The discourse regarding the importance of stirring in chemical reactions has been reignited by a contentious study, eliciting reactions from chemical engineers in the Netherlands. The conversation revolves around whether mixing is vital in chemical processes, particularly after research suggested that stirring might not have a substantial effect on solution-phase organic reactions. Nevertheless, Dutch scientists maintain that stirring is critical, especially in heterogeneous or industrial-scale reactions where reproducibility, selectivity, and scalability are crucial.
Historically, the employment of stirrer bars has faced scrutiny due to problems such as contamination resulting in inaccurate readings, and their positional effects on experimental reproducibility. Concerns emerged following a recent study that posited mixing could be unnecessary for certain organic reactions, sparking widespread discussion. Zhong-Quan Liu and his team from Nanjing University of Chinese Medicine investigated 329 organic reactions, concluding that stirring does not influence outcomes. This claim quickly spread within scientific circles and on social media.
Timothy Noël from the University of Amsterdam expressed astonishment at how swiftly the belief that “stirring is insignificant” gained popularity. He emphasizes that this holds true only under very specific circumstances. He warns against dismissing stirring’s vital role, particularly when scaling reactions or facing multi-phase, fast, or exothermic reactions, where inadequate mixing could lead to serious hazards, such as dangerous hot spots and runaway reactions.
In a constructive initiative, Noël and his team released a preprint to elucidate when mixing is important, detailing the roles of diffusion and convection in chemical transport. They point out that insufficient mixing can result in problems such as localized gradients or precipitation, thereby affecting reaction quality and safety. Noël provides an example involving exothermic reactions in an ice bath—manageable at smaller scales but possibly perilous at larger volumes unless effective stirring is employed.
Mixed Reactions
Liu defends the conclusions of his research, claiming that extensive studies support stirring’s limited effect on organic reactions. However, Noël criticizes this overarching conclusion, suggesting that while diffusion may be adequate in small-scale, homogeneous, and slow reactions, it does not apply universally. The amplification on social media has misleadingly minimized the importance of stirring, which Noël and experts like Alex Speed from Dalhousie University caution against. Speed acknowledges that while stirring may not always be imperative, its omission could overlook significant insights into reactions, especially in quicker or heterogeneous reactions, or exothermic and large-scale processes.
Noël highlights the disparity between practices in synthetic chemistry and engineering principles concerning diffusion, convection, and heat transfer. He stresses the necessity of considering these aspects, recognizing chemists’ increasing emphasis on reproducibility, safety, and process robustness as beneficial developments. The vigorous discussion underscores essential fundamentals and reinforces the need for balance between chemical practicality and engineering knowledge to ensure successful reactions.