Russian chemists have studied how the position of a reaction vessel above a magnetic stirrer affects the course of heterogeneous reactions. It turned out that the formation of palladium nanoparticles and the Suzuki reaction proceed at different rates and yields depending on how far the reaction vessel is from the center of the stirrer. This can lead to the publication of non-reproducible results, the authors warn in the journal JACS Au.
In the laboratory, reaction mixtures are usually stirred using magnetic stirrers. The chemist inserts a magnetic anchor, usually cylindrical, into a flask or other reaction vessel, and then secures the flask in a stand above the magnetic stirrer. The stirrer can be adjusted to set the number of revolutions per minute of the anchor, and then turn on the stirring. The anchor begins to rotate and stir the reaction mixture.
It is well known that in reactions involving large amounts of starting reagents, mixing plays an important role, and if the mixing is poor, the reaction may not proceed at all or may lead to other products. But it is generally accepted that for small vessels of a few milliliters, which chemists usually use in experiments, the effects of mixing are less important.
Chemists led by Valentin P. Ananikov from the Zelinsky Institute of Organic Chemistry of the Russian Academy of Sciences had doubts about this. During their work, they noticed that some heterogeneous reactions are difficult to reproduce, and decided to study the reasons for this phenomenon. As it turned out, in some cases this is due to the mixing of the reaction mixture.
To demonstrate how small differences in mixing affect the course of the reaction, the chemists 3D-printed a rack for small jars with screw-on lids. The rack had three height levels, each of which could accommodate five jars. The chemists then placed 15 jars with a palladium complex and carbon nanotubes in the rack and observed how the reaction of palladium deposition on the nanotubes proceeded depending on the position of the jar. It turned out that not only the reaction rates differed from jar to jar, but also the distribution of the resulting palladium nanoparticles by size. For example, in one of the jars in the second row, particles of four nanometers in size predominated, and in one of the jars in the third row, particles of about one nanometer in size. The chemists observed similar differences in the reaction of the formation of palladium nanoparticles without nanotubes.
The scientists then examined how the position of the vessel above the stirrer affected the conversion of the Suzuki reaction catalyzed by commercial palladium on carbon. In this case, nine reaction mixtures were placed at the same height but in different positions relative to the center of the stirrer. As a result, the average conversion of the starting material at the center of the stirrer was about 45 percent, and at the most distant position, about 26 percent.
Thus, for some reactions, small differences in stirring can lead to large differences in results. As scientists have shown, reactions most often proceed faster and with a better yield if the reaction vessel is located closest to the center of the stirrer. This is how the authors of the article advise other chemists to conduct chemical reactions. They also believe that in the experimental part of publications on chemistry, it is worth indicating the parameters of the magnetic stirrer and magnetic anchor, and also attaching a photo of the reaction mixture.
The results of chemical experiments can be affected by very small differences in the purity of the starting materials, the reaction vessel, and the magnetic anchor. We talked in more detail about how difficult it is to study catalytic reactions because of this in the article “The Darkness of Catalysis.”
