Petr Zuman

Polarographic reduction of aldehydes and ketones: Part XXIII. Electroreduction of 1-methylpyridiniumcarboxaldehydes

Abstract All three isomeric 1-methylpyridiniumcarboxaldehydes are present in aqueous solutions predominantly in the hydrated form (PyCH(OH)2). At pH smaller than about 7, the protonated form of the free aldehyde is reduced in two one-electron steps following the sequence H+, e, H+, e. At pH greater than about 8 the free aldehydic form is reduced in two one-electron steps following the sequence, e, H+, e, H+. Up to pH about 10 the reducible species is formed by dehydration of PyCH(OH)2, at higher pH by elimination of hydroxide ion from PyCH(OH)O−. The increase in polarographic reduction current between pH about 8 and 10 is due to a base-catalyzed dehydration, involving a rate-determining formation of PyCH(OH)O−. The polarographic dissociation curve (i1 vs. pH) is in this case shifted to lower pH-values when compared with the conditional dissociation curve ([PyCH(OH)2] vs. pH). Such behavior shows that the effective reduction of the conjugate base of an electroinactive acid (PyCH(OH)2) occurs. The increase in polarographic reduction current at pH smaller than about 1 is due to an acid-catalyzed dehydration involving formation of PyCH(OH)(OH2+). The rate of protonation governs the limiting current, since the increase in current with increasing acidity occurs in a region one to two acidity units higher than pK1 values obtained spectrophotometrically, using acidity functions HC(OH)2. The free carbonyl species produced by dehydration is reduced following a sequence H+, e, H+, e.

Polarographic reduction of aldehydes and ketones: Part XXII. Reduction and oxidation of α,β-unsaturated aldehydes: Acrolein, tiglaldehyde and substituted cinnamaldehydes

Abstract The protonated form of acrolein is reduced at pH 9 a more negative wave (i4) is observed in the potential region where propionaldehyde is reduced. Identification of this wave was complicated by formation of β-hydroxypropionaldehyde in a bulk reaction of acrolein with hydroxide ions. Controlled-potential electrolysis at the limiting current of wave i2 or i3 at pH 5 and 7 yielded propionaldehyde, identified by gas-liquid chromatography (glc) and using waves of semicarbazones. Absence of formation of allyl alcohol was proved by glc and bromine titration. Voltammetric curves obtained with a hanging mercury drop electrode indicated that the mechanism of reduction at a mercury electrode with a constant surface differs from that at the dropping mercury electrode (DME). Polarographic reduction at the DME in the two-electron step yields predominantly saturated aldehyde, as was observed for the majority of α,β-unsaturated ketones and for cinnamaldehyde, rather than an unsaturated alcohol, as proved for crotonaldehyde. Substitution of cinnamaldehyde by a methyl group in the α-position or by a phenyl group in the β-position does not result in a change in the preferred protonation site. Saturated aldehydes are still the predominating product found at the DME. Similarly, introduction of a methyl group in the α-position of crotonaldehyde does not affect the affinity to protonation. Tiglaldehyde is reduced at the DME to an α,β-unsaturated alcohol. Electrolysis products obtained with a mercury pool electrode should not be used for interpretation of the electrode processes at the DME without a proof of analogous processes.

Polarographic reduction of aldehydes and ketones: Part XXVIII. 1,2-cyclohexanedione

Abstract 1,2-Cyclohexanedione exists in aqueous solutions partly as a monoenol. The protonated form of the monoenol (I) is reduced polarographically at H0 = −2.5 to pH 7 in the wave i3, the unprotonated form (IIa) at pH > 6 in the wave i4. Wave i3 at pH 7 at potentials almost 1.0 V more positive than those of the monoenol form. The diketo form (IIb) is strongly hydrated. Waves i1 and i2 are small reaching maximum height at pH about 12 corresponding to 13% of the total current. Their height is limited by the rate of dehydration. The pH dependence of height of waves i1 and i2 is attributed to the base catalysis of dehydration.

A novel type of effect of an acid-base equilibrium preceding electroreduction: Polarographic reduction of the conjugate base of a non-reducible acid

Abstract Polarographic reduction currents of pyridinecarboxaldehydes and their N -methyl-derivatives increase (with increasing pH) at pH>7 in the shapes of a dissociation curve. This increase occurs in a pH-range several pH-units lower than that of the p K for the dissociation of the corresponding geminal diol. This behavior is interpreted by assuming that the rate of dissociation yielding the geminal diol anion is the rate-determining step in the electrode process followed by fast elimination of a hydroxide ion. By comparing the shapes of the i 1 -pH plots it was concluded that whereas for N -methylpyridiniumcarboxaldehydes the protolytic dissociation (5b) predominates, for 4-pyridinecarboxaldehyde, the predominating path is the hydrolytic dissociation (5a). It has been demonstrated that in these cases the conjugate acid is electroinactive and the conjugate base is effectively reduced (after a rapid transformation into an electroreducible species). Such a mechanism is indicated by the shift of the polarographic dissociation curve to pH-values lower than the true p K .

Polarographic reduction of aldehydes and ketones: Part XXVII. 1-Phenyl-1,2-propanedione

Abstract Polarographic reduction of the symmetrical diprotonated form of 1-phenyl-1,2-propanedione (I) occurs in waves i′1 and i1 at pH The latter can be further reduced at more negative potentials in waves i4 and i5. At pH 8.5 to 9.5 the asymmetrical monoprotonated form of I is reduced in the irreversible wave i2 directly to α-ketol which is electroinactive under conditions used. At pH > 9.5 the symmetrical unprotonated form is reversibly reduced into enediolate dianion. Base catalyzed conversion of the enediolate into electroinactive species causes the decrease of the anodic wave obtained with the Kalousek commutator method at pH > 11. The ratio [α-ketol]:[β-ketol] under polarographic conditions is kinetically controlled, while during controlled potential electrolysis it is thermodynamically controlled. The diketo form of compound I is present in about 60% in the monohydrated form. Acid catalyzed dehydration results in an increase in wave i′1 in acidic media, base catalyzed in an increase in wave i3 at pH > 9. The overall pKa-value of the diprotonated form is pKH12 = −1.0, that of the geminal diol dissociation is pKOH = 10.35 and that corresponding to carbanion-enolate formation is pKB = 11.6. Possibility of separation of individual pK-values is shown and assumptions on which such separation is based are stated.

Competitive reduction of the carbonyl group and of the C−S bond in 3-thio-2-oxoalkanoic acids and their derivatives at the dropping mercury and mercury pool electrodes

3-Thio-2-oxoalkanoic acids I, III and IV (AH3) undergo dissociation and form the carboxylate (AH2), the thiolate-carboxylate dianion (AH) and for I even a trianion (A), when AH acts as a C-acid. Formation of a carbanion-enolate was confirmed by following the dissociation of thioether II. Generally, the hydration decreases in the sequence AH3>AH2>AH. Similarly for the thioether II, the conjugate acid is more strongly hydrated than the carboxylate anion. In ester V the conjugate acid is also more strongly hydrated than the thiolate anion. Hydration of ester V is comparable to the hydration of the acid form H3A of I. Introduction of an isopropyl group in α-position in IV decreases the hydration similar to the replacement of the SH group by a SC2H5 grouping. In acidic solutions polarographic reduction of acid form AH3 occurs after pre-protonation of the carbonyl group, either by electron transfer to the carbonyl group followed by elimination of the sulfide anion or by hydrogenolytic cleavage of the C−S bond. At pH 3–6 the monoanion AH2 is furthermore protonated on the carboxyl group and then reduced as AH3. At pH 6–11 the monoanion AH2 is reduced, either at the CO group or with cleavage of the C−S bond. At pH 8–11 this monoanion is formed by a rapid protonation of the dianion AH. For the thioether II where such acid-base equilibrium involving the thiol form is impossible, the half-wave of wave i2A potential at pH>6 is pH-independent. In more alkaline media, base-catalyzed dehydration takes place, occurring via geminal diol anion. Changes in the rate of dehydration resulting from structural changes considerably affect the pH-dependences of polarographic waves, particularly of wave i1 at pH<6. Differences between the behavior of 3-thio-2-oxobutanoic acid (III) (see Fig. 6) and 3-thio-4-methyl-2-oxopentanoic acid (IV) (see Fig. 7) similarly as for those between 3-thio-2-oxopropanoic acid (I) (see Fig. 1) and its 3-ethylthioether (II) (see Fig. 3) are most strking. Whereas at the DME the competition between C=O reduction and C−S cleavage is governed by differences in the rates of establishment of the acid-base equilibria, yields of preparative reductions obtained with a stirred mercury pool electrode are given by the position of these equilibria in the bulk of the solution. At the pool electrode AH3 is predominantly reduced on the CO group, monoanion AH2 shows comparable reduction of both the CO group and cleavage of the C−S bond, whereas the dianion AH is predominantly reduced at the C−S bond.

Polarographic reduction of rubeanic acid and the catalytic waves observed in the presence of ruthenium trichloride

Abstract An automatic oscillometric apparatus for the control of the separation of components in laboratory processes and for industrial control is described. It is based on the principle which regulates the fall in anodic current in the TG-TP type of oscillometer 1,2 . Novel application of the apparatus to extraction and distillation processes 3,4 demonstrates the versatility and advantages of this original equipment.

Development and publication of new polarographic and related methods of analysis

A review is made of the factors that should be investigated in the development of new polarographic methods of analysis. Recommendations are made concerning the preparation for publication of the results of such investigations.

Polarographic reduction of aldehydes and ketones: XX. Electroreduction of benzocyclenones

Summary Reduction of indanone (I), tetralone (II) and benzosuberone (III) was studied by d. c. and pulse polarography, cyclic voltammetry and controlled potential electrolysis with a dropping mercury electrode in aqueous solutions containing 2% ethanol at various pH values. In acidic media the protonated form, at higher pH values the free carbonyl form is reduced. The reduction of the unprotonated form is accompanied by protonation of the radical anion formed. The behavior of all three ketones I–III in this pH region is similar and differs only in the half-wave potential values and the rates of protonation of the radical anion. The reduction of the protonated form is affected both by differences in potentials of the ketones I–III and by differences in the rates of protonation, but most significantly by the half-wave potential of the radical formed in the first electron uptake. For indanone (I) this reduction is so positive that only one two-electron step is observed. For benzosuberone (III) the half-wave potential of this reduction is sufficiently more negative than the first electron uptake and two one-electron separated waves result. For tetralone (II) the reduction of the radical occurs at so negative a potential that it is overlapped by the hydrogen evolution current. Only one one-electron wave is thus observed. The effect of ring size on potentials and protonation reactions is discussed from the point of view of ring strain, adsorbability and radical stability.

Polarographic reduction of benzylidenemalononitrile

The mechanism of reduction of benzylidenemalononitrile (1), C6H5CH=C(CN)2, at the dropping mercury electrode in aqueous solutions containing 50% methanol changes with pH. In acidic solutions the reduction proceeds by a four-electron transfer to the monoprotonated species, yielding an aminomethyl derivative. In alkaline solutions two one-electron steps involving the vinyl double bond reduction are observed. Hydrolysis of dinitrile 1 to form benzaldehyde complicates the study at high pH values. The system is unique in that at pH 5–6 the unprotonated form is reduced at more positive potentials than the protonated form, reflecting a change in the mechanism of the reduction process.