Unknown butyl and pentyl acetate ester synthesis by Fischer esterification produces twelve potential products. Physical methods of analysis (boiling points and refractive indexes) provide limited identification information since the values are often similar for the various unknown esters; GLC affords purity data but does not contribute to ester identity. Spectrometric analysis using electron-impact mass spectrometry provides fragmentation data; however, lack of a discernible molecular ion makes this of limited and complicated use for ester identification. Infrared spectroscopy validates ester production but does not afford specific ester identity. Identification of every potential butyl and pentyl acetate ester can be accomplished using a compilation of PMR spectral data (number of signals/sets of signals, chemical shift data, integrals, and splitting patterns) and CMR spectral data: CMR (number of carbon signals and chemical shift data), gated-decoupled CMR (carbon signal-splitting data), and DEPT (methyl, methylene, methine, and quaternary-carbon signal identification). 2D NMR experiments are, however, required to provide unequivocal assignment for all PMR and CMR signals of the unknown esters. The necessary 2D NMR experiments include HSQC (1-bond C,H), INADEQUATE (1-bond C,C), COSY (2- to 3-bond H,H), HMBC (2- to 3-bond C,H), and TOCSY (spin system related H,H). This study provides a plethora of experiences in the laboratory for advanced organic chemistry students. These include (1) synthesis and purification of unknown butyl and pentyl acetate esters, (2) conclusive identification of the unknown esters using a combination of physical (bp, η, GLC), spectrometric (MS), and spectroscopic (IR, 1D NMR and 2D NMR) techniques, and (3) utilization of both 1D NMR and 2D NMR experiments to afford unequivocal assignment for all PMR and CMR signals of the unknown esters.

Unequivocal assignment for all PMR and CMR signals of unknown butyl and pentyl acetate esters from Fischer esterification using 2D NMR experiments

249th National Meeting (2015)

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