peroxide gives rise to more efficient cross-linking, but only the PE phase in the blends is cross-linked. The DSC results indicated that LDPE and wax are probably miscible in the crystalline phase at low wax concentrations, but at higher wax concentrations the wax is only partially miscible in the crystalline phase.
On-line determination of hydrochloric acid in process effluent streams by potentiometric sequential injection acid-base titrationSource: South African Journal of Chemistry 55, pp 43 –55 (2002)More Less
An on-line potentiometric sequential injection acid-base system for the titration of a hydrochloric acid solution with a standard sodium hydroxide solution in process effluent streams is proposed. A solution of 0.1 mol/l sodium chloride is used as carrier. The sample is sandwiched between two titrants in a holding coil, with the volume of the first base being twice to that of the second one and channeled by flow reversal through a reaction coil to the potentiometric sensor. A linear relationship between peak width and logarithm of the hydrochloric acid concentration was obtained in the range 0.025 mol/l - 0.05 mol/l of hydrochloric acid when a solution of 0.001 mol/l sodium hydroxide was used for the titration. Samples from process effluent streams were used to evaluate the feasibility of the method with that of an automated and manual titration. The results showed good agreement between the different methods. The percentage relative standard deviation (RSD %) was found to be less than 0.22. The sample frequency is 30 samples per hour.
Source: South African Journal of Chemistry 55, pp 56 –66 (2002)More Less
5, 5'-Diformyl-2, 2'-difuran has been synthesized in 60% yield by the palladium acetate-catalysed aryl coupling of furfural in acetonitrile in the presence of dioxygen under pressure. Various reaction conditions have been exploited, and mechanistic aspects of the reaction are discussed.
Analytical characterization of volatile active principles from the leaves of the Alboroseum Backer plantSource: South African Journal of Chemistry 55, pp 67 –71 (2002)More Less
The study presents for the first time the data concerning the qualitative and quantitative determination of the volatile active principles from the leaves of the Alboroseum Backer plant (Crassulaceae). After the extraction of volatile active principles in water, analytical separation and quantitative determination using a GC/MS technique was performed. The compounds detected, are belonging to the following classes: aldehydes, ketones, aromatic hydrocarbons and alcohols.
Internal standard method for the determination of Au and some platinum group metals using inductively coupled plasma optical emission spectrometrySource: South African Journal of Chemistry 55, pp 72 –86 (2002)More Less
A method is described for the determination of Au, Pt, Pd, Ru and Rh in a converter matte sample, using inductively coupled plasma optical emission spectrometry (ICP-OES), with Y or Sc as internal standard. The results obtained by this method are discussed and compared with values obtained by an independent laboratory. The efficiency of internal standardization was evaluated by varying the operating conditions of the ICP-OES. Changes in the amount of energy transferred from the plasma to analyte, the amount of aerosol reaching the plasma and analyte residence time, were studied in terms of their effect on the emission intensity of the analyte and the internal standard. It was found that changes in the emission signals of the PGMs due to power variations could be effectively compensated for, using either Sc, Y or Ar lines as internal standards. For variations in aerosol gas flow rate, both Sc and Y could only compensate for Pd and Au between flow rates of 0.60 and 0.80 l min-1 and for Pt, Ru and Rh at 0.80 l min-1. The effect of sodium on the determination of Au and the PGMs, and the use of Sc and Y as internal standards, to compensate for Na matrix effects on accuracy were also studied. It was found that emission intensities of the analytes decreased with increasing Na concentration. Furthermore, Sc as an internal standard could compensate for Pt, Ru and Rh up to 5 g-1 Na, while Y could only compensate for the same elements up to 1 g l-1 Na.
Protection of the amide side-chain of asparagine with the 1-tetralinyl group in the solid-phase peptide synthesis of lysine-vasopressinSource: South African Journal of Chemistry 55, pp 87 –96 (2002)More Less
Lysine-vasopressin, a nonapeptide, was synthesised on a benzhydryl-resin using solid-phase peptide synthesis via the Boc-strategy. The benzyl group was used in the protection of the side-chains of tyrosine and cysteine, while 1-tetralinyl, benzhydryl and benzyloxycarbonyl groups were used in the protection of the side-chains of asparagine, glutamine and lysine, respectively. Thioanisole-1, 2-ethanedithiol-trifluoroacetic acid-trifluoromethanesulphonic acid (2:1:20:2 v/v) was used to cleave the peptide-resin under different temperature conditions. The cleavage at 40 °C for two hours gave lysine-vasopressin in a one-pot reaction; the yield after reversed-phase HPLC purification was 62%.
Isolation and crystal structure of 3-aryl-1-(2-hydroxyphenyl)-3-hydroxy-1-propanones derived from Claisen-Schmidt condensation of 2-hydroxyacetophenone with benzaldehyde derivativesSource: South African Journal of Chemistry 55, pp 97 –110 (2002)More Less
2-Hydroxyacetophenone reacted with benzaldehyde derivatives under alkalinic conditions followed by addition of dilute hydrochloric acid solution to afford mixtures of the known 2-hydroxychalcone derivatives and the corresponding previously undescribed 3-aryl-1-(2-hydroxyphenyl)-3-hydroxy-1-propanones. The isolation of the β-hydroxyketone derivatives depends on the work-up conditions. Their structures were established using a combination of NMR, IR and mass spectroscopic techniques. The conformation of the β-hydroxyketones was probed using 1H NMR spectroscopy and X-ray crystallography.
Source: South African Journal of Chemistry 55, pp 111 –118 (2002)More Less
The efficient synthesis of 2-arylamino-2-imidazolines, 2-heteroaryl-substituted benzimidazoles, and their morpholin-4-ylmethyl derivativesSource: South African Journal of Chemistry 55, pp 119 –123 (2002)More Less
2-Heteroaryl-substituted 1H-benzimidazoles were synthesized in good yields by heating the HSO3 adducts of heteroaromatic aldehydes with o-phenylenediamine in DMF under reflux. This procedure is more general and shorter than earlier methods. 2-Arylamino-2-imidazolines were prepared by heating dimethyl aryldithioimidocarbonates and ethylenediamine under reflux. The imine-enamine tautomerization of 2-arylamino-2-imidazolines was investigated by means of 1H-NMR spectroscopy. Morpholin-4-ylmethyl derivatives of the benzimidazole and imidazoline products were synthesized regioselectively by treatment with morpholine and formaldehyde.
Source: South African Journal of Chemistry 55, pp 125 –131 (2002)More Less
The direct electrochemical oxidation of 4-t-butyltoluene at graphite electrodes in methanol, using NaClO4 as electrolyte, as been investigated in order to obtain an insight into the mechanism and kinetics of the anodic reactions taking place. It is shown that an increase in current density affects the product distribution, leading to a better yield of the target product 4-t-butylbenzaldehyde dimethyl acetal and a lower percentage of unknowns at the expense of an increase in electrical charge. It is suggested that oxidation takes place by two mechanisms : a direct one involving oxidation of the substrate at the electrode, and an indirect one where a pool of solvent radicals assists in the oxidation process. Further evidence is provided for reaction mechanisms in studies where the substrate loading is varied, and also where the ratio of substrate to solvent is varied ; when the concentration of the substrate is high, there is evidence of the formation of dimers and other unknowns. Under certain conditions, these anodic reactions can be regarded as parallel consecutive second order reactions. The role played by the solvent is confirmed by kinetic data.
Source: South African Journal of Chemistry 55, pp 132 –135 (2002)More Less
The synthesis and isolation of one diastereomer of 1-benzyl-7, 9-dimethyl-8-oxo-1-azaspiro[4.4]nonane-6-carbonitrile 11 was accomplished by the diiron nonacarbonyl-assisted spirocyclization reaction of 2-(1-benzyl-2-pyrrolidinylidene)acetonitrile 10 and 2, 4-dibromo-3-pentanone 12.NOESY NMR spectroscopy experiments of 11 showed it to be the (5R*, 6S*, 7S*, 9S*)-diastereomer.
Complex kinetics in the reaction of taurine with aqueous bromine and acidic bromate : a possible cytoprotective role against hypobromous acidSource: South African Journal of Chemistry 55, pp 136 –143 (2002)More Less
The most abundant aminoacid in the human body, 2-aminoethanesulphonic acid (H2NCH2CH2SO3H), is surprisingly stable and reacts exceedingly slowly even with the most powerful oxidizing agents like acidic bromate. Oxidation occurs only on the nitrogen centre to give the corresponding N-derivatives. No activity is observed at the sulphonic acid group and no cleavage of the C-S bond is observed. The stoichiometry of the oxidation of 2-aminoethanesulphonic acid by bromate is complex, yielding a mixture of monobromo- and dibromotaurines, oximes as well as the corresponding dimeric azo-compounds. In the presence of added bromide, the stoichiometry of the reaction is :
2BrO3- + 3H2NCH2CH2SO3H + 6H+ + 4Br- 3Br2NCH2CH2SO3H + 6H2O.
Monobromotaurine is formedas an intermediate product before formation of the dibromotaurine. Aqueous bromine reacts quantitatively with 2-aminomethanesulphonic acid according to the stoichiometry :
H2NCH2CH2SO3H + 2Br2 -> Br2NCH2CH2SO3H + 2Br- + 2H+.
This reaction is strongly inhibited by acid due to the deactivation of the amino group to electrophilic attack by protonation. The formation of N-bromotaurines is suggested as a possible mechanism by which taurine can moderate the oxidative toxicity of bromine and hypobromous acid in the slightly basic physiological environments.
The synthesis of novel enclathration compounds : Bis(9-amino-9-aryl-9H-thioxanthenes) and investigations of their host-guest inclusion potentialSource: South African Journal of Chemistry 55, pp 144 –148 (2002)More Less
Four novel bis(9-amino-9-aryl-9H-thioxanthenes) were synthesized and their inclusion potential assessed by allowing them to crystallize slowly from organic solvents (single and binary mixtures). Only two of the compounds studied displayed inclusion ability and enclathrated several of the solvents studied. The stoichiometries of the resultant inclusion complexes were determined by 1H-NMR studies, and the favoured host:guest ratios were found to be predominantly 1 : 1, with ratios 2 : 1, 4 : 3 and 3 : 2 occurring less frequently. The stability of the clathrate complexes has been investigated by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). Competition experiments were carried out with the compounds that successfully hosted solvent molecules in order to determine their relative affinity for these solvents. One of the hosts was also exposed to two vaporous guests in two separate experiments to assess its ability for entrapping molecules from the vapour phase.