The influence of cathode material on the reduction of aryl carbonyl compounds - formation of redicals
The influence of cathode material on the reduction of aryl carbonyl compounds - formation of redicals
The cathodic reduction of seven aromatic carbonyl compounds including aldehydes and ketones with unsaturated side chains has been studied in acidic water/methanol mixtures. The voltammetry depends strongly on cathode material. The influence of various parameters such as mass transport, pH, concentration has been defined at a lead cathode. The influence of these parameters on the reduction was studied at a vitreous carbon. At pH ~ 1, benzaldehyde always reduces in two 1e- steps and the first step, corresponding to the formation of the radical intermediate, is always reversible. The first reduction step is uninfluenced by the cathode material. On the other hand, the potential for the second reduction wave, corresponding to the formation of alcohol, varies strongly with the cathode material. It was shown that at a vitreous carbon cathode, the second step occurs at much more negative potential than at a lead or a mercury electrode, giving much better resolution of the two reduction waves. In citrate buffer, pH ~ 4, the first reduction wave is shifted negative. As a result, at a lead electrode, a single 2e- wave is seen whereas at a vitreous carbon, two 1e- steps are still observed. This difference in the potential for the second electron transfer is most probably explained in terms of catalysis of the second electron transfer by the lead surface. Experience also showed that the in situ lead plated electrodes led to greatly enhanced reproducibility for voltammograms, over bulk lead electrodes.
This better wave resolution at a vitreous carbon voltammetry would be a substantial advantage for synthetic reactions, which require the 1e- reduction to a radical intermediate. Electrolyses were therefore carried out for various aldehydes and ketones at a vitreous carbon cathode and at a carbon felt cathode and in various cells. In the case of simple aldehydes (benzaldehyde and 2-methoxybenzaldehyde), the corresponding pinacols were obtained in good yields, confirming the presence of the radical intermediate. However, traces of the corresponding alcohols were always found. This is interpreted in terms of hydrogen abstraction by the radical. The generally accepted mechanism states that the alcohol is only formed by a 2e- reduction and this is incorrect. In the case of aldehydes and ketones containing a substituent with an unsaturated side chain, the cyclised compounds were not isolated. Indeed, no products from the cathode reaction were identified and further work is necessary to understand these electrolyses.
University of Southampton
1999
Libot, Cecile
(1999)
The influence of cathode material on the reduction of aryl carbonyl compounds - formation of redicals.
University of Southampton, Doctoral Thesis.
Record type:
Thesis
(Doctoral)
Abstract
The cathodic reduction of seven aromatic carbonyl compounds including aldehydes and ketones with unsaturated side chains has been studied in acidic water/methanol mixtures. The voltammetry depends strongly on cathode material. The influence of various parameters such as mass transport, pH, concentration has been defined at a lead cathode. The influence of these parameters on the reduction was studied at a vitreous carbon. At pH ~ 1, benzaldehyde always reduces in two 1e- steps and the first step, corresponding to the formation of the radical intermediate, is always reversible. The first reduction step is uninfluenced by the cathode material. On the other hand, the potential for the second reduction wave, corresponding to the formation of alcohol, varies strongly with the cathode material. It was shown that at a vitreous carbon cathode, the second step occurs at much more negative potential than at a lead or a mercury electrode, giving much better resolution of the two reduction waves. In citrate buffer, pH ~ 4, the first reduction wave is shifted negative. As a result, at a lead electrode, a single 2e- wave is seen whereas at a vitreous carbon, two 1e- steps are still observed. This difference in the potential for the second electron transfer is most probably explained in terms of catalysis of the second electron transfer by the lead surface. Experience also showed that the in situ lead plated electrodes led to greatly enhanced reproducibility for voltammograms, over bulk lead electrodes.
This better wave resolution at a vitreous carbon voltammetry would be a substantial advantage for synthetic reactions, which require the 1e- reduction to a radical intermediate. Electrolyses were therefore carried out for various aldehydes and ketones at a vitreous carbon cathode and at a carbon felt cathode and in various cells. In the case of simple aldehydes (benzaldehyde and 2-methoxybenzaldehyde), the corresponding pinacols were obtained in good yields, confirming the presence of the radical intermediate. However, traces of the corresponding alcohols were always found. This is interpreted in terms of hydrogen abstraction by the radical. The generally accepted mechanism states that the alcohol is only formed by a 2e- reduction and this is incorrect. In the case of aldehydes and ketones containing a substituent with an unsaturated side chain, the cyclised compounds were not isolated. Indeed, no products from the cathode reaction were identified and further work is necessary to understand these electrolyses.
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Published date: 1999
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Local EPrints ID: 464126
URI: http://eprints.soton.ac.uk/id/eprint/464126
PURE UUID: 20ea3a54-afbc-4f8e-aa9d-e1634720391e
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Date deposited: 04 Jul 2022 21:19
Last modified: 04 Jul 2022 21:19
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Author:
Cecile Libot
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