J. Heyrovský, Japan, and organic polarography.

Polarography is an electroanalytical technique based on recording current-voltage curves using a dropping mercury as the working electrode. It can be used for investigations of both reductions and oxidations of inorganic and organic species. Before WWII the developments of this technique linked Prague (in the then Czechoslovakia) with Kyoto (in Japan, where reductions of organic compounds were first observed). After WWII wide use of this technique has developed, so that in the 1950s and 1960 it became the fifth most frequently used analytical technique. More recently the analytical applications were limited to those of heterogeneous solutions and analyses of some drugs. Its applications in physical organic chemistry involve studies of structure-reactivity relationships and applications to investigations of equilibria and kinetics of rapidly or slowly established processes.

Reactions of orthophthalaldehyde with ammonia and 2-aminoethanol.

Reactions of orthophthalaldehyde (OPA) with amines are used in the determination of amino acids and in applications of OPA as a biocide. To contribute to the understanding of processes involved, the reactions of OPA with ammonia, which are conveniently slow, were studied. In a set of rapidly established equilibria, the 1,3-dihydroxyindole and the product of its dehydration are formed (Scheme 1). The individual equilibria were identified and equilibrium constants determined using DC polarography and UV spectra. The ring closure involves the carbinolamine; the imine formation is a side reaction. Both the ring formation and the dehydration of the carbinolamine are generally acid catalyzed. In the finally established overall equilibria between OPA and the isoindole derivative, the concentrations of intermediates are negligible. The same applies to the reaction of OPA with 2-aminoethanol, in which the initial formation of a carbinolamine and of an imine are too fast to be followed. Very slow reactions taking place during periods of hours or days, which probably result in the formation of dimeric species, have also been observed. This contribution demonstrates the advantages of combinations of polarographic and spectrophotometric techniques in the investigation of complex reactions of some organic compounds.

Electrochemical determination of HIV drug Abacavir based on its reduction.

Abacavir (I), a drug used in the treatment of HIV, is electrochemically reduced at the dropping mercury electrode in a four-electron process, similar to structurally related adenine (III) and adenosine triphosphate (IV). To undergo the reduction, the species is protonated in the vicinity of the electrode. The protonations take place on the 6-amino group and on one of the pyrimidine ring nitrogens. The role of covalent hydration of the pyrymidine ring has been interpreted. Best suited as supporting electrolytes for analytical purposes are solutions of 0.1-1.0 M sulfuric, perchloric, or hydrochloric acids. Procedures of analyses of tablets containing I were established and validated, based on peak currents obtained by linear sweep, differential pulse, or square-wave voltammetry with a hanging mercury drop electrode as indicator electrode. The procedure proved to be more sensitive and more reliable than that based on oxidation on a glassy carbon electrode, proposed previously.

Existence and reactivity of three forms of orthophthalaldehyde in aqueous solutions. Polarographic, voltammetric, and spectrophotometric study.

Orthophthalaldehyde (1,2-dicarboxaldehyde) (OPA) forms in the presence of a strong nucleophile with amino acids isoindole derivatives. The reaction is used in fluorometric determination of amino acids. The mechanism of these processes is not understood. OPA is present in aqueous solutions in three forms: unhydrated (I(a)), monohydrated acyclic (I(b)), and cyclic hemiacetal (I(c)). The absence of data for the molar absorptivities of these forms, together with overlap of their absorption bands, limits the application of spectrophotometry. Measurement of polarographic limiting currents of forms I(a) and I(b) enables determination of equilibrium constants K1 (formation of I(b)) and K2 (for the ring formation). The presence of these forms was supported by 1H NMR and 13C NMR. The rate of hydration of OPA is general-acid-base-catalyzed, but that of dehydration shows only specific-acid-base catalysis. The rate of hydration is controlled by general-acid-base-catalyzed addition of water to I(a). The rate of dehydration depends on the opening of the ring in I(c), which is specific-acid-base-catalyzed. At pH > 10 OPA undergoes a complex set of acid-base reactions (Scheme 3). The presence of polarographic anodic waves and oxidation on the gold electrode indicates the importance of the presence of a geminal diol form (II(a)). Establishment of equilibria among the three forms of OPA together with reactions at pH > 10 has to be considered in elucidating the reaction scheme of procedures using OPA as a reagent in the determination of amino acids.

Electroreduction of aromatic oximes: diprotonation, adsorption, imine formation, and substituent effects.

Aromatic oximes are reduced in aqueous solution in a four-electron process. The reducible species in the pH range 5-8 is a diprotonated form of the oxime. This species is generated in the course of electrolysis in the vicinity of the electrode surface from the adsorbed neutral form of the oxime. The reduction is initiated by a cleavage of the N-O bond. The diprotonation facilitates the reduction process by the preformation of OH2+ as a good leaving group and by a positive charge on the azomethine nitrogen. Diprotonation has been proven based on shapes of i = f(pH) plots, by observed shifts of half-wave potentials with pH and by comparison with the reduction of nitrones. Some observed deviations from theoretical i = f(pH) plots were attributed to the role of adsorption on the rate of protonation. Adsorption is also responsible for dips on some of the i-E curves. Adsorption plays a role at concentrations as low as 1 x 10(-5) M, when the electrode surface is still not fully covered. This indicates that catalyzed protonation occurs on islets of adsorbed materials. At pH 2-5 the studied oximes in the vicinity of the electrode are predominately present in a protonated form, which is less strongly adsorbed. In this pH range the protonation takes place in a homogeneous reaction layer of the electrode. It yields a monoprotonated form, which is reduced. The separation of two two-electron waves observed for some oximes in acidic media serves as an experimental proof of the formation of imines as reduction intermediates. This separation is caused by the differences in pKa values of protonated forms of oximes and imines. The effects of substituents in the para position on the benzene ring are characterized by correlation with the Hammett substituent constant sigmax. This has been proven at pH 1.5 for substituted benzaldehyde oximes and at pH 5.0 for substituted acetophenone oximes.

Nucleophilic attack by bromide ions as a first step of conversion of Se(VI) to Se(IV).

Numerous commonly used analytical methods allow only determination of a total amount of selenium in a given sample. Electroanalytical methods as well as those based on hydride generation or on formation of piazselenol allow only determination of Se(IV). To determine Se(VI) by these procedures, present alone or in mixtures with Se(IV), it is first necessary to convert Se(VI) to Se(IV). Such conversion is effective in the presence of excess of halides in acidic media or by photoreduction. In the often used conversion of Se(VI) in the presence of chlorides or less frequently of that of bromides, it has been assumed that the halide ion acts as a reducing agent. Kinetic studies of conversion of Se(VI) in acidic solutions containing an excess of bromide ions indicated that the rate determining first step of the reaction with Se(VI) is a nucleophilic substitution of the OH(2)(+) group in the protonated form of H(2)SeO(4) by bromide ions. For the overall reaction with rate -d[Se(VI)]/dt=k(1)[H(+)][Br(-)](1.15)[Se(IV)] the rate constant 1 x 10(-3)L(2)mol(-2)s(-1) was found. The following formation of Se(IV) from the bromo derivative is a fast reaction probably resulting in elimination of HBrO.

Strong covalent hydration of terephthalaldehyde.

Spectrophotometric and electroanalytical studies indicate that one of the formyl groups of terephthalaldehyde in aqueous solution is present in about 23% as a geminal diol. Stronger covalent hydration of CHO in terephthalaldehyde than in p-nitrobenzaldehyde is attributed to a strong resonance interaction between the two formyl groups.