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Issue 5, 2002

A single aqueous reference equilibrium constant for amide synthesis–hydrolysis

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Abstract

Experimentally measured equilibrium constants at a given pH in part reflect the contributions of ionisation of acidic and basic groups present. These contributions can be isolated from the equilibrium constant by expressing all reactant concentrations in terms of the uncharged forms only. This article presents methods to calculate uncharged reference equilibrium constants for amide synthesis/hydrolysis reactions. For zwitterions in particular these methods are not always straightforward. It is explained how (microscopic) pKa values can be estimated where experimental values are not available. A large number of equilibrium data are analysed for hydrolysis or synthesis of protected and unprotected di- and tri-peptides, beta-lactam antibiotics, and acyl acids and amides. This reveals just how similar the reference equilibrium constants are when ionisation is properly accounted for (K0ref = 103.6 M−1) regardless of the molecular form of the reactants involved.

Graphical abstract: A single aqueous reference equilibrium constant for amide synthesis–hydrolysis

Article information


Submitted
05 Sep 2001
Accepted
22 Feb 2002
First published
22 Mar 2002

J. Chem. Soc., Perkin Trans. 2, 2002, 1024-1028
Article type
Paper

A single aqueous reference equilibrium constant for amide synthesis–hydrolysis

R. V. Ulijn, B. D. Moore, A. E. M. Janssen and P. J. Halling, J. Chem. Soc., Perkin Trans. 2, 2002, 1024 DOI: 10.1039/B108041E

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