Desktop NMR spectroscopy for real-time monitoring of an acetalization reaction in comparison with gas chromatography and NMR at 9.4 T.
Monitoring of chemical reactions in real-time is in demand for process control.  Different methods such as gas chromatography (GC), mass spectroscopy, infrared spectroscopy, and nuclear magnetic resonance (NMR) are used for that purpose.  The current state-of-the-art compact NMR systems provide a useful method to employ with various reaction conditions for studying chemical reactions inside the fume hood at the chemical workplace.  In the present study, an acetalization reaction was investigated with compact NMR spectroscopy in real-time.  Acetalization is used for multistep synthesis of the variety of organic compounds to protect particular chemical groups.  A compact 1 T NMR spectrometer with a permanent magnet was employed to monitor the acid catalyzed acetalization of the p-nitrobenzaldehyde with ethylene glycol.  The concentrations of both reactant and product were followed by peak integrals in single-scan 1H NMR spectra as a function of time.  The reaction conditions were varied in terms of temperature, agitation speed, catalyst loading, and feed concentrations in order to determine the activation energy with the help of a pseudo-homogeneous kinetic model.  For low molar ratios of aldehyde and glycol, the equilibrium conversions were lower than for the stoichiometric ratio.  Increasing catalyst concentration leads to faster conversion.  The data obtained with low-field NMR spectroscopy were compared with data from GC and NMR spectroscopy at 9.4 T acquired in batch mode by extracting samples at regular time intervals.  The reaction kinetics followed by either method agreed well.  The activation energies for forward and backward reactions were determined by real-time monitoring with compact NMR at 1 T were 48 ± 5 and 60 ± 4 kJ/mol, respectively.  The activation energies obtained with gas chromatography for forward and backward reactions were 48 ± 4 and 51 ± 4 kJ/mol.  The equilibrium constant decreases with increasing temperature as expected for an exothermic reaction.  The impact of dense sampling with online NMR and sparse sampling with GC was observed on the kinetic outcome using the same kinetic model.  Graphical abstract Acetalization reaction kinetics were monitored with real-time desktop NMR spectroscopy at 1 T. Each data point was obtained at regular intervals with a single shot in 15 s. The kinetics was compared with sparsely sampled data obtained with GC and NMR at 9.4 T.