Analysis of Minor Components by Ultrahigh Resolution NMR. 2. Detection of 0.01percent Diacetone Alcohol in "Pure" Acetone and Direct Measurement of the Rate of the Aldol Condensation of Acetone

Article abstract of DOI:10.1021/ja00256a008

Ultrahigh resolution NMR methodology permits the detection of as little as 0.01percent CH3COCH2(OH)C(CH3)2 (2) in a sample of "pure" acetone (1) of natural isotopic composition, with the use of the methyl resonances in the proton-decoupled (13)C NMR spectrum, even though the resonances of 2 are very close to those of 1.Also, all three peaks of the triplet of the (13)CH2D carbon of (13)CH2D-(12)CO-(12)CH3 are fully resolved, even though their intensity is only 0.015percent of that of the main contributor to the spectrum, (13)CH3-(12)CO-(12)CH3, and one of the peaks of the triplet is only 7 Hz away from the main acetone peak.The intensities of the (13)CH2D triplet serve as a calibrator of dynamic range accuracy in (13)C NMR measurements of relative proportions of major and minor components.After addition of aqueous sodium hydroxide to the acetone sample, the growth of the methyl carbon resonances of 2 provides a direct measurement of the rate constant for the aldol dimerization of 1 and also the equilibrium constant for this reaction.This example demonstrates the ability of ultrahigh resolution NMR to study directly the kinetics of reactions that have equilibria highly displaced toward the reagents, instead of the traditional procedure of monitoring only the kinetics of the reverse reaction by starting with pure "products".Artifacts that may interfere with the study of minor components by ultrahigh resolution NMR are discussed.