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<5 in two one-electron waves ( i 1, i 2) replaced at higher pH values by a single two-electron wave ( i 3) corresponding to the reduction of the unprotonated form. At pH>9 a more negative wave ( i 4) 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 i 2 or i 3 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.