Scielo RSS <![CDATA[South African Journal of Chemistry]]> http://www.scielo.org.za/rss.php?pid=0379-435020090001&lang=pt vol. 62 num. lang. pt <![CDATA[SciELO Logo]]> http://www.scielo.org.za/img/en/fbpelogp.gif http://www.scielo.org.za <![CDATA[<b>The use of Dowex-M43 ion-exchange resin in the determination of Pt and Pd in converter matte by ICP-OES</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502009000100001&lng=pt&nrm=iso&tlng=pt The method of choice for the separation of platinum group metals (PGMs) from base metals and easily ionizable elements (EIEs) is usually ion-exchange. This paper describes an anion-exchange procedure using Dowex-M43 for the separation of EIEs in geological samples (converter matte) from PGMs and Au. Aqueous commercial standards were used to study the adsorption capacity of the Dowex-M43 resin for both Pd and Pt metals (0-1000µg mL-1). The concentrations of Pd and Pt were determined using inductively coupled plasma optical emission spectroscopy (ICP-OES). The results from the analysis of solutions prepared from commercial standards after ion-exchange treatment showed quantitative recoveries of both Pt (97%) and Pd (99%) from Dowex-M43 resin using 0.1 mol L-1 thiourea solution. Good agreement was observed between results obtained in this method and those obtained with the use of the conventional acid dissolution method for the determination of Pd and Pt in converter matte. <![CDATA[<b>Synthesis and characterization of transition metal (Hg(II), Ag(I) and Cd(II)) complexes of some new phosphorus ylides</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502009000100002&lng=pt&nrm=iso&tlng=pt Reaction of two new phosphorus ylides Ph3PCHCOC10H7 (Y¹) and Ph3PCHCOC4H3S(Y²) with mercury(II) halides and a previously reported ylide (p-tolyl)3PCHCOOCH2Ph (Y³) with CdCl2 in equimolar ratios using methanol as solvent yielded binuclear complexes of the type [(Y).HgX2]2 (Y = Y¹ and Y², X = Cl, Br and I) and a complex of [(Y³).HgCl2]2. The latter ylide reacts with Hg(NO3)2.H2O in the same solvent with equimolar ratios to give a polynuclear complex [Hg(Y³)(NO3)(µ,-NO3)]n. Also, the reaction between ylides and AgNO3 in 1:2 molar ratios gave mononuclear complexes. Characterization of these products was carried out by elemental analysis, IR and multinuclear NMR techniques. <![CDATA[<b>Solventless substitution reaction of solid phosphine ligands with Ru(C<sub>5</sub>H<sub>4</sub>R)(CO)<sub>2</sub>I (R = H, Me)</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502009000100003&lng=pt&nrm=iso&tlng=pt The solventless reaction between Ru(C5H4R)(CO)2I (R = H, Me) and solid phosphine ligands [PPh3/ P(p-MeOC6H4)3, P(m-MeC6H4)3,P(p-FC6H4)3,P(p-ClC6H4)3] was undertaken at various temperatures with all the reactions occurring in the melt phase. The reaction rates and type of products isolated are dependent on the electronic properties of the Cp-substituent and the para-substituent on the phosphine ligands. For the C5H4R moiety, when R = H the ligand substitution product Ru(C5H5)(CO)(PR3) I was obtained, and the ligand addition product [Ru(C5H4Me)(CO)2(PR3)]I was predominantly obtained when R = Me. The reaction rates are enhanced by para-substituted electron-donating phosphines. These results are similar to those obtained from studies utilizing benzene and xylene as solvents. The products have been characterized by FTIR and NMR spectroscopy and by MS. The complex Fe2(C5H5)2(CO)4 was found to act as a catalyst in all the melt reactions. <![CDATA[<b>Excess molar volumes and partial molar volumes of binary systems (ionic liquid + methanol or ethanol or 1-propanol) at <i>T</i> = (298.15, 303.15 and 313.15) K</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502009000100004&lng=pt&nrm=iso&tlng=pt Excess molar volumes have been evaluated from density measurements over the entire composition range for binary systems of an ionic liquid (IL) and an alcohol at T = (298.15, 303.15 and 313.15) K. The IL is 1-butyl-3-methylimidazolium methylsulphate [BMIM]+[MeSO4]- and the alcohols are methanol, ethanol or 1-propanol. The Redlich-Kister smoothing polynomial equation was used to fit the excess molar volume data and the partial molar volumes were determined from the Redlich-Kister coefficients. For all the systems studied, the excess molar volumes were negative over the entire composition range at all temperatures. The results are interpreted in terms of the alcohol chain length and the intermolecular interactions. <![CDATA[<b>Synthesis and antimicrobial activity of 2-(aminoacid ester)-3-(6-methyl-2-pyridyl)-3,4-dihydro-2<i>H</i>-1,3,2 λ<sup>5</sup>-benzoxazaphosphinin-2-thiones</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502009000100005&lng=pt&nrm=iso&tlng=pt Synthesis of 2-(aminoacid ester)-3-(6-methyl-2-pyridyl)-3,4-dihydro-2H-1,3,2λ5-benzoxazaphosphinin-2-thiones (3a-j) was accomplished through a two-step process. It involves the prior preparation of 2-chloro-3-(6-methyl-2-pyridyl)-3,4-dihydro-2H-1,3,2λ5-benzoxazaphosphinin-2-thione monochloride (2) and its subsequent reaction with the aminoacid ester hydrochlorides in dry tetrahydrofuran-toluene in the presence of triethylamine at various temperatures. These compounds were characterized by IR, ¹H, 13C, 31P NMR and mass spectral data. <![CDATA[<b>One-pot synthesis of substituted pyrroles with <i>N</i>,<i>N</i>,<i>N'</i>,<i>N'</i> -tetrachlorobenzene-1,3-disulphonamide and <i>N</i>,<i>N'</i>-Diiodo-<i>N</i>,<i>N'</i>-1,2-ethanediyl<i>bis</i> (<i>p</i>-toluenesulphonamide) as novel catalytic reagents</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502009000100006&lng=pt&nrm=iso&tlng=pt In this research, N/N/N',N'-tetrachlorobenzene-1,3-disulphonamide as novel catalytic reagent and N,N'-diiodo-N,N'-1,2-ethanediylbis(p-toluenesulphonamide) as new catalyst were used for the synthesis of N-substituted pyrroles in good to excellent yields under mild conditions. These reusable reagents were compared with existing reagents and it is clear that this study is a welcome addition in the field of pyrrole synthesis. <![CDATA[<b>Highly efficient formylation of alcohols, thiols and aniline derivatives by a heterogeneous (HCOOH/SiO<sub>2</sub>) system under microwave irradiation and solvent-free conditions</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502009000100007&lng=pt&nrm=iso&tlng=pt A simple, rapid and efficient microwave-assisted procedure for the formylation of aniline derivatives and alcohols, using a heterogeneous (HCOOH/SiO2) system under solvent-free conditions is reported. The method is applied to a set of amines, alcohols and thiols and short reaction times (<10 min) with high yields are reported. This protocol introduces a practical and viable green technology of solvent-free and catalyst-free reactions. <![CDATA[<b>Solid silica-based sulphonic acid as an efficient green catalyst for the selective oxidation of sulphides to sulphoxides using NaCIO in aqueous media</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502009000100008&lng=pt&nrm=iso&tlng=pt A range of sulphides can be selectively oxidized to the corresponding sulphoxides in good yields using NaClO/silica sulphonic acid as an efficient and recyclable solid acid catalyst, in both water and 50/50 water/EtOH as solvents. The new method compares favourably with previous methods in the literature. <![CDATA[<b>Computational and experimental studies on the hetero-Diels-Alder reactions of cross-conjugated enaminones with sulphene</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502009000100009&lng=pt&nrm=iso&tlng=pt Ab initio and density functional theory (DFT) calculations have been used to investigate the feasibility of cycloaddition reactions of enaminones 1 with sulphene. Specifically, the geometry optimizations, frequency calculations and self-consistent reaction-field (SCRF) solvent simulations in combination with higher-quality relative energies carried out at the DFT level using the 6-31 + G(d) basis set suggests that the product obtained is both thermodynamically and kinetically preferred, indicating the feasibility of this reaction. Additionally, experimental studies carried out on the reactions of these enaminones 1 with sulphene were also found to be in agreement with the theoretical predictions resulting in the synthesis of a variety of novel oxathiine derivatives having great biological and medicinal importance. <![CDATA[<b>The influence of palladium on the structure and catalytic activity of lanthanum based mixed oxide</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502009000100010&lng=pt&nrm=iso&tlng=pt LaSrCo0.9Pd0.1O4 and LaSrCoO4 mixed oxides were prepared and characterized by XRD, FTIR, BET and XPS, then studied for the reduction of NO by CO. The results show that all samples display characteristic peaks of crystal structure A2BO4 and show the rather strong diffraction pattern for LaSrCo0.9Pd0.1O4 (reduced and oxidized) and weak signals of lanthanum oxide for LaSrCo0.9Pd0.1O4 (reduced). The noble metal Pd has a significant influence on the specific surface areas, pore volumes, binding energies and activities of the samples. For NO reduction by CO, catalytic activity shows complete conversion below 360 °C and the order of activity is LaSrCo0.9Pd0.1O4 (reduced and oxidized) > LaSrCo0.9Pd0.1O4 (reduced) > LaSrCoO4 (reduced). Kinetic analysis for NO reduction by CO indicates that the specific reaction rate is significantly affected by the synthesis procedure. <![CDATA[<b>Application of (<i>S</i>,<i>S</i>)-pentacycloundecane bis(4-phenyloxazoline) as a novel chiral ligand for catalysis of the asymmetric Diels-Alder reaction of cyclopentadiene with 3-acryloyl-2-oxazolidinone</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502009000100011&lng=pt&nrm=iso&tlng=pt The synthesis of the novel C1 symmetric (S,S)-pentacycloundecane bis(4-phenyloxazoline) ligand 5 and its evaluation as a chiral Lewis acid catalyst in the benchmark asymmetric Diels-Alder reaction between 3-acryloyloxazolidin-2-one (6)and cyclopentadiene (7) is reported. From the various metal salts screened the anhydrous magnesium perchlorate complex emerged as the best catalyst. The endo-cycloadduct product 8 was afforded in 81% enantiomeric excess with an endo:exo ratio of 98:2. An extensive screening of various metal ions as complexing agents was performed and is also reported. <![CDATA[<b>The effect of synthesis parameters on the catalytic synthesis of multiwalled carbon nanotubes using Fe-Co/CaCO<sub>3</sub> catalysts</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502009000100012&lng=pt&nrm=iso&tlng=pt Fe-Co bimetallic catalysts supported on CaCO3 were prepared by a wet impregnation, a deposition-precipitation and a reverse micelle method. The sizes of the Fe and Co particles were not affected by the Fe and Co sources (nitrate, acetate) when the wet impregnation and deposition-precipitation methods were used. 'Clean' multi-walled carbon nanotubes (MWCNTs) were obtained from all three Fe-Co synthesis procedures under optimal reaction conditions. The CNTs produced gave yields ranging from 623 % to 1215 % in 1 h under the optimal conditions, with similar outer diameters (o.d.) of 20-30 nm and inner diameters (i.d.) ~10 nm. The Fe/Co catalyst formed in the wet impregnation method revealed that the yield, diameter and purity of the CNTs were influenced by the C2H2/N2 ratio, time and temperature. All the methods gave high-quality CNTs after short reaction times but the quality deteriorated as the synthesis time was increased from 5 to 360 min. Indeed, the most influential parameter in controlling CNT purity, length and o.d. was found to be the synthesis time. The as-synthesized CNTs were purified using a single-step mild acid treatment process (30 % HNO3), which readily removed the support and metal particles. <![CDATA[<b>A mechanistic study of hydroboration of 1-octene with 1,3,2-dithiaborolane and 1,3,2-dithiaborinane: Part 1. Synthesis and kinetic studies</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502009000100013&lng=pt&nrm=iso&tlng=pt Alkylthioboranes 1,3,2-dithiaborolane and 1,3,2-dithiaborinane have been synthesized from the reaction of BH3-SMe2 with 1,2-ethanedithiol and 1,3-propanedithiol, respectively. These heterocyclic boranes disproportionated significantly during their synthesis. The rate constants, and the enthalpies and entropies of the hydroboration reaction of 1-octene with 1,3,2-dithiaborolane and 1,3,2-dithiaborinane have been investigated, and we have shown that hydroboration with these boranes is slow and proceeds via an associative mechanism. <![CDATA[<b>A mechanistic study of hydroboration of 1-octene with 1,3,2-dithiaborolane and 1,3,2-dithiaborinane: Part 2. A DFT study of disproportionation and hydroboration</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502009000100014&lng=pt&nrm=iso&tlng=pt The hydroboration reactions of propene with 1,3,2-dithiaborolane and 1,3,2-dithiaborinane in their ground states have been studied using density functional theory (DFT) at the B3LYP/3-21 + G and B3LYP/6-31 + G(d) levels. Hydroboration and disproportionation transition states have been determined and activation energies for these transition states were compared. It has been shown that hydroboration reactions require slightly higher activation energies than disproportionation, and yield thermodynamically more stable products. <![CDATA[<b>An approach to dealing with the difficulties undergraduate chemistry students experience with stoichiometry</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502009000100015&lng=pt&nrm=iso&tlng=pt Chemistry for first year students has been identified by Tshwane University of Technology as one of the subjects with a low pass rate. It is apparent that students often memorize formulae and definitions, without understanding the underlying concepts required to work with abstract units of measure. We have found that the majority of students at this university are unable to balance reaction equations satisfactorily. They are also unable to predict the reaction yield, or identify limiting reagents. It is imperative that these and other related problems are overcome before any meaningful change to the high failure rate at first year level will be realized. All conventional forms of lecture presentation failed to make any significant impact on the success rate. Structured worksheets were developed and used, together with tactile models, to address the problems and the initial findings showed a marked improvement. It was discovered that the students' problems originated from their inability to understand the meaning of subscripts and coefficients in chemical equations. The worksheets and the impact they have made on the students' understanding of stoichiometry are shared in this paper. <![CDATA[<b>Hydrogenation reactions in ionic liquids: The efficient reduction of nitroarenes, including nitroferrocenyl derivatives, to the corresponding aminoarenes in [bmim][BF4]</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502009000100016&lng=pt&nrm=iso&tlng=pt <![CDATA[<b>KHSO<sub>4</sub>-catalyzed three-component synthesis of α-aminonitriles</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502009000100017&lng=pt&nrm=iso&tlng=pt A simple, convenient and general method has been developed for the synthesis of α-aminonitriles by a one-pot three-component condensation of aldehydes, amines and trimethyl silyl cyanide under solvent-free conditions in the presence of a catalytic amount of KHSO4 in good yields. The new method compares well in terms of yields with alternative methods. In general the reaction times are much shorter and the work up much easier. <![CDATA[<b>Synthesis and biological activity of some Triazolothiadiazoles</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502009000100018&lng=pt&nrm=iso&tlng=pt The synthesis of a series of novel 3-pyridyl-6-aryl-s-triazolo[3,4-b]-[1,3,4]-thiadiazoles is described. Fourteen new compounds were synthesized and characterized by spectral and elemental analyses. Some compounds were screened for antibacterial activity against S. aureus, E. coli, B. subtilis and P. aeruginosa. Compounds containing aryl substituents at position 6 and the 1,2,4-triazole moiety at position 1 or 2 showed reasonable antibacterial activity. <![CDATA[<b>An efficient and green procedure for the preparation of acylals from aldehydes catalyzed by alum [KAl(SO<sub>4</sub>)<sub>2</sub>-12H<sub>2</sub>O]</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502009000100019&lng=pt&nrm=iso&tlng=pt Alum [KAl(SO4)2-12H2O] is an inexpensive, efficient, non-toxic and mild catalyst for the preparation of acylals from aromatic and heteroaryl aldehydes with acetic anhydride at room temperature under solvent-free conditions. This method gives remarkable advantages such as excellent chemoselectivity, mild reaction conditions, short reaction times and excellent yields, and offers a green synthetic solution by avoiding toxic catalysts and hazardous solvents. <![CDATA[<b>Synthesis of novel 3-hydroxy-3-pyridylcamphor derivatives</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502009000100020&lng=pt&nrm=iso&tlng=pt The synthesis of a series of five novel precursor compounds derived from (R)-(+)-camphor is reported. From these precursors, a further four novel pyridyl alcohol ligands were synthesized. The molecules represent the first reported examples where the pyridyl and hydroxyl moieties are pendant on the C3 position of the camphor skeleton. The regioselective synthesis was investigated and an efficient method was identified. The final ligands were obtained in moderate yield with absolute regio- and stereoselective control. The molecules were screened as catalysts in the alkylation of aldehydes with diethylzinc in order to compare the new arrangement of donor groups with previously reported results obtained with C2 pendant ligands. The results demonstrated a significant improvement for the synthesized C3 pendant ligands over previous C2 examples with moderate yields and up to 85 % ee being obtained. <![CDATA[<b>The evaluation of novel camphor-derived ligands as catalysts in the asymmetric Henry reaction</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502009000100021&lng=pt&nrm=iso&tlng=pt The evaluation of a series of camphor-derived ligands as catalysts in the asymmetric Henry reaction is reported. The synthesis of two novel derivatives is detailed and these molecules are also screened as catalysts in this reaction. The single crystal X-ray structure of one of the novel compounds is reported. The reaction is catalyzed with moderate to excellent yields and moderate enantioselectivity. <![CDATA[<b>The evaluation of novel camphor-derived pyridyl ligands as catalysts in the asymmetric Diels-Alder reaction of cyclopentadiene with 3-acryloyl-2-oxazolidinone</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502009000100022&lng=pt&nrm=iso&tlng=pt A series of camphor-derived pyridyl ligands were evaluated in the Diels-Alder reaction of 3-acryloyl-2-oxazolidinone 1 with cyclopentadiene 2. High yields with good endo:exo selectivity, but only moderate enantioselectivities (43 % ee), were obtained. The structures of the copper (II) complexes of the ligands were calculated using ONIOM density functional theory and the results suggest that chiral induction to the alkene functional group is indeed lacking. This explains the moderate experimental selectivities obtained. <![CDATA[<b>Synthesis and biological evaluation of some anthranilic acid and 2-phenylquinazoline-4(3<i>H</i>)-one analogues</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502009000100023&lng=pt&nrm=iso&tlng=pt In the present investigation a novel series of N-(phenyl) chalconyl anthranilic acids containing pyrazolines (4a-j), tetra-hydropyrimidines (4k-o), tetrahydrothiopyrimidines (4p-t) and 2-phenylquinazolin-4(3H)-ones containing pyrazolines (8a-f), isoxazolines (8g-l), tetrahydropyrimidines (8m-r) and tetrahydrothiopyrimidines (8s-x) were synthesized and characterized by elemental analysis, FT-IR, ¹H NMR and mass spectroscopy. The title compounds (4a-t) and (8a-x) were investigated for their analgesic, anti-inflammatory, antimicrobial and in vitro protein denaturation activities. Compounds 4j and 8x were identified as lead compounds with optimum analgesic, anti-inflammatory and antimicrobial activities. <![CDATA[<b>Synthesis of oxo- and thio-analogues of 2-oxo-2<i>H</i>-chromen-7-yl dimethylcarbamates</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502009000100024&lng=pt&nrm=iso&tlng=pt A range of novel 2-oxo-2H-chromen-7-yl dimethylcarbamates was synthesized containing either an oxygen or sulphur atom in the α-position to the carbonyl or thiocarbonyl group of the amide moiety. The synthesis and spectroscopic data of these compounds are reported. Microwave synthesis was essential for the successful synthesis of some of the sulphur-containing carbamates. The synthesized compounds will be used in a subsequent study on the influence of the α -substituent on the amide rotational barrier. <![CDATA[<b><i>Ab initio</i></b><b> molecular orbital studies of the vibrational spectra of some van der Waals complexes: Part 4. Complexes of sulphur dioxide with carbon dioxide, carbonyl sulphide, carbon disulphide and nitrous oxide</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502009000100025&lng=pt&nrm=iso&tlng=pt The binary complexes formed between sulphur dioxide, as electron donor, and the series carbon dioxide, carbonyl sulphide and carbon disulphide, as electron acceptors, have been studied by means of ab initio molecular orbital theory. The optimized structures, the interaction energies and the vibrational spectra have been determined, and the effect of successive substitution of sulphur for oxygen atoms in the electron acceptor molecules has been established. Nitrous oxide, which is isoelectronic with carbon dioxide, has also been included among the electron acceptors, but the properties of the complex formed between sulphur dioxide and nitrous oxide are substantially different from those of the other three complexes. <![CDATA[<b>Alternative synthesis of 2,4-substituted-1,3-thiazines and 2,5-substituted-thiazole derivatives</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502009000100026&lng=pt&nrm=iso&tlng=pt An easy and convenient route for the synthesis of 2,4-substituted thiazine (3-6) and 2,5-substituted thiazole (16) derivatives from phenacetamidines and glycine methyl ester is reported. To the best of our knowledge, this is the first report in which phenacetamidines have been utilized as precursors for thiazine synthesis. The syntheses of novel cyclic fused 1,3-diazabutadienes 13 and 2-aza-1,3-butadiene 17, and iodocyclization of 17 leading to the formation of thiazole 16a are also reported. In this paper a total of twenty novel compounds are reported. <![CDATA[<b>A new sterol diglycoside from <i>Conyza floribunda</i></b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502009000100027&lng=pt&nrm=iso&tlng=pt Chemical investigation of CH2Cl2 and MeOH extracts of Conyza floribunda have led to the isolation of (24S)-ethylcholesta-5, 22E, 25-trien-3-0-glucosyl (1" → 4')-rhamnoside (1), (24S)-ethylcholesta-5,25-dien-3-0-β-glucoside (2), cyasterone (3), stigmasta 5,22-dien-3-acetate (4), stigmasta 5,22-dien-3-ol (5), spinasta 7,22-dien-3-ol (6), 3-oxofriedooleanane (7), 3-hydroxyfriedooleanane (8), 3-acetoxyfriedooleanane (9), betullinic acid (10), quercetin 3-0-β-glucoside (11), quercetin (12), myricetin (13) and kaempferol (14). Their structures were determined using spectroscopic and physical methods as well as comparison with data available in the literature. <![CDATA[<b>Glass electrode calibration for use in the voltammetric determination of stability constants under extreme acidic conditions</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502009000100028&lng=pt&nrm=iso&tlng=pt A glass electrode (GE) can be successfully employed to measure pH in the study of metal-ligand equilibria by voltammetry at extremely low pH (between 0 and 2); two consecutive strong acid-strong base titrations involving different base concentrations (recommended to avoid corrosion of the GE in very basic solutions) are best suited to establish the response parameters of a GE. A novel approach of using a combined linear and binomial GE calibration was developed; this procedure allows measurements between pH 0 and 2 with uncertainty better than ±0.01 pH unit. From an extensive error analysis, it has been established that the uncertainties of about ±0.5 mV in the response slope and ±1.3 mV in E°' might result in an absolute error in pH of about 0.02 which should not generate errors larger than 0.3 % in optimized stability constants (as log K values) determined by voltammetry at extremely low pH. A test of GE suitability for the study of metal complexes by voltammetry is also proposed; it should be implemented only for suspect electrodes that show response parameters outside the limits recommended in this work. <![CDATA[<b>Intellectual skills needed for the effective learning and application of chemical knowledge</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502009000100029&lng=pt&nrm=iso&tlng=pt Many students' difficulties in learning and applying chemical knowledge are associated with their being incompetent in a few widely applicable intellectual skills and strategies. This paper discusses the results of a study that was done to test first year university students' competence in some types of intellectual skills that are important in chemistry. The skills tested include language skills, mathematical skills, graphical skills, three-dimensional visualization skills, information processing skills and reasoning skills. The study showed that the competence of most students in intellectual skills is very poor. This lack of competence could be expected to lead to negative attitudes and a lack of self-confidence that would seriously handicap their learning, and may also be an important reason for the observed high failure rate of students in science courses. The study also showed a significant correlation between success in first year university courses and competence in intellectual skills and strategies. It is strongly suggested that much greater emphasis should be placed in our courses on the development of students' intellectual skills and strategies. Such training should be integrated with the teaching of subject content. <![CDATA[<b>A facile catalyst-free Pudovik reaction for the synthesis of α-amino phosphonates</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502009000100030&lng=pt&nrm=iso&tlng=pt Some imines were synthesized by the reaction of 5-amino 1-naphthol with substituted aromatic aldehydes in ethanol under reflux conditions. Dialkyl phosphites underwent addition with aromatic imines to give novel α-amino phosphonates. All the title compounds were characterized by elemental analysis, IR, ¹H, 13C, 31P NMR and mass spectral data. All the newly-synthesized compounds (4a-j) exhibited moderate antibacterial and antifungal activity. <![CDATA[<b>Synthesis and cytotoxicity screening of some novel benzofuranoyl-pyrazole derivatives against liver and cervix carcinoma cell lines</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502009000100031&lng=pt&nrm=iso&tlng=pt A series of new pyrazole, thiazole and thiazolinone derivatives incorporated into benzofuran were synthesized by using 3-(benzofuran-2-yl)-1-phenyl-1H-pyrazol-4-carboxaldehyde as starting material. A total of 41 novel compounds were synthesized. Some of these synthesized compounds were evaluated for cytotoxicity activity against HEPG2 (liver carcinoma cell line) and HELA (cervix carcinoma cell line). The tested compounds (1, 2c, 7c, 8b, 9b, 13b and 14b) showed better activities at low concentration against the commonly used human carcinoma cell lines. A detailed synthesis, spectroscopic data and cytotoxicity screening for the new compounds are described. <![CDATA[<b>Inhibitor for the corrosion of mild steel in H<sub>2</sub>SO<sub>4</sub></b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502009000100032&lng=pt&nrm=iso&tlng=pt An extract of Terminalia chebula fruits was investigated as a corrosion inhibitor of mild steel in 0.5 M H2SO4 by means of conventional mass loss, electrochemical polarization, electrochemical impedance spectroscopy and scanning electron microscopy. The mass loss results showed that the extract of Terminalia chebula is an excellent corrosion inhibitor, electrochemical polarization data revealed the mixed mode of inhibition and the results of electrochemical impedance spectroscopy showed that the change in the impedance parameters, charge transfer resistance and double layer capacitance with the change in concentration of the extract is due to the adsorption of active molecules leading to the formation of a protective layer on the surface of mild steel. Scanning electron microscopic studies provided confirmatory evidence of an improved surface condition, due to adsorption, for corrosion protection. <![CDATA[<b>Silica-supported Preyssler nanoparticles as new catalysts in the synthesis of 4(3<i>H</i>)-quinazolinones</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502009000100033&lng=pt&nrm=iso&tlng=pt A new and efficient method for the preparation of 4(3H)-quinazolinones from the condensation of anthranilic acid, orthoester and substituted anilines, in the presence of catalytic amounts of silica-supported Preyssler nanoparticles is reported. The catalyst performs very well in comparison with other catalysts reported before. An important advantage of this catalyst is the ease of separating it from the reaction mixture, as well as the fact that it could be recycled a number of times. <![CDATA[<b>A GC/MS profile of the volatile constituents of the aerial parts of <i>Artemisia abrotanum</i> L. (Asteraceae) from Serbia</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502009000100034&lng=pt&nrm=iso&tlng=pt The diethyl ether extract of the aerial parts of Artemisia abrotanum L. was analyzed by GC and GC/MS. The main identified constituents were silphiperfol-5-en-3-one A (14.6 %), ascaridole (13.1 %), 1,8-cineole (10.5 %), α-bisabolol oxide A acetate (8.7 %), germacrene D (6.5 %) and borneol (6.0 %).