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

Abstract

Listen

Translate article

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.