Gas Chromatography-Mass Spectrometric Analysis of Derivatives of Dibenzalacetone Aldol Products

The aldol condensation reaction is one of the many synthesis reactions carried out in second semester organic chemistry laboratories. This reaction was integrated into a novel experiment requiring students to synthesize dibenzalacetone derivatives using the crossed-aldol condensation by reacting different types of benzaldehyde derivatives with acetone, followed by analyzing those products using spectrometric and spectroscopic methods, especially gas chromatography- mass spectrometry (GC-MS). The main objective of this 2 week lab experiment compared to other procedures in the literature was the analysis of the final aldol product using GC-MS methods rather than solely nuclear magnetic resonance (NMR) spectroscopic methods. During the first week's 3 h laboratory period, students completed the reaction with a known benzaldehyde derivative and acetone to synthesize their aldol product. In the second week, students calculated the percent yield of their product, determined the product's melting point range, and analyzed the product by thin-layer chromatography, NMR, and GC-MS methods. Students also strengthened their skills of spectral interpretation using NMR spectroscopy, especially distortionless enhancement by polarization transfer NMR, an NMR technique rarely covered in undergraduate organic laboratory sections. Students spent more classroom time during week two, analyzing GC-MS data. Students were shown how to use molecular model kits to understand the fragmentation of compounds in the mass spectrum. As a result, the students who performed this experiment increased their general knowledge of GC-MS spectrometric techniques, including identification of compounds based on their fragmentation pattern.