Exploring C–H Activation in Arc Plasma-Initiated Microdroplets: A Green Chemistry Experiment for Undergraduate Students

This work describes a green undergraduate laboratory experiment that introduces undergraduate students to C(sp2)–H and C(sp3)–H activation chemistry in arc-plasma-initiated microdroplets. Mass spectrometry is employed as the primary analytical tool because it enables real-time monitoring of microdroplet reaction progress through the detection of molecular ion peaks and acquisition of fragment ions via tandem mass spectrometry for structural elucidation. Students first introduce toluene and dibutylamine separately into the arc plasma-microdroplet system to generate their respective microdroplets and acquire mass spectra. Subsequently, the two solutions are introduced simultaneously, allowing an arc-plasma-initiated C–H activation reaction to occur in the microdroplets, and the mass spectrum of the resulting mixed microdroplets is recorded. By comparing the three sets of mass spectra, students identify changes in characteristic peaks, determine whether a new product has formed, and interpret the mass spectrometric evidence for the formation of 2-[(dibutylamino)methyl]phenol (2-DMP). Furthermore, the product was subjected to MS2 analysis to obtain its fragmentation pattern for structural identification. The exact position of the hydroxyl substitution is then determined by comparing the high-performance liquid chromatography (HPLC) retention time of the product to those of authentic positional standards. A group discussion guided by the instructor addresses the proposed mechanism of C–H activation and product formation. This experiment enables students to connect advanced concepts in C(sp2)–H and C(sp3)–H activation with hands-on instrumental analysis, mass spectrometric data interpretation, mechanistic reasoning, and green chemistry evaluation. The activity provides students with opportunities to develop skills in safely operating the arc plasma-mass spectrometry system, performing microdroplet reactions and acquiring mass spectrometry data, interpreting mass spectrometric and chromatographic evidence for product identification, and evaluating the sustainability of experimental protocols using green chemistry principles. The student-facing activities can be completed within a single standard laboratory period, making them suitable for integration into an undergraduate organic chemistry laboratory curriculum.