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A voltammetric study of the 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) mediated oxidation of benzyl alcohol in tert-butanol/water

A voltammetric study of the 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) mediated oxidation of benzyl alcohol in tert-butanol/water
A voltammetric study of the 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) mediated oxidation of benzyl alcohol in tert-butanol/water
The mechanism of the 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) mediated oxidation of benzyl alcohol in alkaline tert-butanol/water mixtures has been investigated in order to understand the preferred conditions for the mediated oxidation of alcohols in an electrochemical microflow reactor. It is shown that TEMPO and the corresponding hydroxylamine both undergo facile oxidation to the oxoammonium ion at the potential and pH used for the mediated oxidation although the hydroxylamine/TEMPO couple is electrochemically irreversible. The initial reaction between the oxoammonium ion and benzyl alcohol is a rapid reaction but the overall rate of the catalytic cycle, and hence the rate of conversion of alcohol to aldehyde, is determined by a deprotonation step of an intermediate formed in this reaction. The mediated oxidation is never mass transport controlled with respect to the alcohol but the rate of the reaction increases with pH. The oxoammonium ion also reacts with hydroxide and hence the selection of pH for electrosynthesis is a compromise between the increased rate of the catalytic cycle and the loss of mediator due to reaction of the oxoammonium ion with hydroxide as the pH is increased. At a pH of ?11.5, the rate of the conversion to benzaldehyde is high enough for the reaction to be synthetically useful and much of the TEMPO can be recovered after electrolysis
0013-4686
550-556
Green, Robert
b94d81d2-c00e-4fcb-bbcd-2998250be0e9
Hill-Cousins, Joseph
615ef358-b4f3-4736-b5cf-3d05355277f2
Brown, Richard
21ce697a-7c3a-480e-919f-429a3d8550f5
Pletcher, Derek
f22ebe69-b859-4a89-80b0-9e190e6f8f30
Leach, Stuart
80f0a3ee-1890-4f47-9b78-3f407ce2907c
Green, Robert
b94d81d2-c00e-4fcb-bbcd-2998250be0e9
Hill-Cousins, Joseph
615ef358-b4f3-4736-b5cf-3d05355277f2
Brown, Richard
21ce697a-7c3a-480e-919f-429a3d8550f5
Pletcher, Derek
f22ebe69-b859-4a89-80b0-9e190e6f8f30
Leach, Stuart
80f0a3ee-1890-4f47-9b78-3f407ce2907c

Green, Robert, Hill-Cousins, Joseph, Brown, Richard, Pletcher, Derek and Leach, Stuart (2013) A voltammetric study of the 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) mediated oxidation of benzyl alcohol in tert-butanol/water. Electrochimica Acta, 113, 550-556. (doi:10.1016/j.electacta.2013.09.070).

Record type: Article

Abstract

The mechanism of the 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) mediated oxidation of benzyl alcohol in alkaline tert-butanol/water mixtures has been investigated in order to understand the preferred conditions for the mediated oxidation of alcohols in an electrochemical microflow reactor. It is shown that TEMPO and the corresponding hydroxylamine both undergo facile oxidation to the oxoammonium ion at the potential and pH used for the mediated oxidation although the hydroxylamine/TEMPO couple is electrochemically irreversible. The initial reaction between the oxoammonium ion and benzyl alcohol is a rapid reaction but the overall rate of the catalytic cycle, and hence the rate of conversion of alcohol to aldehyde, is determined by a deprotonation step of an intermediate formed in this reaction. The mediated oxidation is never mass transport controlled with respect to the alcohol but the rate of the reaction increases with pH. The oxoammonium ion also reacts with hydroxide and hence the selection of pH for electrosynthesis is a compromise between the increased rate of the catalytic cycle and the loss of mediator due to reaction of the oxoammonium ion with hydroxide as the pH is increased. At a pH of ?11.5, the rate of the conversion to benzaldehyde is high enough for the reaction to be synthetically useful and much of the TEMPO can be recovered after electrolysis

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e-pub ahead of print date: 8 October 2013
Published date: 15 December 2013
Organisations: Chemistry

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Local EPrints ID: 364118
URI: http://eprints.soton.ac.uk/id/eprint/364118
ISSN: 0013-4686
PURE UUID: 326216e3-f472-4b90-9ce3-32d0e804b69a
ORCID for Richard Brown: ORCID iD orcid.org/0000-0003-0156-7087

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Date deposited: 04 Apr 2014 15:32
Last modified: 10 Dec 2019 01:53

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Contributors

Author: Robert Green
Author: Joseph Hill-Cousins
Author: Richard Brown ORCID iD
Author: Derek Pletcher
Author: Stuart Leach

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