Introduction

Flow chemistry undoubtedly plays a significant role in laboratory-scale organic syntheses due to quick reaction optimization [1−8], rapid scope expansion [9,10], safer [11−20] and even faster reactions compared to batch synthesis [21,22,23]. Because of these, and more additional benefits of flow chemistry [24], not only individual organic reactions but also complete continuous flow API syntheses were published in the literature (e.g., rufinamide [25], amitriptyline [26], diphenhydramine [27], imatinib [28], prexasertib [29] and others [30,31,32,33,34,35,36,37]). Parallel to this, several industrial companies realized the long-term advantages of flow chemistry and started pilot or even industrial scale flow research [38,39].

Consequently, there is and there will be a great demand in the pharmaceutical industry for chemical engineering graduates, who are proficient in flow chemistry and familiar with basic laboratory-scale flow devices. However, even to this day, apart from a handful of examples [40], little emphasis is given to the subject of flow chemistry to the undergraduates at universities, as the curricula usually focus on round-bottom flask syntheses at the laboratory practices. Even so, this disappointing fact encouraged us to present a new flow chemistry-based laboratory practice to chemical engineering BSc students at the Budapest University of Technology and Economics. During the practice, the undergraduates perform the flow synthesis of paracetamol, a common painkiller by two different N-acylations: reacting the p-aminophenol substrate with acetic anhydride and, in the second case, with ammonium acetate. Apart from the practical aspects, students also receive a clear vision, why flow chemistry can be important in the synthesis of active pharmaceutical ingredients (API). Advantages, as well as disadvantages of flow chemistry are presented to the students, compared to traditional batch synthetic methods. In addition, various synthetic routes are presented to undergraduates to obtain paracetamol (Fig. 1) [41,42].

Fig. 1
figure 1

Various synthetic routes to obtain paracetamol

Full size image

Numerous analytical methods are compatible with a modern flow system, allowing the reaction to be monitored in real time using Process Analytical Technology (PAT). PAT enables rapid, non-invasive and continuous analysis of various process parameters, enabling the real-time monitoring of production operations [43]. By analyzing the flow leaving the reactor in situ, valuable information can be obtained almost immediately about the composition of the reaction mixture and its change over time. In this way, even the reaction mechanism may be determined. The data provided by PAT can be used for real-time process control, ensuring that the quality of the output is within the desired range. In the case of on-line analyses, such as Mass Spectrometry (MS) [44] and High Performance Liquid Chromatography (HPLC) [45], sampling from the reaction mixture is needed in order to determine the composition. However, in the case of in-line analytical methods, the whole reaction mixture passes through the measuring cell of the device. In-line reaction monitoring can be performed using PAT sensors such as near infrared (NIR) spectroscopy [46,47], Raman spectroscopy [48,49], ultraviolet-visible (UV-VIS) spectroscopy [50], Fourier transformation infrared (FTIR) spectroscopy [51] and nuclear magnetic resonance (NMR) spectroscopy [52,53].

The aim of the laboratory practice is getting familiar with flow chemistry, its devices and modules, as well as with the various flow synthetic methods of paracetamol. During this practice, students acquire the skill to not only conduct a flow synthesis, but also to use attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) for reaction monitoring. These experiments allow students to explore the importance of in-line reaction monitoring during pharmaceutical processes. Last, but not least, the concept of end-to-end continuous process is outlined for the students (Fig. 2), where the API is produced from the starting materials and reactants in a telescoped multi-step flow reactor system, which is then subjected to on-line or in-line analysis and ultimately, to formulation, all in a continuous manner. However, crystallization can be a crucial step, because the whole process can only be considered truly continuous, if the product remains in the mother liquor, while the solid impurities stay on the filter [54].

Fig. 2
figure 2

The concept of end-to-end continuous medicine manufacturing

Full size image

Results and discussion

Experiment 1: N-acylation of p-aminophenol with acetic anhydride

The first continuous-flow experiment involved the reaction of p-aminophenol and acetic anhydride at room temperature allowing a fast, efficient and selective nucleophile N-acylation of the amino group (Fig. 3A). Due to the high reactivity of the acylating agent, 5 min of residence time resulted paracetamol in a complete conversion.

Preparing the starting solutions of the compounds, a concentration of 0.5 M was applied. The p-aminophenol was dissolved in acetic acid-aqueous solution (1:4), while the excess of acetic anhydride was diluted with acetonitrile.

The schematic setup of the flow system assembly can be seen on Fig. 3B. A tube reactor was used, which was made of polytetrafluoroethylene (PTFE, ID = 1.5 mm, volume 4 mL, Syrris Asia®). Two single syringe pumps (Chemyx Nexus 6000) were connected to the tube reactor with a T-mixer (PEEK, IDEX®) and standard microfluidic tube fittings (Syrris Asia®). Before starting the first reaction, the flow system has to be thoroughly washed with AcOH:H2O = 1:4 mixture (5–5 mL, 1 mL/min flow rate each) to ensure a clean system. Once the setup was assembled, both solutions made with the proper concentrations were drawn into two, 5 mL plastic syringes (BBraun Ltd.) without any air bubbles and connected to the two, single syringe pumps (Fig. 3C). After 5 min of residence time, students were asked to collect a sample, dilute and analyse it by thin-layer (TLC) and gas chromatography (GC).

The flow chemical system was connected to Bruker ALPHA II FTIR spectrometer with a Diamond Crystal ATR, which enables the in situ analysis of the reaction mixture’s composition exiting the flow reactor. The reaction mixture exiting the flow reactor is scanned in real time. Spectra were collected with an 8 cm− 1 resolution in the range 4000–900 cm− 1 using 16 and 8 scans for background and samples, respectively. For better visualization of the spectra, the spectral data were preprocessed using standard normal variate (SNV) and Savitzky-Golay smoothing (31 points window width, 2nd order polynomial). By observing the characteristic peaks of the specific components, conclusions can be drawn regarding the state of the reaction. Students can scan the spectra of the starting materials, as well as the reaction mixture during the process, and they can evaluate the latter by comparing the two with each other.

Fig. 3
figure 3

Continuous-flow synthesis of paracetamol in the reaction of p-aminophenol and acetic anhydride (A: Reaction scheme, B: Schematic flow setup, C: Photo about the flow chemical assembly)

Full size image

A three-dimensional (3D) surface plot of the FTIR spectra captured during the complete reaction is shown on Fig. 4. In order to better visualize the changes in the position of the characteristic bands, three spectra from various stages of the reaction were compared. Initially, only the solvent mixture (acetic acid and water) was present in the tube system. The characteristic peak of the hydrogen-bonded water molecules was identified in the spectral range of 2900–3700 cm− 1. The band observed at 1640 cm− 1 was attributed to the angular (scissor vibrational) deformation of water. The C = O stretching vibrations of the acetic acid were detected at ca. 1700 cm− 1. During the reaction, a shift in the 1488–1568 cm− 1 range was observed at 70 s, when the reagents reached the FTIR spectrometer through the tubes. Later, 350 s into monitoring the process a 1133 cm− 1 band was identified. The appearance of the 1133 cm− 1 peak indicates the presence of paracetamol.

Fig. 4
figure 4

Three-dimensional surface plot for complete reaction monitoring and comparison of three selected IR spectra at various stages of the reaction. (A.U.: arbitrary units)

Full size image

Experiment 2: Transamidation with in situ formed acetamide

The second experiment conducted during the laboratory practice involved the reaction between p-aminophenol and ammonium acetate (Fig. 5A). In this reaction, the in situ formation of acetamide from ammonium acetate caused the direct N-acylation of the amino group. The experiment was carried out at 120 °C with a residence time of 30 min in acetic acid solution.

Preparing the starting solutions of the compounds, the same concentration of 0.5 M was applied. Both the p-aminophenol and the excess of ammonium acetate was dissolved in acetic acid. In the case of the p-aminophenol, a few drops of distilled water were also needed to obtain a homogenous solution.

The schematic setup of the chemical assembly can be seen on Fig. 5B. The same tube reactor was used as in Experiment 1 (PTFE, ID = 1.5 mm, volume 4 mL, Syrris Asia®), which was placed into an oil bath to ensure a temperature of 120 °C. The two single syringe pumps (Chemyx Nexus 6000) were connected to the tube reactor with a T-mixer (PEEK, IDEX®) and standard microfluidic tube fittings (Syrris Asia®). Carrying out the reaction over the boiling point of acetic acid, a back pressure regulator (BPR) was also built into the system to maintain the desired temperature and inner pressure. Once the setup was assembled, both solutions made with the proper concentrations were drawn into two, 5 mL plastic syringes (BBraun Ltd.) without any air bubbles and placed onto the two, single syringe pumps (Fig. 5C). After 30 min of residence time, students were asked to collect a sample, dilute and analyse it by thin-layer (TLC) and gas chromatography (GC).

Fig. 5
figure 5

Continuous-flow synthesis of paracetamol in the reaction of p-aminophenol and ammonium acetate (A: Reaction scheme, B: Schematic flow setup, C: Photo about the flow chemical assembly)

Full size image

Assessment

These two experiments allow students to synthesize a compound of high interest. The pedagogical goals of these experiments include the acquirement of the basics of flow chemistry, an identification of preparing molecules through N-acylations and a comparison of the reactivity of different nucleophilic and electrophilic functional groups present in p-aminophenol, acetic anhydride and acetamide. The students were also permitted to acquire several analytical methods (TLC, GC, FTIR) essential for the evaluation of an organic synthesis. In order to assess the students’ understanding about the main concepts of the laboratory practice, they were asked to response some questions regarding the theoretical background of flow chemistry, paracetamol synthesis, analytics and safety regulations. Moreover, after the practice, every student had to write a lab report including a short description about the investigated reactions with schemes and schematic technological figures, calculations regarding the measurement of the reactants, flow rate and productivity, and an evaluation of the analytical measurements with a complete conclusion. This practical course was developed in January 2023 for Hungarian and international undergraduate students in the framework of the four-hour long Organic Chemical Technology Laboratory Practice at Budapest University of Technology and Economics. In the first year, the experiments were performed by about 40 students.

Hazards

During the laboratory practice, students must wear safety gear (lab coat, safety goggles and nitrile gloves). For all chemical’s links to appropriate safety hazards data are included in the Supporting Information.

Conclusions

A flow chemistry laboratory practice was developed at the Budapest University of Technology and Economics for chemical engineering BSc students. The undergraduates obtained a common painkiller, paracetamol via two different continuous flow routes. In addition, in-line FTIR spectrometer was implemented to observe the formation of the desired product in realtime. Thus, students got acquainted with the setup of a flow system, were able to calculate flow rates, residence times, stoichiometry and productivity and could also experience the benefits of flow chemistry themselves. In our experiments, inexpensive reagents, syringe pumps and home-made tube reactors were utilized. All in all, we believe that these tasks related to teaching continuous flow API synthesis could be implemented at other universities and institutes as well.