Geminal Diol Anion Research Articles

Importance of Electrode Surface and Addition of OH− Ions in Oxidation of Orthophthalaldehyde and Some Benzaldehydes in Alkaline Solutions

Formation of a pre‐monolayer of a slightly soluble metal hydroxo complexes on electrode surface facilitates the electrooxidation. This factor, together with establishment of equilibria between the aldehyde and a geminal diol anion (which is the electroactive form) play important roles in electrooxidation of aromatic aldehydes. Better understanding of chemical and electrochemical processes involved, enables application of electroanalytical methods for investigation of reactions of orthophthalaldehyde—an important analytical reagent—in alkaline solution.

Comparison of Solution Chemistries of Orthophthalaldehyde and 2,3-Naphthalenedicarboxaldehyde

Polarography was used to obtain the concentrations of the dialdehydic (10%), monohydrated acyclic (5%), and cyclic hemiacetal form (85%) of orthophthalaldehyde (OPA). For 2,3-naphthalenedicarboxaldehyde (NDA) these values were estimated to be 15, 7, and 78%. Addition of water in unbuffered solutions followed first-order kinetics with rate constants 0.0018 s-1 for OPA and 0.0012 s-1 for NDA. Dehydration to form both the dialdehyde and the monohydrate is both acid- and base-catalyzed. Both dialdehydes yield on reaction with OH- ions geminal diol anion, which is electro-oxidized to a carboxylic acid. In the most frequently used pH range for the determination of amino acids, NDA can undergo reaction with OH- ions, but OPA does not. In aqueous solutions, NDA is less strongly hydrated than OPA.

Polarographic reduction of aldehydes and ketones: Part XXV. Electroreduction of pyridinecarboxaldehydes in aqueous buffered solutions

The mechanisms of electroreduction of 2-, 3-, and 4-pyridinecarboxaldehydes have been studied in aqueous buffered solutions of pH between 2 and 12 by means of dc and differential pulse polarography, linear-sweep voltammetry and controlled-potential electrolysis. Spectroscopic measurements confirmed that these compounds in aqueous solutions exist in equilibria involving both the hydrated and unhydrated forms. The degree of hydration depends strongly on protonation both of the pyridine ring and of the side-chain. Dehydration reactions producing the aldehyde species occur prior to the electron (and possibly the proton) transfer. The loss of water can take place from a protonated geminal diol form or from a neutral diol, depending on pH. Alternatively, the geminal diol anion can eliminate hydroxide ion. A diprotonated pyridinecarboxaldehyde is the electroactive species at pH < 7, the monoprotonated and unprotonated forms are reduced at higher pH-values. For the 2- and 4-aldehyde base-catalyzed dehydration occurs at pH > 8 manifested by an increase of the total limiting current. In this reaction the rate-determining step is the dissociation of a proton from the geminal diol forming a geminal diol anion, which is followed by a rapid elimination of a hydroxide ion to yield the reducible aldehyde. Reaction schemes involving sequences of hydration-dehydration and acid-base reactions as well as electron transfers at mercury electrodes have been proposed for the three isomers. The predominating processes in individual pH-ranges have been pointed out. Electroreduction of the 3-aldehyde occurs up to pH 10 in a single two-electron step. The 2- and 4-aldehydes are reduced between pH 2 and 12 in two, one-electron steps. Electrodimerization occurs at the potential of the limiting current of the first one-electron wave for the 2- and 4-isomer, similarly as for the 3-isomer at pH > 10, where its reduction occurs in a single one-electron wave. Surface dimerization or reaction of the electrolysis products with the parent aldehyde was considered in interpretation of the reduction of the 4-pyridinecarboxaldehyde at pH > 7. Electropolymerization accompanies the reduction of 2-pyridinecarboxaldehyde in alkaline media.

Polarographic reduction of aldehydes and ketones: Part XXVI. Electroreduction and electrooxidation of pyridinecarboxaldehydes in alkaline solutions

The electroreductions and electrooxidations of 2-, 3-, and 4-pyridinecarboxaldehyde have been investigated in solutions of sodium hydroxide of concentration up to 10 M ( J = 16.7) using dc and differential pulse polarography, linear sweep voltammetry, and controlled potential electrolysis. Both reductions and oxidations of these compounds were influenced by the formation of geminal diol anions (PyCH(OH)O −) in the bulk of the solution and in the vicinity of the electrode. The electroreductions of the 3- and 4-aldehydes proceed by a one-electron uptake, followed by a radical dimerization yielding pinacols. The two-electron reduction of the 2-aldehyde is complicated by competing dimerization and polymerization. In strongly alkaline solutions, the Cannizzaro reaction completes with bulk electrolysis. To undergo a two-electron oxidation in alkaline media to pyridinecarboxylic acids, pyridinecarboxaldehydes must be converted into PyCH(OH)O −. At J − < 14 this anion is present in equilibrium with both the free aldehydes (PyCHO) and their geminal diol forms (PyCH(OH) 2). To be electrooxidized the anion Py(OH)O − is either transported to the electrode from the bulk of the solution or formed in the vicinity of the electrode either by addition of OH − ions to PyCHO or by dissociation of PyCH(OH) 2. Measurements of kinetic currents of oxidation of 3-pyridinecarboxaldehyde at pH 10–12 made it possible to estimate rate constants for the formation of PyCH(OH)O −. At J −> 14 the geminal diol anion reacts with another hydroxide ion. This reaction results in a shift of half-wave potentials of the anodic wave to more negative values with incrasing a OH−.

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.

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 (AH 3) undergo dissociation and form the carboxylate (AH 2), 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 AH 3>AH 2>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 H 3A 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 SC 2H 5 grouping. In acidic solutions polarographic reduction of acid form AH 3 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 AH 2 is furthermore protonated on the carboxyl group and then reduced as AH 3. At pH 6–11 the monoanion AH 2 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 i 2A 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 i 1 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 AH 3 is predominantly reduced on the CO group, monoanion AH 2 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.

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

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 19 . Hydration-Dehydration Equilibria and their Effect on Reduction of α,α,α-Trifluoroacetophenone

In acidic media, at pH <5, the carbonyl group of α,α,α-trifluoroacetophenone (I) is reduced in the protonated form to give alcohol and pinacol, whereas at pH > 7 the first reduction step involves hydrogenolysis of the trifluoromethyl group of the unprotonated form of I to yield acetophenone, which can undergo further reduction at more negative potentials. Reduction of both the carbonyl group at lower pH and of the C—F bonds in the unprotonated form are accompanied by hydration-dehydration equilibria. At pH > 7 these reactions involve addition of water and of hydroxide ions. The increase of current at pH > 8 is due to a shift of the equilibria in favor of the geminal diol anion of I; the decrease at pH > 10 is due to an increase in the rate of the addition of hydroxide ions with increasing pH. Spectrophotometric measurements at μ = 0.1 gave the following values for the acid dissociation constant of the geminal diol of I: K 2 = 6.4 × 10 -11 in 2% ethanol and 5 × 10 -12 in 40% ethanol. Formation of the hemiketal results in an increase of the absorbance of the free carbonyl form with increasing ethanol concentration. Formation of the hydrate and hemiketal of I in dimethylformamide is compared.

Oxydation électrochimique des aldéhydes en solution aqueuse: I. Oxydation de solutions aqueuses basiques d'aldéhydes à des électrodes d'or et de platine

Voltemmetric experiments have been performed upon 9 aldehydes in aqueous basic solutions from pH 9 to pH 14, at rotating gold and platinum electrodes. The potential of the first oxidation process, which is either a wave or a peak, depends linearly on pH. It is identical for all the studied aldehydes but differs at both electrodes. From the relation between the maximal intensity and pH, it can be concluded that the electroactive species is in all cases the geminal diol anion RCHOHO −. In order to explain all the experimental results we propose the following assumption: aldehyde oxidation proceeds from a chemical reaction between adsorbed monoionized aldehyde hydrate and superficial metal oxides.

Oxydation électrochimique des aldéhydes en solution aqueuse: II. Oxydation de solutions aqueuses basiques d'aldéhydes R−CO−CHO a l'électrode de mercure

The oxidation of hydrated aldehydes R−CO−CH(OH) 2 at the mercuric electrode in basic aqueous solution has been studied. The electrochemical process corresponds to the oxidation of the geminal diol anion to carboxylate ion. In order to explain our experimental results, we propose the following assumption: oxidation of aldehyde implies a chemical reaction between monoionized hydrate of aldehyde and mercury species rather than a direct electron transfer at the mercury anode.

Effect of hydration on polarographic reduction of some carbonyl compounds

Hydration and acid-base equilibria of the hydrated form govern the reduction and oxidation of numerous carbonyl compounds. Aliphatic and aromatic aldehydes are reduced in the free carbonyl form and oxidized as geminal diol anions. In haloacetaldehydes and haloacetones the C−X bond can be reductively cleaved both in the free carbonyl and in the hydrated form, oxidized form is again the geminal diol anion. Schemes (6), (7), (10), (11), (12) and (13) summarize the mechanism of the hydration and its relation to electrooxidation and reduction. Differential pulse of polarographic waves enabled measurement of heights of waves close to the initial or final rise of current and following the variations of their height with changes in activity of the solution.

Effect of hydration on polarographic reduction of some carbonyl compounds

Hydration and acid-base equilibria of the hydrated form govern the reduction and oxidation of numerous carbonyl compounds. Aliphatic and aromatic aldehydes are reduced in the free carbonyl form and oxidized as geminal diol anions. In haloacetaldehydes and haloacetones the C−X bond can be reductively cleaved both in the free carbonyl and in the hydrated form, oxidized form is again the geminal diol anion. Schemes (6), (7), (10), (11), (12) and (13) summarize the mechanism of the hydration and its relation to electrooxidation and reduction. Differential pulse of polarographic waves enabled measurement of heights of waves close to the initial or final rise of current and following the variations of their height with changes in activity of the solution.

ChemInform Abstract: POLAROGRAPHIC REDUCTION OF ALDEHYDES AND KETONES PART 15, HYDRATION AND ACID‐BASE EQUILIBRIA ACCOMPANYING REDUCTION OF ALIPHATIC ALDEHYDES

Summary Aldehydes R−CHO give, at 20°C or 25°C, polarographic waves the height of which reaches a limiting value with increasing pH, but at 2°C show a decrease at pH>12. This behaviour was explained by the scheme (2)–(4) which involves formation of the hydrate monoanion, R−CH(OH)O − , as an intermediate. The same scheme can be used for re-interpretation of formaldehyde reduction. Formation of CH 2 (OH)O − has been proved spectroscopically. The dehydration reaction cannot be interpreted by a single rate-determining pH dependent step operating over the entire pH range. The electroactive free carbonyl form is generated by a system of competitive reactions. The rate of formation of RCHO from the geminal diol follows first order kinetics with overall rate constant k overall =( k H 2 O − + k H 2 O + )+ k 3 /(1+[H + ]/ K 2 ) − k −3 [OH − ] where k 3 is the rate constant of cleavage of the geminal diol anion, k −3 the rate constant of the nucleophilic attack of OH − ion on the carbonyl. Shifts of half-wave potentials of aldehydes R−CHO with the nature of R are expressed by a modified Taft equation. Catalytic and anodic waves of aldehydes were observed.

Polarographic reduction of aldehydes and ketones: XV. Hydration and acid-base equilibria accompanying reduction of aliphatic aldehydes

Aldehydes R−CHO give, at 20°C or 25°C, polarographic waves the height of which reaches a limiting value with increasing pH, but at 2°C show a decrease at pH>12. This behaviour was explained by the scheme (2)–(4) which involves formation of the hydrate monoanion, R−CH(OH)O−, as an intermediate. The same scheme can be used for re-interpretation of formaldehyde reduction. Formation of CH2(OH)O− has been proved spectroscopically. The dehydration reaction cannot be interpreted by a single rate-determining pH dependent step operating over the entire pH range. The electroactive free carbonyl form is generated by a system of competitive reactions. The rate of formation of RCHO from the geminal diol follows first order kinetics with overall rate constant koverall=(kH2O−+kH2O+)+k3/(1+[H+]/K2) −k−3 [OH−] where k3 is the rate constant of cleavage of the geminal diol anion, k−3 the rate constant of the nucleophilic attack of OH− ion on the carbonyl. Shifts of half-wave potentials of aldehydes R−CHO with the nature of R are expressed by a modified Taft equation. Catalytic and anodic waves of aldehydes were observed.