Evaluation of acidity of strong acid catalysts I. Derivation of an acidity function from carbon-13 NMR measurements

• Published in: Journal of catalysis 1992, Vol.134 (1), p.118-125
• Main Authors: Fǎrcaşiu, Dan, Ghenciu, Anca, Miller, Glen
• Format: Article
• Language: English
• ISSN: 0021-9517; 1090-2694
• DOI: 10.1016/0021-9517(92)90215-4

The limitations of the Hammett indicator method for acidity measurements of liquid acid catalysts of practical importance and the inapplicability in principle of the Hammett acidity concept to solid acid catalysts are discussed. Evaluation of acidities from the hydronation equilibrium of two simple alcohols, methanol and ethanol, and an α,β-unsaturated ketone, 4-methyl-3-penten-2-one (mesityl oxide, I), by carbon-13 NMR is demonstrated for sulfuric acid-water mixtures. The latter compound is particularly useful because the signal for its carbon atom C-4 shifts downfield upon hydronation by almost 50 ppm, whereas the signal for C-3 moves upfield by 3 ppm. The comparison of the chemical shift difference δ(C-4)-δ(C-3) (Δδ) for two different solutions allows a comparison of acidities of the two solutions from which other medium effects upon chemical shifts have been cancelled out. The variation of Δδ with concentration of I, or the ratio of base ( I) to acid, is linear, at least between 1 and 0.05 M of I. Extrapolation of Δδ of I to [B]/[AH] = 0 ( Δδ 0) allows the use of 13C NMR spectroscopy to establish a thermodynamically meaningful acidity scale, such as the traditional acidity function H 0. The slope of Δδ vs [B]/[AH] plot changes with acid strength; it is lowest (most negative) for the acidity at which I is half-protonated. The acidity required for half-protonation can thus be accurately determined from this slope. On the other hand, Δδ can be also measured at the stoichiometric ratio of the indicator base to acid molecules or sites. The latter approach must be used for comparison of strength of solid acids.