POLAROGRAPHIC STUDIES ON SOME ARYLAZO DERIVATIVES OF ^-DIKETONES AND SUBSTITUTED AZINES

Abstract

During the present course of study, the electrochemical reduction of some important hydrazo compounds has been carried out. These compounds have attracted wide attention because of their applications in the synthesis of biologically and medici nally important heterocyclic compounds having a phenylazo substituent in an appropriate position. The polarographic method has been particularly useful in understanding the mechanism of metabolic and biochemical redox reactions taking place at the solution-cell membrane of the interface. In a few cases this method can be successfully employed to ascertain their structu ral characteristics. The second chapter of the thesis has been devoted to the polarographic reduction of the precursors of biologically important heterocyclic confounds such as pyrazoles, isoxazoles, pyrazolones etc. The structure of the coupled product of aryldiazonium chloride with ^-diketones is in doubt. Since this can be represented both by hydrazono and azo structures, therefore an attempts have been made to establish their correct structure by polarographic technique. A number of coupled products, 3-arylhydrazono-pentane-2, ^-diones having substituents in the benzene ring, viz., H, 2-Chloro, lf-chloro, 2-methyl, ^t-methyl, 2,3-dimethyl, ^-ethoxy, 2-methoxy, 'i-methoxy, 2,5-dichloro and 3-nitro were synthesised in the laboratory by the literature method. These were

subjected to detailed polarographic analysis at the dropping mercury electrode. These coupled products gave a single well defined, ^-electron transfer, irreversible diffusion controlled-reduction wave in B.R. buffers in the pH range 2.0 - 11.8 except nitro derivative in which one more ^electron transfer irreversible diffusion controlled reduction wave was observed at a more positive potential over the entire pH range. The heights of the waves were found to be pH independent. Hydrazono structure for the coupled products has been proposed in view of the if-electron reduction. This is in contrast to the azo structure for which the electrode mechanism would involve participation of two electrons. In these coupled products an interesting variation in the presence of different substituents on the Ey values under a similar set of conditions was observed. It was found that an electron donating group would strengthen-NH-N-bond resulting in the shift of E,,? towards more negative potential whereas an withdrawing groiap would facilitate reduction by decreasing electron density at -NH-N-bond. Such a behaviour was observed for all the 3-arylhydrazonopentane-2,Lt—diones. The effect of ionic strength on E, ,„ and i-, was studied by varying the concentration of the supporting electrolyte from 0.01M to 0.56M. No marked effect on these two polarographic parameters (E, ,_ and i, ) was observed. On the other hand with increase in size of the cation, the value of specific reaction (in) constant (/°) increased. The shift in \/2 to more negative potential follows the order? Mg"^ > NH^ >K+ > Na+ > Li+ Furthermore the effect of different concentration of depolarizer, different heights of mercury column, solvent comp ositions and surfactants on the polarographic characteristics of these compounds have been discussed. En/2 shifted towards a negative potential with increase in solvent composition followed by a change in limiting current. This effect was explained on the basis of decrease in adsorbability of the depolarizer. The reduction was also studied In presence of surfactants and no effect was observed on E,/p and I, by the presence of anionic or non-Ionic surfactants. However, in the presence of cationic surfactant viz., cetyltrimethylammonium bromide (CTAB) the E1/2 shifted towards a more negative potential accompanied by a decrease in limiting current. In absence and presence of surf actant (CTAB) values of K and aa for all these compounds were calculated and have been taken as a proof inner sphere or outer sphere path of the ele strode reaction. A survey of the literature had revealed that another categoxy of azomethine compounds viz., azines have not been Investigated in detail. The third chapter of the thesis has been devoted to a systematic and comprehensive study of the reduction of some substituted benzalazines with emphasis on the mechanism of electrode reaction, a quantitative relationship between E1/2 and effect of substituents. The effect of different (iv) solvent compositions, various cations, anions and surfactants on the reduction has been discussed. The compounds under inves tigation showed following polarographic characteristics ? All these azines except nitro derivative gave well defined, single ^-electron transfer, irreversible, diffusion controlled wave in B.R. buffers of pH range 2.0-12. As in the case of the nitro-derivative the appearance of a ^-electron reduction wave at a more positive potential is attributed to the reduction of the nitro group. The value of E-j/p changed towards more negative potential with the increase in the concen tration of depolarizer and with increase of pH upto 8.U-6 and afterwards E^p became indepandent of pH,the plots of E,/p Vs pH were linear and the value of 'p' , number of protons, involved In the rate determining step was also calculated from the slope r of these plots. The value of kinetic parameter viz., ah, K? h was calculated from the slope and intercept of the plots of Ed.e VsflQg J^T - °»5te log tl . With increase in the concentration of organic solvents, the E1/2 is shifted towards more negative potential side by side influencing the wave height. A four electron transfer mechanism has been proposed for the reduction of these compounds. The value of reaction constant (/> = 0.166V) obtained from the plot of E±/ Vs Hammett substituent constant (cO was found to be positive, indicates a nucleophilic mechanism of the electrode process. In the fourth chapter of the thesis, adsorption phenome non of surfactants and different solvent compositions on (v) reduction of azine at the mercury solution Interface has been studied by electrocapillary method. Interfacial tension of the mercury solution interface was measured as a function of poten tial and surfactant concentrations. It was also measured as a function of potential and different solvent compositions. With increase in the concentration of surfactants viz., tween 20, tween U-0, tween 60 or solvents viz., dimethylformamide, acetonitrile, methanol from benzalazlne solution using IM KC1 as suppo rting electrolyte, the interfacial tension decreases due to adsorption of surfactants and solvents at the mercury surface. Further the electrocapillary maxinfa^ have been shifted towards more positive side on the potential axis with increasing the concentration of the surfactant. This behaviour suggested that the adsorption of the surfactant at the mercury interface takes place at the beginning, when applied potential at d.m.e. is zero, whereas in the case of solvent electrocapillary maxima was independent of concentrations at about h0/'. and then slightly shift towards positive potential with further addition of the solvent. The surface excess of the adsorption isotherm were calculated by Gibb's equation at different potentials and conce ntrations of surfactants and solvents. The shift in p.z.c. (potential at zero charge) was plotted against the logarithm concentrations of the sufactant or solvent and with surface excess . It was observed that a sudden increase occurs in the change of p.z.c. potential and this abrupt behaviour was explained on the basis of new mole cular orientation. (vi) . The last chapter of the thesis has devoted to the mesomeric effect of substituents on the reduction of -C=Ngroup. For explaining the mesomeric effect between the two groups where n - it or tt - o~ conjugation occurs between them, quantum treatment was employed. The energy of lowest unoccupied molecular orbital (UJMO), tt energy, HMO energy (Huckel molecular orbital), self consistent field energy (SCF) and triplet state ( tt _ tt36 ) energy of these compounds were calculated by using Huckel molecular orbital and Pariser-Parr-Pople (PPP) methods. These calculations were carried out on a computer deck-20. Computation of wave functions, energy levels i.e. coefficients of charge densities, bond order , KJMO energies and tt- energies were carried out using a computer programme written by Green wood. The linear relationships were obtained between E, /p and DJMO energy E, ,p and tt-energy, E,/2 and SCF-energy, E,/2 and triplet energy and E-j/p and HMO energy.