Mechanistic studies on the galvanic replacement reaction between multiply twinned particles of Ag and HAuCl4 in an organic medium

This article presents a mechanistic study on the galvanic replacement reaction between 11- and 14-nm multiply twinned particles (MTPs) of Ag and HAuCl4 in chloroform. We monitored both morphological and spectral changes as the molar ratio of HAuCl4 to Ag was increased. The details of reaction were different from previous observations on single-crystal Ag nanocubes and cuboctahedrons. Because Au and Ag form alloys rapidly within small MTPs rich in vacancy and grain boundary defects, a complete Au shell did not form on the surface of each individual Ag template. Instead, the replacement reaction resulted in the formation of alloy nanorings and nanocages from Ag MTPs of decahedral or icosahedral shape. For the nanorings and nanocages derived from 11-nm Ag MTPs, the surface plasmon resonance (SPR) peak can be continuously shifted from 400 to 616 nm. When the size of Ag MTPs was increased to 14 nm, the SPR peak can be further shifted to 740 nm, a wavelength sought by biomedical applications. We have also investigated the effects of capping ligands and AgCl precipitate on the replacement reaction. While hollow structures were routinely generated from oleylamine-capped Ag MTPs, we obtained very few hollow structures by using a stronger capping ligand such as oleic acid or tri-n-octylphosphine oxide (TOPO). Addition of extra oleylamine was found to be critical to the formation of well-controlled, uniform hollow structures free of AgCl contamination thanks to the formation of a soluble complex between AgCl and oleylamine.     

Controllable growth of silver nanostructures by a simple replacement reaction and their SERS studies

Hierarchical silver nanostructures, consisting of dendritic (symmetric branched) and fractal patterns (randomly ramified), were synthesized very easily by dropping a droplet of AgNO₃-HF solution on silicon wafers. Scanning electron microscopy (SEM), X-ray diffraction (XRD) and open circuit potential-time (Ocp-t) measurement demonstrated that the two nanostructures converted with the reaction composition. The structural evolution was tentatively explained with the theory that oriented growth was determined by the anisotropy of the solid–liquid interfacial energy and the oriented attachment-based aggregation mechanism. Results on surface-enhanced Raman scattering (SERS) signals of the silver films with hierarchical nanostructures demonstrate that SERS is sensitive to silver nanostructures.     

Novel silver nanostructures from silver mirror reaction on reactive substrates

Novel silver clusters have been prepared by simply carrying out the silver mirror reaction on certain reactive substrates. Leaflike fractal silver microstructures and perpendicularly aligned silver nanosheets were produced on a commercially available copper foil and sandpaper-rubbed copper foil, respectively. The surface features of copper foils and the chemical state of Cu atoms play important roles in regulating the morphological structures of the resulting silver clusters. Silver nanoclusters with various morphologies ranging from the leaflike to flowerlike hierarchical structures can be produced from the silver mirror reaction on commercially available copper foils after being treated with a dilute aqueous HCl solution under different conditions. The aqueous solution of silver nanosheets shows an optical absorption spectrum with a broad light-scattering peak at about 350 nm, compared to a corresponding surface plasmon absorption band around 430 nm for silver nanoparticles from the conventional silver mirror reaction on glass.     

Mechanistic study of iodide catalysed oxidation of l-glutamic acid by cerium(IV) in aqueous sulphuric acid medium

A minute quantity (10⁻⁶ mol dm⁻³) of iodide catalysed oxidation of l-glutamic acid by Ce^IV has been studied in H₂SO₄ and SO ₄ ²⁻ media. The reaction was first order each in [Ce^IV] and [I⁻]. The order with respect to [l-glutamic acid] was less than unity (0.71). Increase in [H₂SO₄] decreased the reaction rate. The added HSO ₄ ⁻ and SO ₄ ²⁻ decreased the rate of reaction. The added product, succinic acid, had no effect on the reaction rate, whereas added Ce^III retarded the reaction. The ionic strength and dielectric constant did not have any significant effect on the rate of reaction. The active species of oxidant was Ce(SO₄)₂. A suitable mechanism was proposed. The activation parameters were determined with respect to the slow step of the mechanism. The thermodynamic quantities were also determined and discussed.     

Mechanistic studies on the reaction between glutathionylcobalamin and selenocysteine

Abstract

The reaction between glutathionylcobalamin (GSCbl), a complex of Co(III)-cobalamin with glutathione, and selenocysteine (Sec) was investigated using ultraviolet-visible (UV–vis) spectroscopy. The interaction results in the formation of cob(II)alamin and proceeds via two pathways: (i) a rapid formation of complex between GSCbl and Sec followed by the rate-determining substitution of glutathionyl-ligand by Sec and rapid electron-transfer from Se-atom to Co(III)-ion and (ii) a nucleophilic attack of Co(III)-S bond by Sec.     

Quantitative study of the reaction between quinones and iron(II) in phosphoric acid medium

A simple method is described for the determination of some quinones with ammonium iron(II) sulphate, by titration potentiometrically or with cacotheline, Methylene Blue, Methylene Green, thionine, Toluidine Blue, Azure A, Azure C or Azure-2-eosin as redox indicators. The reverse titrations can be used for estimation of iron(II). Alternatively, excess of iron (II) can be added and the iron(III) produced determined colorimetrically with thiocyanate.     

Transmetalation reaction between hydrophobic silver nanoparticles and aqueous chloroaurate ions at the air-water interface

The transmetalation reaction between a sacrificial nanoparticle and more noble metal ions in solution has emerged as a novel method for creating unique hollow and bimetallic nanostructures. In this report, we investigate the possibility of carrying out the transmetalation reaction between hydrophobic silver nanoparticles assembled and constrained at the air-water interface and subphase gold ions. We observe that facile reduction of the subphase gold ions by the sacrificial silver nanoparticles occurs resulting in the formation of elongated gold nanostructures that appear to cross-link the sacrificial silver particles. This transmetalation reaction may be modulated by the insertion of an electrostatic barrier in the form of an ionizable lipid monolayer between the silver nanoparticles and the aqueous gold ions that impacts the gold nanoparticle assembly. Transmetalation reactions between nanoparticles constrained into a close-packed structure and appropriate metal ions could lead to a new strategy for metallic cross-linking of nanoparticles and generation of coatings with promising optoelectonic behavior.     

Mechanistic study of the Mitsunobu reaction

The Mitsunobu reaction occurs typically with inversion of configuration in secondary alcohol derivatives. In this paper, a mechanistic explanation for lactonizations of hindered alcohols under Mitsunobu conditions with retention is proposed. This involves the intermediacy of an acyloxyphosphonium salt followed by acyl transfer to the alcohol.     

Kinetics of the reversible reaction between arsenious acid and aqueous iodine

The kinetics of the reversible reaction have been studied spectrophotometrically in acid solution under conditions in which both the forward and reverse reactions go to virtual completionand in which the reaction comes to a practical equlibrium. The rates of theforward (R_f) and reverse (R_r) reactions are given by where f, g, h, u, and v have the values (4 ± 1) × 10^?5 mole/1.·s, (4.2 ± 0.2) × 10^?5 mole²/1.²·s, (5.0 · 0.3) × 10^?7 mole³/1.³·s, (1.1 ± 0.1) × 10^?3 1.²/mole²·s, and (3.7 ± 0.2) × 10^?3 1.³/mole³·s at 298.2°K and at an ionic strength of 2.00M maintained by adding sodium chloride. The stoichiometric equilibrium constant under similar conditions is 0.022 ± 0.003. Differentvalues of these parameters were obtained when sodium perchlorate and sodiumnitrate were used to control ionic strength. The results are compared with those from previous reports and a mechanism is proposed based upon an initial rapid equilibrium followed by a rate-determining attack of water upon H₃AsO₃I⁺, H₂AsO₃I, and HAsO₃I^?.     

Mechanistic information on the copper-catalysed autoxidation of mercaptosuccinic acid in aqueous solution

Copper(II) ions react rapidly with sulfur from thiol groups, forming two distinct, intensely absorbing, short-lived intermediates, which decompose in a subsequent redox reaction to produce reduced copper and disulfides. In this study we report the results of a mechanistic study on the reaction between mercaptosuccinic acid, HO(2)CCH(2)CH(SH)CO(2)H, and Cu(2+)(aq) and [Cu(tren)H(2)O](2+), tren = tris(2-aminoethyl)amine. Spectroscopic and kinetic data indicate that in the presence of an excess of thiol, at least two distinct complexes are formed, with very different decomposition rate constants and an absorption maximum at 346 nm. Upon addition of thiol to [Cu(tren)H(2)O](2+)(1:1), a transient with a maximum at 380 nm appears, whereas in an excess of thiol this complex decomposes and again the 346 nm band is observed. The use of [Cu(tren)H(2)O](2+) enables to study the reaction of thiol with copper also in alkaline solution, where the rate of the overall process is slowed down greatly. The reactions were studied in detail, including the effect of dioxygen, and a possible reaction mechanism for the catalysed autoxidation process is proposed and discussed in reference to available literature data.     

Tandem bis-aza-Michael addition reaction of amines in aqueous medium promoted by polystyrenesulfonic acid

An efficient and environmentally benign tandem bis-aza-Michael addition of amines catalyzed by polystyrenesulfonic acid (PSSA) is described. This operationally simple high yielding microwave assisted synthetic protocol proceeded in water in the absence of any organic solvent.     

Novel ascorbic acid based ionic liquids for the in situ synthesis of quasi-spherical and anisotropic gold nanostructures in aqueous medium

A series of newly designed ascorbic acid based room temperature ionic liquids were successfully used to prepare quasi-spherical and anisotropic gold nanostructures in an aqueous medium at ambient temperature. The synthesis of these room temperature ionic liquids involves, first, the preparation of a 1-alkyl (such as methyl, ethyl, butyl, hexyl, octyl, and decyl) derivative of 3-methylimidazolium hydroxide followed by the neutralization of the derivatised product with ascorbic acid. These ionic liquids show significantly better thermal stability and their glass transition temperature (Tg) decreases with increasing alkyl chain length. The ascorbate counter anion of these ionic liquids acts as a reducing agent for HAuCl4 to produce metallic gold and the alkylated imidazolium counter cation acts as a capping/shape-directing agent. It has been found that the nature of the ionic liquids and the mole ratio of ionic liquid to HAuCl4 has a significant effect on the morphology of the formed gold nanostructures. If an equimolar mixture of ionic liquid and HAuCl4 is used, predominantly anisotropic gold nanostructures are formed and by varying the alkyl chain length attached to imidazolium cation of the ionic liquids, various particle morphologies can formed, such as quasispherical, raspberry-like, flakes or dendritic. A probable formation mechanism for such anisotropic gold nanostructures has been proposed, which is based on the results of some control experiments.     

Kinetics of reaction of sodium phenylarsonite with chloroacetic acid in aqueous alkaline medium

The reaction order with respect to its components, the rate constants, and thermodynamic parameters of the reaction of sodium phenylarsonite with chloroacetic acid in aqueous alkaline medium were elucidated. The effect of substituents at the arsenic atom on the nucleophilic reactivity of arsenite ion in the Meyer reaction is discussed.     

Kinetics and mechanism of the reaction between silver(III) and thiourea in aqueous alkaline media

The tetrahydroxoargentate(III) ion, Ag(OH)₄⁻, is rapidly reduced by thiourea (tu) in accordance with the three term rate law RATE = {k₁+(k₂+k₃[OH⁻])[tu]}[Ag^III] where k₁ = 1.08 s⁻¹, k₂ = 1.46 x 10³ M⁻¹ s⁻¹, and k₃ = 2.02 x 10³ M⁻² s⁻¹. The k₁ path occurs via the rate-determining aquation of Ag(OH)₄⁻ while the other two paths involve axial attack of thiourea on silver. The higher values of k₂ and k₃ compared to the ethylenediamine reaction, which obeys the same rate law, is a reflection of the greater nucleophilicity of tu.

Following the redox reaction, solutions become brown in a reaction that obeys pseudo-first-order kinetics. Similar behaviour is observed when tu is replaced by Na₂S or thio-acetamide and when Ag^I reacts with any of these sulphur containing compounds. We attribute this process to the Ag^I promoted formation of sulphide species which eventually precipitate as Ag₂S.     

Kinetics of reaction between acetic acid and Ag2+ in nitric acid medium

The reaction kinetics between acetic acid and Ag²⁺ in nitric acid medium is studied by spectrophotometry. The effects of concentrations of acetic acid (HAc), H⁺, NO^?₃, and temperature on the reaction are investigated. The rate equation has been determined to be –dc(Ag²⁺)/dt = kc(Ag²⁺)c(HAc)c^?1(H⁺), where k = (610 ± 15) (mol/L)^?1 min^?1 with an activation energy of about (48. 8 ± 3.5) kJ mol^?1 when the reaction temperature is 25°C and the ionic strength is 4.0 mol L^?1. The reduction rate of Ag²⁺ increases with the increase in HAc concentration and/or temperature and the decrease in HNO₃ concentration. However, the effect of NO^?₃ concentrations within 0.5–2.5 mol L^?1 on the reaction rate is negligible. © 2012 Wiley Periodicals, Inc. Int J Chem Kinet 45: 47–51, 2013     

Mechanistic study on the Knorr pyrazole synthesis-thioester generation reaction

A novel Fmoc-SPPS compatible peptide thioester generation method leveraging Knorr pyrazole synthesis was reported recently. C-terminal peptide hydrazides, pentane-2,4-dione and excess arylthiol were added in one-pot to efficiently produce peptide thioesters in acidic aqueous solution at room temperature. To elucidate the detailed mechanism of this reaction and the origin of the effect of solution acidity, a theoretical investigation on the Knorr pyrazole synthesis-thioester generation reaction was carried out. Our computational results suggest that the reaction generally proceeds through three stages: hydrazone formation, pyrazole formation and thioester formation. The rate-determining step is the CO bond cleavage step in the pyrazole formation stage. The formed pyrazole is readily converted to thioester in the presence of excess thiophenol. The effect of solution acidity originates from the need for protonation of oxygen atoms to increase the electrophilicity of carbonyl group or the leaving ability of hydroxyl group.     

Multifunctional bioconjugation by Morita-Baylis-Hillman reaction in aqueous medium

An efficient approach for modular assembly of multifunctional bioconjugates from oligosaccharides, peptides and proteins with fluorescent probes/affinity tags based on Morita-Baylis-Hillman (MBH) reaction in aqueous medium has been developed.