Steroid–acid colour reactions. Part I. Carbon-13 nuclear magnetic resonance spectra of various protonated aliphatic ketones and their pKBH+ values

Abstract

The ¹³C n.m.r. spectra of cyclopentanone, diethyl ketone, di-isopropyl keton cyclohexanone, dicyclopropyl ketone, cyclopropyl methyl ketone, and cyclohex-2-enone in sulphuric acid of varius concentrations have been recorded. The maximum downfield change in the chemical shift, the apparent effect of complete protonation of the carbonyl group, is ca. 30 p.p.m. A plot of chemical shift against acidity shows that there is a gradual increase in apparent protonation but the form is not that of a normal titration curve. Solvation of the protonated ketone is suggested as explanation. A plot of log I against –H₀ has a slope of ca. 0.3. Values of H₀ at apparent halfprotonation are reported and ¹³C n.m.r. is shown to be an excellent method for the determination of the apparent basicity of a ketone. The results are treated by the method of Bunnett and Olsen and values of the thermodynamic pK_BH⁺ are obtained. The protonation of steroids is discussed.