Estudio cinético de la fotodegradación del ión p-hidroxibencenodiazonio en medio polar / Kinetic study of the photodegradation of p-hydroxybenzenediazonium ion in polar media

Se ha realizado un estudio sobre la fotodescomposición del ión p-hidroxibencenodiazonio (PDQ) basado en los datos espectrofotométricos y cromatográfi cos obtenidos con disoluciones de PDQ expuestas a irradiación UV (254 nm) en medio de acetonitrilo y agua. Los resultados de HPLC y HPLC-masa (HPLC/MS) indican que el 4-acetamidofenol es el principal producto que se forma tras la irradiación de PDQ en acetonitrilo. Esto se explica como consecuencia de la formación inicial del catión arilo, que posteriormente participa en una reacción de Ritter. El análisis cinético de los datos espectrofotométricos revela que la fotodegradación de PDQ es más rápida en acetonitrilo (constante de velocidad observada, kobs, = 0,1442 s-1) que en acetonitrilo acidifi cado (kobs = 0,009 s-1), lo que indica una mayor fotoestabilidad de la especie protonada derivada de PDQ. La constante de segundo orden (0,062 M s-1) encontrada para la fotodescomposición de PDQ en tampón fosfato (pH 7) se justifi ca por el establecimiento de un equilibrio entre las especies protonada y no protonada procedentes de la disociación ácida de PDQ

A study on the photodecomposition of p-hydroxybenzenediazonium ion (PDQ) has been made using chromatographic and spectrophotometric data obtained from UV-irradiated (254 nm) PDQ solutions in acetonitrile and aqueous media. The HPLC and HPLC-mass results indicate that 4-acetamidophenol is the main product formed after the irradiation of PDQ in acetonitrile. This is explained as a consequence of the initial formation of the aryl cation which is later involved in a Ritter’s reaction. A kinetic analysis of the spectrophotometric data reveals that PDQ photodegradation is faster in acetonitrile (observed rate constant (kobs) = 0.1442 s-1) than in acidifi ed acetonitrile (kobs = 0.009 s-1) indicating a higher photostability of the protonated species derived from PDQ. The second order constant (0.062 M s-1) found for the PDQ photodecomposition in phosphate buffer (pH 7) is explained in term of the equilibrium between protonated and non-protonated species coming from the acid dissociation of PDQ

Oxidative effects induced by dediazoniation of the p-hydroxybenzenediazonium ion in a neutral aqueous medium.

The toxicity of arenediazonium ions is believed to result from the appearance of very reactive compounds during the dediazoniation process. In the case of the p-hydroxybenzenediazonium ion (PDQ), radical species generated during dediazoniation could potentially initiate lipid peroxidation. The data obtained in spectrophotometric experiments suggest that an interaction between PDQ and linoleic acid (LA) gives rise to the characteristic absorption of oxidized products deriving from LA, both in the presence and absence of a mixed micellar medium containing the surfactant Tween 20 (Tw20). Spectroscopic evidence also clearly points to the interference of these processes in the dediazoniation of PDQ. Analysis by reverse-phase, high-pressure liquid chromatography (HPLC) confirms that the decomposition of PDQ in a mixed micellar medium induces the peroxidation of both LA and methyl linoleate (MEL), thus causing the appearance of peaks characteristic of dienic conjugated hydroperoxides. The same products are observed after interaction between LA and the water-soluble 2,2'-azobis (2-amidinopropane), a frequently used initiator of lipid peroxidation. The proportion of isomers produced during the peroxidation process agrees well with that reported for reactions mediated by free radicals. A further chromatographic analysis of the decomposition of PDQ in the presence of 2-methylcyclohexa-2,5-diene-1-carboxylic acid (CHD) shows that phenol and quinone are the main products of the reaction. These results are discussed on the understanding that aryl and peroxyl radicals abstract a hydrogen atom from CHD, in accordance with our general scheme for PDQ dediazoniation described in a previous publication.

Degradation of ampicillin in the presence of cadmium (II) ions.

Cadmium (II) ion-catalyzed degradation of ampicillin in methanol at 20 degrees C has been studied. It has been observed that the rate values tend to saturate when the concentration of ampicillin or the metal ion is increased. The results obtained in the present study suggest that ampicillin degradation occurs through the formation of a 1:1 (SM) and 2:1 (S(2)M) ampicillin-metal complexes. These complexes decompose giving a single product (absorption maximum at 285 nm; ((p)=1.82x10(4) l mol(-1) cm(-1)) that has been isolated and identified (Cd(II) (L(2-))(2) (H(2)O)(4) Na(2)). The appearance of this product reflects a first order reaction with respect to the 1:1 complex, with a rate constant of 3.87x10(-2) min(-1) and the existence of an equilibrium between the 1:1 and 2:1 initial complexes. The equilibrium constant value, calculated from kinetic data, is 1.7x10(3) l mol(-1).

Reaction of sodium amoxicillin with Cu(II) ion in a methanolic medium.

The present article considers several physicochemical aspects of the complexation reaction of sodium amoxicillin and Cu(II) ion in a methanolic medium. Analysis of spectrophotometric data demonstrated the formation of two complex species with amoxicillin:Cu(II) ion mole ratios of 1:1 and 2:1. Stability constants (beta) and molar absorptivities at 610 nm (epsilon) of the complexes (20 degrees C) in methanol were calculated simultaneously by a computer program on the basis of absorbance data obtained at 610 nm. The values thus calculated for the 1:1 complex were as follows: log beta 1 = 5.48 +/- 0.21 L.mol-1, epsilon 1 = 70 +/- 2 L.mol-1.cm-1. The values for the 2:1 complex were as follows: log beta 2 = 8.98 +/- 0.17 L2.mol-2 and epsilon 2 = 138 +/- 4 L.mol-1.cm-1. The amoxicillin:Cu(II) ion complexes were quite stable over time.