Jump to main content
Jump to site search

Issue 15, 2004

Photochemistry of the three carboxypyridines in water: a pH dependent reaction

Author affiliations

Abstract

The photochemistry of ortho, meta and para-carboxypyridines (pKa1 = 1.0–2.1 and pKa2 = 4.7–5.3) in aqueous medium was studied by laser-flash photolysis and product studies. At pH < pKa1, hydroxylated compounds are produced with low quantum yields. Within the pH range 4–7, ortho and meta isomers undergo dimerization together with decarboxylation with a quantum yield showing a very sharp maximum around pKa2 (ϕmax = 0.09 and 0.01, respectively) while the para isomer is photostable. End-of-pulse transients assigned to triplet states were detected by laser-flash photolysis at pH < pKa1 and pH > 4. Additionally, the carboxypyridinyl radicals were detected as secondary intermediates at pH < pKa1 and 4 < pH < 7 and the OH-adduct radicals at pH < pKa1. This is in favour of an electron transfer reaction between triplet and starting compound producing a charge transfer species. The radical anion would escape as carboxypyridinyl radical while the radical cation may add water at pH < pKa1 yielding the OH-adduct radical or may undergo decarboxylation at pH > 4. The high quantum yield of phototransformation of the ortho isomer at pH > 4 is due to an easy decarboxylation process. A reaction scheme is proposed accounting for the dependences of ϕ on both the pH and the carboxypyridines concentration. This study points out the distinct pattern of reactivity of carboxypyridines depending on the ionisation state of starting compounds and isomeric substitution.

Graphical abstract: Photochemistry of the three carboxypyridines in water: a pH dependent reaction

Article information


Submitted
02 Feb 2004
Accepted
26 May 2004
First published
23 Jul 2004

Org. Biomol. Chem., 2004,2, 2253-2261
Article type
Paper

Photochemistry of the three carboxypyridines in water: a pH dependent reaction

F. Rollet, C. Richard, J. Pilichowski and B. Aboab, Org. Biomol. Chem., 2004, 2, 2253 DOI: 10.1039/B401551G

To request permission to reproduce material from this article, please go to the Copyright Clearance Center request page.

If you are an author contributing to an RSC publication, you do not need to request permission provided correct acknowledgement is given.

If you are the author of this article, you do not need to request permission to reproduce figures and diagrams provided correct acknowledgement is given. If you want to reproduce the whole article in a third-party publication (excluding your thesis/dissertation for which permission is not required) please go to the Copyright Clearance Center request page.

Read more about how to correctly acknowledge RSC content.


Social activity

Search articles by author

Spotlight

Advertisements