2‐thiobarbituric acid test for lipid oxidation in food: Synthesis and spectroscopic study of 2‐thiobarbituric acid‐malonaldehyde adduct

Synthesis, purification, elemental analysis, and spectroscopic studies were undertaken to characterize the structure of the red adduct 2:1 thiobarbituric acid (TBA)‐malonaldehyde involved in the evaluation of oxidative rancidity in fats and oils. Thin‐layer chromatography, infrared and ultravioletvisible absorption, 1H (1H NMR) and 13C nuclear magnetic resonance (NMR) spectra were used. A yield of 93% was obtained in the synthesis. The results of elemental analysis agree with the formula for the chloro‐monohydrated form, C11H11N4O5S2Cl. Three characteristic absorption maxima at 532, 310, and 245 nm, respectively, were shown in acid aqueous medium (pH 2.9). The characteristic vibrations assigned to the ‐NH, ‐OH, CαH (exocyclic) and ‐C=S groups were confirmed in the infrared spectra. There was no evidence of thioenolization. 1H NMR data at δ 5.10 (‐CONH‐ group, H2O and HCl molecules); δ 11.54 (‐OH group of keto‐enol tautomer, ‐NH group); and δ 176.4 (‐CONH‐ group) also were observed. The experimental results obtained were consistent with the existence of two spectral equivalent tautomeric structures. The colored adduct was compared with other TBA‐aldehyde compounds.