All stirred up: chemical engineers refute claims that ‘stirring doesn’t matter’

 Chemical engineers in the Netherlands have weighed in on the debate over whether to stir chemical reactions or not, following the publication of a study that suggested mixing doesn’t matter. In response, the team argues that stirring remains critical for reproducibility, selectivity and scalability, particularly in heterogeneous or industrially relevant systems.

Recent years have seen stirrer bars scrutinised, with some research raising issues, such as contamination causing false negatives, the bars themselves acting as ‘phantom catalysts’ or their position affecting reproducibility. More recently, a study claimed that stirring could be unnecessary for certain solution-phase organic reactions, prompting a flurry of social media posts about whether mixing mattered or not. Zhong-Quan Liu and his colleagues at Nanjing University of Chinese Medicine, investigated 329 classic organic reactions from 25 different categories and concluded that stirring didn’t affect the outcomes.

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‘We were surprised to see how quickly the claim that “stirring doesn’t matter” spread through the community and social media,’ says Timothy Noël at the University of Amsterdam. ‘This was, at best, only true under very narrow conditions. Once you scale up, introduce multiple phases, or deal with fast or exothermic reactions, mixing is absolutely decisive. To suggest otherwise risks misleading chemists into overlooking a critical experimental parameter and, in some cases, creating hazardous situations.’

Noël and his colleagues felt compelled to set the record straight and provide an easy-to-read framework to think about when mixing does or doesn’t matter. Publishing a preprint out of urgency, the team laid out the fundamental mechanisms of diffusion and convection in the transport of chemical species in solution and how poor mixing can lead to a range of issues, such as localised concentration gradients, or ‘hotspots’, that promote side reactions or unwanted precipitation.

‘Take a simple example: at small scale, an exothermic reaction cooled in an ice bath may appear perfectly manageable, even without much stirring,’ explains Noël. ‘But at larger scale (this can be even in the laboratory environment on a – rather modest – 250ml–1l scale), the heat generated in the centre of the reactor can quickly exceed what diffusion or passive cooling can handle. Without active mixing, local hotspots can develop, which may trigger runaway reactions and, in extreme cases, explosions.’