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Hydroxamic acids as weak base indicators: protonation in strong acid media
B García1, S Ibeas, F J Hoyuelos
1Dipartimento di Chimica e Chimica Industriale dell'Università di Pisa, Via Risorgimento 35, 56126 Pisa, Italy. begar@ubu.es
The Journal of Organic Chemistry
|November 28, 2001
Summary
This study investigated the protonation of four hydroxamic acids in strong acid media. Results indicate similar solvation needs for amides and hydroxamic acids, with mineral acid type influencing protonation equilibria.
Area of Science:
- Physical Organic Chemistry
- Acid-Base Equilibria
- Spectroscopy
Background:
- Hydroxamic acids are important ligands and intermediates.
- Understanding their behavior in strong acid media is crucial for various chemical applications.
- Protonation equilibria dictate reactivity and stability.
Purpose of the Study:
- To investigate the protonation equilibria of four substituted hydroxamic acids.
- To analyze the influence of concentrated mineral acids (sulfuric, hydrochloric, perchloric) on these equilibria.
- To compare the solvation requirements of hydroxamic acids with those of amides.
Main Methods:
- UV-Vis spectral measurements at 25°C.
- Application of Hammett equation, Bunnett-Olsen, and excess acidity methods.
- Correction for medium effects using Cox-Yates and vector analysis.
Main Results:
- The H(A) acidity function based on benzamides yielded optimal results.
- Hydroxamic acids exhibit similar solvation characteristics (m coefficient) to amides.
- Protonation variations were attributed to solvent-separated ion pairs and solvation of the protonated species (BH+).
Conclusions:
- Hydroxamic acids and amides share comparable solvation requirements in strong acid media.
- The catalytic efficiency sequence of mineral acids (HCl > H(2)SO(4) > HClO(4)) correlates with observed protonation changes.
- This study provides insights into the acid-base behavior of hydroxamic acids in complex chemical environments.