N. M. Made Gowda

Spectrophotometric methods for the determination of certain catecholamine derivatives in pharmaceutical preparations

Two simple, rapid and sensitive spectrophotometric methods for the determination of catecholamine derivatives (pyrocatechol, dopamine, levodopa and methyldopa) are developed. The first method involves the oxidation of o-dihydroxybenzene derivatives by N-bromosuccinimide followed by oxidative coupling with isoniazid leading to the formation of a red-coloured products of maximum absorbance (lambda(max)=480-490 nm). The second method is based on the formation of green to blue complex (lambda(max)=635-660 nm) between o- dihydroxybenzene derivatives and sodium nitroprusside in the presence of hydroxylamine hydrochloride. All measurements of the two procedures are carried out in an alkaline medium at room temperature. The two methods are successfully applied for the determination of dopamine hydrochloride, levodopa and methyldopa in injections and tablets of pharmaceutical preparation. The common excipients used as additives in pharmaceuticals do not interfere in the proposed methods. The reliability of these methods are established by parallel determination with the reported and official methods.

Extractive spectrophotometric methods for the assay of sildenafil citrate (Viagra) in pure form and in pharmaceutical formulations

Two simple and sensitive extractive spectrophotometric methods for the determination of sildenafil citrate (SC) are proposed. The methods are based on the formation of ion-association complexes of sildenafil citrate with bromocresol green (BCG, method A) and with chromoxane cyanine R (CCR, method B) in aqueous acidic buffer. The complex species, extractable to chloroform phase, were quantitatively measured at 415 and 460 nm for methods A and B, respectively. Beers law was obeyed in the SC concentration range 1.25-25 mug ml(-1) with a limit of detection 0.16 mug ml(-1) and 1.5-60 mug ml(-1) with a limit of detection 0.18 mug ml(-1), respectively, for methods A and B. The methods have been successfully applied to the analysis of bulk drug and its tablets. No interference was observed from common pharmaceutical adjuvants.

Stability indicating RP-LC determination of sildenafil citrate (Viagra) in pure form and in pharmaceutical samples

A simple, precise, sensitive and stability-indicating reverse-phase high performance liquid chromatographic (RP-HPLC) method has been developed for the quantitation of sildenafil citrate (SC) in pure form and its pharmaceutical formulations. Method employs water and acetonitrile (48:52 v/v) as mobile phase with flow rate of 1 ml min(-1), LiChrospher C18-5 microm (25 x 0.46 cm) column and UV detection set at 245 nm. The internal standard method using piroxicam (PX) as the internal standard is used. The linear dynamic range of SC was found to be 0.05-7.5 microg ml(-1). The proposed method is successfully employed for the determination of SC in the tablets. The excipients present in the formulations do not interfere with the assay procedure. Analytical parameters were calculated and full statistical evaluation is included.

Oxidation of isoniazid by N-haloarenesulfonamidates in alkaline medium: A kinetic and mechanistic study

The kinetics of oxidation of Isoniazid (INH) by sodium N-haloarenesulfonamidates, chloramine-T (CAT), bromamine-T (BAT), chloramine-B (CAB), and bromamine-B (BAB), has been studied in alkaline medium at 303 K. The oxidation reaction follows identical kinetics with a first-order dependence on each [oxidant] and [INH] and an inverse fractional-order on [OH−:]. Addition of the reaction product (p-toluenesulfonamide or benzenesulfonamide) had no significant effect on the reaction rate. Variation of ionic strength and addition of halide ions have no influence on the rate. There is a negative effect of dielectric constant of the solvent. Studies of solvent isotope effects using D2O showed a retardation of rate in the heavier medium. The reaction was studied at different temperatures, and activation parameters have been computed from the Arrhenius and Eyring plots. Isonicotinic acid was identified as the oxidation product by GC-MS. A two-pathway mechanism is pro-posed in which RNHX and the anion RNX− interact with the substrate in the rate-limiting steps. The mechanism proposed and the derived rate laws are consistent with the observed kinetics. The rate of oxidation of INH increases in the order: BAT > BAB > CAT > CAB. This effect is mainly due to electronic factors.

Kinetics and mechanism of oxidation of aspirin by bromamine-T, N-bromosuccinimide, and N-bromophthalimide

The kinetics of the oxidation of aspirin (ASP) by bromamine-T (BAT), N-bromosuccinimide (NBS), and N-bromophthalimide (NBP) has been studied in aqueous perchloric acid at 303 K. The oxidation reaction follows identical kinetics with first-order in [oxidant], fractional-order in [ASP], and inverse fractional-order in [H+]. Under identical experimental conditions the extent of oxidation with different oxidizing agents is in the order: NBS>BAT>NBP. The rate decreased with decreasing dielectric constant of the medium. The variation of ionic strength and the addition of the reaction products and halide ions had no significant effect on the reaction rate. The solvent isotope effect was studied using D2O. Kinetic parameters were evaluated by studying the reaction at different temperatures. The reaction products were identified by GC–MS. The proposed reaction mechanism and the derived rate law are consistent with the observed kinetic data. Formation and decomposition constants for ASP-oxidant complexes have been evaluated. Decarboxylation, bromination, and loss of acetic acid gave 2,4,6-tribromophenol.

Synthesis, characterization, and spectral studies of lanthanide(III) nitrate complexes of promethazine

Abstract A new series of lanthanide(III) nitrate complexes of 10-(2-dimethylaminopropyl)phenothiazine (promethazine or PM) with the general formula [Ln(PM)2(NO3)2]NO3, where Ln=La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu, have been prepared and characterized based on elemental analysis, molar conductivity, magnetic susceptibility, spectroscopic data, and thermal studies. Conductance study indicates a 1:1 ionic ratio for the complexes in solution. The infrared spectra show that promethazine acts as a bidentate ligand using the heterocyclic nitrogen atom and tertiary nitrogen atom of the side chain as the sites of coordination. Only two of the nitrate ions are coordinated bidentately to the central metal ion while another nitrate ion remains outside the coordination sphere. A tentative structure has been proposed for the complexes.

Oxidation of L-glutamine by manganese(iii) in aqueous sulfuric acid, acetic acid, and pyrophosphate media: A kinetic and mechanistic study

Manganese(III) solutions were prepared by known electrochemical methods in sulfuric acid, acetic acid, and pyrophosphate media. The nature of the oxidizing species present in manganese(lll) solutions was characterized by spectrophotometric and redox potential measurements. Kinetics of oxidation of L-glutamine by manganese(III) in sulfuric acid (1.5 M), acetic acid (60% v/v), and pyrophosphate (pH = 1.3) media at 313 K, 323 K, and 328 K, respectively, have been studied. Three different rate laws have been obtained for the three media. Effects of varying ionic strength, solvent composition, and added anions, such as fluoride, chloride, perchlorate, pyrophosphate, and bisulfate, have been investigated. There is evidence for the existence of free radicals as transient species. Activation parameters have been evaluated using Arrhenius and Eyring plots. Mechanisms consistent with the observed kinetic data have been proposed and discussed

OSMIUM(VIII) CATALYZED KINETICS AND MECHANISM OF INDOLES OXIDATION WITH ARYL-N-HALOAMINES IN ALKALINE MEDIUM

The kinetics of oxidation of the title substrates by sodium N-haloarylsulfonamides (or ary-N-haloamines), chloramine-T (CAT), bromamine-T (BAT), chloramine-B (CAB), and bromamine-B (BAB), catalyzed by osmium(VIII) in alkaline medium has been studied at 30°C. The corresponding oxindoles and arylsulfonamides have been characterized as reaction products. The reaction rate shows a first-order dependence each on |indole|0 and |oxidant|0, a fractional-order on |Os(VIII)|, and an inverse first-order on |OH−|. Addition of arylsulfon-amide, chloride and bromide, and variation of ionic strength of the medium have no effect on the reaction rate. There is a negative effect of dielectric constant of the solvent. Activation parameters have been calculated from the Arrhenius and Eyring plots. Hammett correlation of substituent effects indicates an LFE relationship with ρ = −1.0, showing the formation of an electron deficient transition state. From enthalpy-entropy relationships and Exner correlations, the isokinetic temperatures (333 K and 326 K) have been determined for the reactions of CAT and BAT, respectively. Proton inventory studies in H2O-D2O mixtures have shown the involvement of a single exchangeable proton of OH− ion in the transition state. A mechanism consistent with the observed kinetics has been proposed.

Palladium(II)‐Catalyzed Oxidation of Primary Amines by Bromamine‐T in Alkaline Medium: A Kinetic and Mechanistic Study

Oxidations of n‐propyl, n‐butyl and n‐hexylamines by bromamine‐T (BAT), catalyzed by Pd(II), in alkaline medium have been kinetically studied at 30 °C. The reaction rate shows a first‐order dependence on [BAT], a fractional‐order dependence each on [NaOH] and [Pd(II)], and an inverse fractional‐order dependence on [Cl–]. The reaction rate is independent of [amine]. The addition of the reduction product of BAT, p‐toluenesulfonamide, and the variation of dielectric constant of the solvent have no effect on the rate. The catalytic constants were calculated at different temperatures. Activation parameters were evaluated.

MECHANISTIC INVESTIGATIONS OF THE OXIDATION OF SUBSTITUTED PHENETHYL ALCOHOLS BY MANGANESE(III) SULFATE CATALYZED BY RUTHENIUM(III) IN ACID SOLUTION

The kinetics of the Ru(III) catalysed oxidation of six derivatives of phenethyl alcohol (X-C6H4-CH2CH2OH where X═H, Cl, Br, CH3, OCH3 or NO2) by manganese(III) sulfate, in sulfuric acid solution at 30°C, were followed spectrophotometrically at λmax =500 nm. The rate shows a first-order dependence on [Mn(III)] and Michaelis-Menten kinetics was observed with the substrate. The rate was of fractional-order in [Ru(III)]. Hammett acidity function (─H0) has been invoked to explain the retardation of the rate by acid. Bunnett plots indicated the absence of water molecule participation in the rate-limiting step. Variation of the ionic strength and the dielectric constant and the addition of the reaction product Mn(II) and anions such as C1−, SO2+ 4, and F− have no effect on the rate. The solvent isotope effect k(H2O)/k(D2O) =1.38 was observed. Proton inventory studies were made in H2O-D2O mixtures. A linear free energy relationship was observed when the results were analyzed by Hammett and Okamoto-Brown equations. Activation parameters were computed. A mechanism consistent with the observed kinetics has been proposed.