On-Flow Diagram Chemical Shift vs. Time

An on-flow experiment is now carried out. 50 pi of a solution of the product mixture (5 mg in 5 mL solvent) are injected and the NMR proton signal accumulation started simultaneously. The time taken for the chromatogram is 17 min. During this time a total of 128 proton NMR spectra are recorded, each with eight scans, i.e. an FID is accumulated approximately every 7 sec. After the Fourier transformation we obtain a two-dimensional representation (Fig. 33) of the on-flow experiment. 

The two axes represent the chromatogram (retention time) and the chemical shift information. The individual NMR spectra can be extracted by the software and viewed individually in the form of normal ID proton NMR spectra. 

33 On-flow experiment carried out on the product mixture. Horizontal axis proton chemical shift. Vertical axis retention time 

The lower spectrum shows an ester group (triplet at 1.2 ppm, quartet at 4.2 ppm) and a singlet at 8.1 ppm, the latter indicating the presence of a formyl group. Peak 1 results from ethyl formate, formed by adventitious hydrolysis of the acetal 4. The middle and top spectra correspond to the two acetals (see equation) the assignment is very simple. 

Acetal 5 contains only one type of methyl group, while acetal 4 has two different types. Thus the spectrum recorded after 3 min corresponds to acetal 5, while that recorded after 7.5 min results from acetal 4. The methylene groups (between 3.6 and 4.2 ppm) and the methine protons (at 4.8 and 5.4 respectively) confirm these assignments. 

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