7. A polarographic study of the electroreduction of acridine

Abstract

The reduction of acridine at the dropping-mercury electrode has been studied over the pH range 0-14.I n aqueous solution, complex anomalous polarograms are obtained. In solutions containing upwards of 60% alcohol, the anomalies disappear, leaving two one-electron reduction steps which are thermodynamically irreversible, and for which diffusion currents are proportional to acridine concentrations in the range 10-9-1W~. The anomalies in aqueous solutions may be satisfactorily explained on BrdiEka's theory as due to adsorption of acridine, mainly in the semiquinone reduced state, on the mercury electrode. Supporting evidence for adsorption has been obtained from a cathode-ray oscillographic study of the current-time relation during the formation of a mercury drop at the electrode. Acridine is found to undergo a two-step electroreduction, both in aqueous and alcoholic solutions, with the formation of an intermediate semiquinone radical, the apparently great stability of which is attributed to resonance.

Reduction of CO2 to Methanol Catalyzed by a Biomimetic Organo-Hydride Produced from Pyridine

“…PyH 0 can also be produced electrochemically at inert electrodes. For instance, a glassy carbon electrode − has been used to electrochemically produce similar neutral radicals from the Py-related species nicotinamide and acridines. − In another case, photochemical production of PyH 0 driven by visible light was recently demonstrated by MacDonnell and co-workers using a surface-free photochemical process in which Ru­(II) trisphenanthroline (chromophore) and ascorbate (reductant) act in concert to reduce PyH + to PyH 0 via 1e – transfer. − The produced PyH 0 radical is actively involved in the observed homogeneous reduction of CO 2 to CH 3 OH (albeit at low yield), − an observation in contrast with recent studies focused on the specific case using a Pt cathode ,,− that rule out participation of homogeneous PyH 0 in Py-catalyzed CO 2 reduction. We stress that we consider a Pt electrode to be a special case.…”

“…Using quantum chemical calculations whose methodology is outlined in Section , we will: (1) demonstrate how Py is transformed into the recyclable organo-hydride PyH 2 , via a sequential PT–ET–PT–ET process (Sections and ). PyH 2 is a 2H + /2e – transfer product of pyridine (Py). − We note that the formation of related dihydropyridines proceeds via sequential PT and ET steps; − (2) establish the hydride nucleophilicity of PyH 2 and related dihydropyridines (Section ); (3) calculate key transition states and reaction free energies to demonstrate that PyH 2 is both kinetically and thermodynamically proficient in reducing CO 2 to CH 3 OH through three successive homogeneous HTPT steps (Sections –); and (4) show that the catalytic hydride transfer reaction by the PyH 2 /Py redox couple is driven by a dearomatization–aromatization process (Section ) . Concluding remarks are given in Section .…”

“…92 We have already pointed out in Section 3.1 that radical self-quenching of PyH 0 can actually yield the productive PyH 2 via disproportionation. 105 In addition, it is relevant to note that Py-related neutral radicals of nicotinamide, 43 acridine, 44 and 3,6-diaminoacridine 45 have been experimentally observed and are key intermediate species en route to forming the related dihydropyridine species.…”

Catalytic Reduction of CO2 by Renewable Organohydrides

“…Electrochemical Pathways - Sequential PT and ET Steps to Pyridines to Form DHPs: The electrochemical reduction of Py’s to DHPs has been reported for various cases on carbon and mercury electrodes. − In one example (eq ), nicotinamide undergoes 2H + /2e – reduction to form the DHP species 1,6-dihydronicotinamide on a Hg electrode . The first PT and ET to nicotinamide were proposed to be simultaneous under the employed experimental conditions, forming the nicotinamide neutral radical; this radical was proposed to then undergo another simultaneous PT and ET to produce 1,6-dihydronicotinamide.…”

Organic Polarography

“…A very careful polarographic study of acridine indicates that the complex behavior is due to adsorption of the acridinium ion, because the anomalies are worse in acid solution, The complex behavior disappears in solutions containing 50% or more of alcohol and is replaced by two one-electron reduction steps. Methylacridinium salts give similar adsorption phenomena in both acidic and basic medium which are not eliminated by alcohol (104). Mercaptobenzothiazole (XXVIII) gives a cathodic wave only in the pH range 2.5 to 7 and an anodic wave in the range 1 to 13. Large scale electrolysis at +0.1 volt gave a white precipitate and very little mercuric ion in solution and suggested the following reaction for the anodic wave (166).…”