1,720,993 research outputs found
INCLUSION OF THE HERBICIDE DIFENZOQUAT (1,2-DIMETHYL-3,5-DIPHENYL-PYRAZOLIUM) BY BETA-CYCLODEXTRIN IN AQUEOUS-SOLUTION
No full textPROPERTIES OF SOME N-HETEROCYCLIC HERBICIDES AND FUNGICIDES AT ELECTROCHEMICAL INTERFACES
No full textThe advantage of electrochemical studies for research on herbicides and fungicides is outlined in selected cases of the elucidation of redox properties and interfacial absorption from aqueous solutions. The compounds investigated here are on the base of pyridinium and pyrazolium electroactive centres
COMPLEX-FORMATION OF METHYL VIOLOGEN AND DIQUAT WITH MACROCYCLIC ETHER DIBENZO-24-CROWN-8
No full textThe complex formation of a coplanar N,N'-ethylene-bipyridinium dication and dibenzo-24-crown-8 in methanol was detected by electrochemical techniques. It is demonstrated here that a coplanar ring arrangement is not required for an efficient charge-transfer interaction between bipyridinium and crown. The much stronger complexation of methyl viologen was investigated by both electrochemical and spectral techniques. The presence of a benzene ring in the crown molecule is a condition for complex formation. The described complex exerts catalytic activity for the reduction of oxygen
ELECTROCHEMICAL EVIDENCE OF THE REORIENTATION OF ALKYNES ON TRIMETALLIC CLUSTERS DURING A 2-ELECTRON REDUCTION
No full textModels of pesticides inside cavities of molecular dimensions. A role of the guest inclusion in the dechlorination process
No full textThe reduction mechanism of the pesticide vinclozoline (3-(3,5-dichlorophenyl)-5-methyl-5-vinyl-1,3-oxazolidine-2,4-dione) was studied in nonaqueous solvents in the confined environment of a cyclodextrin (CD) cavity. The effect of the cavity dimensions on the mechanism of the redox process was evaluated using glucose as a reference and using three cyclodextrin molecules of different cavity sizes, namely, RCD, âCD, and çCD. In the absence of CD the main reduction product of vinclozoline in the first reduction step is dichloroaniline. An addition of glucose leads to a quantitative change of mechanism with 10 products in
total. Addition of CD, however, leads exclusively to dechlorination of the phenyl ring. The degree of
dechlorination depends strongly on the choice of cyclodextrin molecule. The importance of the complex
formation equilibria in the change of the mechanism is supported by a series of semiempirical AM1 quantum-mechanical calculations. Very good correlation (correlation coefficient 0.995) was obtained between the
complex stabilization energy of the inclusion complex and the degree of pesticide dechlorination. Additionally,
we were able to show that the complexes are stabilized by the formation of intermolecular hydrogen bonds between the host and guest species. CD molecules do not simply act as proton donors in a nonaqueous environment, but also protect parts of the molecule included within the cavity and steer the degradation process toward fewer products
Double-layer effects and distance dependence of electron transfer in reduction of nitro aromatic radical anions
No full textThe first example of the effect of an electric double layer on the reduction of electrochemically generated radical
species is reported. The anion radical of methyl 5-(2,4-dichlorophenoxy)-2-nitrobenzoate (pesticide bifenox) is
electrochemically reduced in acetonitrile to a phenylhydroxylamine derivative in a process involving three electrons.
This heterogeneous reaction is strongly influenced by the concentration and nature of the cation of the indifferent
electrolyte. Depending on the type of tetraalkylammonium cation, the redox potential changes by 0.45 V. The kinetic
parameters were obtained for five tetraalkylammonium hexafluorophosphate salts. The Frumkin correction, which
assumes that the outer Helmholtz plane coincides with the reaction site, was applied to kinetic data of the radical anion
reduction. The correction of the apparent rate accounted for the observed effect only in the case of tetramethylammonium
salt. The presence of higher tetraalkylammonium homologues causes deviations from the predicted dependence of
the electron-transfer rate on the æ2 potential of the outer Helmholtz plane. Hence, the nature of the cation of the
electrolyte exerts a further effect extending beyond the electrostatic repulsion only. The corrected rate of electron
transfer decreases exponentially with increasing size of the alkyl chain of the indifferent electrolyte cation in the order
methyl > ethyl > propyl > butyl > hexyl. The rate decay is characterized by an exponent â ) 0.83. This confirms
that the reaction plane for the reduction of the bifenox radical anion is different for each electrolyte. Due to this fact
the Frumkin correction cannot fully account for the observed dependence of the heterogeneous rate on the solution
composition. The observed effect is not specific to the bifenox radical. A similar influence of the concentration and
nature of the cation of the indifferent electrolyte was observed for other nitro compounds, namely, nitrobenzene,
nitrobenzoate, and nitrofen
Inclusion complexes of atrazine with alpha-, beta- and gamma-cyclodextrins. Evidence by polarographic kinetic currents
No full textThe reduction of atrazine in acidic aqueous media on mercury electrodes proceeds only after the protonation reaction. In fact, the efficiency of the reduction process is very low at pi-Is greater than 4. However, the addition of cyclodextrins (CDs) to neutral aqueous solutions of atrazine yields a kinetically controlled polarographic reduction wave, whose limiting current depends on the size of the CD cavity, and increases with the concentration of the CD itself. In particular, the size of the increase follows the order beta-CD < gamma-CD < alpha-CD for the same CD concentration. The half-wave potential shifts toward more negative values when the concentration of CDs increases. These findings lead to the conclusion that atrazine and CDs form an inclusion complex, whose stability constant we have estimated, and also that atrazine undergoes protonation facilitated by complexation with CDs. The stability constants of 1:1 complexes evaluated from polarographic data in 0.1 M-KCl and at neutral pH for alpha-CD, beta-CD and gamma-CD are 4900, 1970 and 19000, respectively. The formation of the inclusion complex was indirectly confirmed by UV-Vis measurements in the presence of methylred and phenolphthalein, which both compete with atrazine in the formation of the corresponding inclusion compounds with CDs
Decomposition reactions of bifenox anion radical involving intramolecular electron transfer
No full textThe nitropesticide bifenox (methyl 5-(2,4-dichlorophenoxy)-2-nitrobenzoate) is reduced in aprotic media in two reduction steps.
The first heterogeneous electron transfer involves the acceptance of one electron leading to the formation of a relatively stable nitro
anion radical that was characterized by EPR spectroscopy. The electrochemically generated radical anion eventually decomposes to
two stable products. Intramolecular electron transfer coupled with chemical steps leads in the first case to the cleavage of the carboxymethyl
group and the formation of the nitropesticide nitrofen ((2,4-dichlorophenyl)(4-nitrophenyl) ether) and in the second case
to a trans-esterification process, i.e., an exchange of the methyl group for an alkyl of the supporting electrolyte. Both these final
products retain pesticidal activity, which explains the high efficiency of bifenox as a pesticide. The electrochemical characteristics
show that the radical anion undergoes a further two-electron reduction at more negative potentials. The second reduction step is
strongly influenced by the nature and concentration of the tetraalkylammonium cation of the supporting electrolyte. The influence
of the electrolyte concentration was accounted for by the effect of the electrode double-layer structure on the electron transfer kinetics
(Frumkin effect) due to repulsion of the radical anion from a negatively charged electrode|electrolyte interface
ELECTROCHEMISTRY OF S-TRIAZINE HERBICIDES - REDUCTION OF ATRAZINE AND TERBUTYLAZINE IN AQUEOUS-SOLUTIONS
No full textThe electrochemical behavior of both herbicides is qualitatively similar. The two-electron reduction of atrazine (2-chloro-4-ethylamino 6-isopropylamino-1,3,5-triazine) proceeds only in acidic media where the preceding protonation takes place (pK = 1.44). In solutions of higher pH (2-4) the observed reduction current is kinetically controlled by protonation and the estimated value of the protonation rate constant is k(r) = 2400 +/- 400 mol(-1) 1 s(-1). The products of reduction were identified by large-scale electrolysis followed by GC-MS analysis. The electron transfer reaction causes cleavage of the Cl atom and loss of the ethyl group yielding the respective products in the ratio approximately 4:1. The electrochemical properties of terbutylazine are characterized by the protonation constant pK = 2.37 and rate constant k(r) = 1900 +/- 200 mol(-1) 1 s(-1)
Growth of compact layers at the interfase. V Adsorption properties of difenzoquat herbicide (1, 2 - dimethyl - 3, 5 - diphenyl pyrazolium.
No full textThe adsorption properties of 1,2-dimethyl-3,5-diphenyl-pyrazolium cation (difenzoquat) on the mercury electrode in aqueous medium are given. Strong adsorption is detectable at concentrations below 10(-6) mol/l and in a wide potential range. The time dependence of the electrode impedance indicates the formation of a compact film controlled by nucleation/growth phenomena. Under specific conditions the film has a fractal character connected with the existence of a constant phase element. An adsorptive stripping method can be worked out for its sensitive determination
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