Electrochemistry of aryl carbonyl compounds in flow cells
Electrochemistry of aryl carbonyl compounds in flow cells
The reduction of benzaldehydes in acidic water/methanol solution has been investigated in a membrane flow cell (the FM01-LC electrolyser) with a lead cathode. Two products, the corresponding hydrobenzoin and benzyl alcohol could be produced selectively by controlling the potential of electrolysis. Parameters such as mass transport, temperature, pH and concentration had almost no effect on the selectivity of the reactions when the electrolyses are carried out at controlled potential. Voltammetry was carried out at a Pb rotating disc as well as in the FM01-LC flow cell. Voltammograms in the two cells are essentially identical and constitute a good basis to predict the selectivity of the reduction. They showed that benzaldehydes reduce in two, mass transport controlled, one electron waves. The first wave was pH dependent and corresponds to the reduction of the protonated aldehyde of the diol. The second wave, at more negative potential, corresponds to the formation of alcohols. These data confirm earlier conclusions that radical intermediates are formed in the first 1e- step.
4-Substituted benzaldehydes have been investigated in similar conditions. The nature of the substituent (e.g.: -F, -OCH3, -CH3, -C(CH3)3) has little effect on the shape of the voltammograms and only small shifts in the half wave potentials for the two reduction waves were observed. The yield and current efficiency for reduction to the corresponding diol and alcohol (depending on the potential of electrolysis) are again excellent.
Unsymmetrical diols (e.g.:4-F-C6H5-CH(OH)-CH(OH)-C6H6) could be produced by the reduction of a mixture of two benzaldehydes provided the electrolysis were carried out at a potential where both aldehydes are reduced to radical intermediates (determined from the voltammetry of the separate reactants). Of course, the symmetrical diols were always formed in parallel to the unsymmetrical diols. In the condition employed, the ratio of the three diols followed the statistical distribution expected from the coupling of the two different radical intermediates produced in the same ratio as their precursors. The yields based on a particular aldehyde could be optimised with some success by using an excess of the second aldehyde.
University of Southampton
1996
Guena, Thierry
(1996)
Electrochemistry of aryl carbonyl compounds in flow cells.
University of Southampton, Doctoral Thesis.
Record type:
Thesis
(Doctoral)
Abstract
The reduction of benzaldehydes in acidic water/methanol solution has been investigated in a membrane flow cell (the FM01-LC electrolyser) with a lead cathode. Two products, the corresponding hydrobenzoin and benzyl alcohol could be produced selectively by controlling the potential of electrolysis. Parameters such as mass transport, temperature, pH and concentration had almost no effect on the selectivity of the reactions when the electrolyses are carried out at controlled potential. Voltammetry was carried out at a Pb rotating disc as well as in the FM01-LC flow cell. Voltammograms in the two cells are essentially identical and constitute a good basis to predict the selectivity of the reduction. They showed that benzaldehydes reduce in two, mass transport controlled, one electron waves. The first wave was pH dependent and corresponds to the reduction of the protonated aldehyde of the diol. The second wave, at more negative potential, corresponds to the formation of alcohols. These data confirm earlier conclusions that radical intermediates are formed in the first 1e- step.
4-Substituted benzaldehydes have been investigated in similar conditions. The nature of the substituent (e.g.: -F, -OCH3, -CH3, -C(CH3)3) has little effect on the shape of the voltammograms and only small shifts in the half wave potentials for the two reduction waves were observed. The yield and current efficiency for reduction to the corresponding diol and alcohol (depending on the potential of electrolysis) are again excellent.
Unsymmetrical diols (e.g.:4-F-C6H5-CH(OH)-CH(OH)-C6H6) could be produced by the reduction of a mixture of two benzaldehydes provided the electrolysis were carried out at a potential where both aldehydes are reduced to radical intermediates (determined from the voltammetry of the separate reactants). Of course, the symmetrical diols were always formed in parallel to the unsymmetrical diols. In the condition employed, the ratio of the three diols followed the statistical distribution expected from the coupling of the two different radical intermediates produced in the same ratio as their precursors. The yields based on a particular aldehyde could be optimised with some success by using an excess of the second aldehyde.
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Published date: 1996
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Local EPrints ID: 460153
URI: http://eprints.soton.ac.uk/id/eprint/460153
PURE UUID: d15e2338-a100-4ff3-ab9b-142b9b27a31f
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Date deposited: 04 Jul 2022 18:02
Last modified: 04 Jul 2022 18:02
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Author:
Thierry Guena
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