Bimolecular Hydrogen Abstraction from Phenols by Aromatic Ketone Triplets†

Photochemistry and PhotobiologyVolume 82, Issue 1 p. 291-300 Bimolecular Hydrogen Abstraction from Phenols by Aromatic Ketone Triplets† Edward C. Lathioor, Edward C. Lathioor Department of Chemistry, McMaster University, 1280 Main Street West, Hamilton, ON, CanadaSearch for more papers by this authorWilliam J. Leigh, Corresponding Author William J. Leigh Department of Chemistry, McMaster University, 1280 Main Street West, Hamilton, ON, Canada To whom correspondence should be addressed: Department of Chemistry, McMaster University, 1280 Main Street West, Hamilton, ON, Canada L8S 4M1, Fax: 905-522-2509; e-mail: leigh@mcmaster.caSearch for more papers by this author Edward C. Lathioor, Edward C. Lathioor Department of Chemistry, McMaster University, 1280 Main Street West, Hamilton, ON, CanadaSearch for more papers by this authorWilliam J. Leigh, Corresponding Author William J. Leigh Department of Chemistry, McMaster University, 1280 Main Street West, Hamilton, ON, Canada To whom correspondence should be addressed: Department of Chemistry, McMaster University, 1280 Main Street West, Hamilton, ON, Canada L8S 4M1, Fax: 905-522-2509; e-mail: leigh@mcmaster.caSearch for more papers by this author First published: 30 April 2007 https://doi.org/10.1562/2005-06-20-RA-581Citations: 58 † This paper is part of a special issue dedicated to Professor J. C. (Tito) Scaiano on the occasion of his 60th birthday. Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat ABSTRACT Absolute rate constants for hydrogen abstraction from 4-methylphenol (para-cresol) by the lowest triplet states of 24 aromatic ketones have been determined in acetonitrile solution at 23°c, and the results combined with previously reported data for roughly a dozen other compounds under identical conditions. The ketones studied include various ring-substituted benzophenones and acetophenones, α,α,α-trifluoroacetophenone and its 4-methoxy analog, 2-benzoylthiophene, 2-acetonaphthone, and various other polycyclic aromatic ketones such as fluorenone, xanthone and thioxanthone, and encompass n,π*, π,π*(CT) and arenoid π,π* lowest triplets with (triplet) reduction potentials (Ered*) varying from about –10 to –38 kcal mol-1. The 4-methylphenoxyl radical is observed as the product of triplet quenching in almost every case, along with the corresponding hemipinacol radical in most instances. Hammett plots for the acetophenones and benzophenones are quite different, but plots of log kQvs Ered* reveal a common behavior for most of the compounds studied. The results are consistent with reaction via two mechanisms: a simple electron-transfer mechanism, which applies to the n,π* triplet ketones and those π,π* triplets that possess particularly low reduction potentials, and a coupled electron-/proton-transfer mechanism involving the intermediacy of a hydrogen-bonded exciplex, which applies to the π,π* ketone triplets. Ketones with lowest charge-transfer π,π* states exhibit rate constants that vary only slightly with triplet reduction potential over the full range investigated; this is due to the compensating effect of substituents on triplet state basicity and reduction potential, which both play a role in quenching by the hydrogen-bonded exciplex mechanism. Ketones with arenoid π,π* states exhibit the fall-off in rate constant that is typical of photoinduced electron transfer reactions, but it occurs at a much higher potential than would be normally expected due to the effects of hydrogen-bonding on the rate of electron-transfer within the exciplex. Citing Literature Volume82, Issue1January 2006Pages 291-300 RelatedInformation