The Chemistry Column

The lazier the greener?

Claire Gormley

If you ever carried out a chemistry practical in school, you are likely to remember the big, brown bottles reserved for waste materials, and the strict instructions from your teacher not to pour anything down the sink. An estimated fifty to eighty per cent of the materials left over after a chemical reaction are solvents that can be harmful to the environment, and must be disposed of by special means (Lim, 2015). Solvents are used to dissolve the solid chemicals that are needed to create the desired end product. When chemicals dissolve, the rate at which they mix and collide— making and breaking bonds in the process —increases, thereby speeding up the overall reaction. Apart from this initial dissolving step, these solvents are not usually involved in the reaction process; at the end, they're just there.

This is the way that chemistry has been done for decades— but it is changing. Known as 'Lazy Man's chemistry' (a term coined by Tomislav Friščič, a leading scientist in the field at McGill University)— or, more recently, as 'Chemistry 2.0' —mechanochemistry is paving the way for a greener, safer, solvent-free approach (Lim, 2015; Friščič, Mottillo, and Titi, 2020).

The solvent-free approach comes in many flavours, but, at its core, mechanochemistry is a reaction between two or more chemicals in their solid state over a few days or weeks. Although the process takes much longer than the solvent-based approach— Chemistry 1.0, if you will —in the end, most or all of the reactants are completely used up, and the only thing left is the desired product (Lim, 2015). Currently, the most popular methods for achieving this involve using shaker or planetary mills, which are both available commercially (Friščič, Mottillo, and Titi, 2020). Alternative methods make use of different additives to augment, direct, or enable reactivity between the chemicals. These additives include a small amount of liquid proportional to the reaction weight (Liquidassisted Grinding, LAG), a metal catalyst such as copper, adding certain wavelengths of light (known as Photo-Mechanochemistry, or Mechanochemical Photocatalytic Reactions), and many others (Friščič, Mottillo, and Titi, 2020).

In fairness to Chemistry 1.0, although solidstate chemistry occurs in nature, it has not always been as possible to achieve in the laboratory as it is today. Advances in imaging techniques like X-ray Crystallography, nuclear magnetic resonance scanning, and electron microscopy have enabled scientists to understand what is happening during these solid-state reactions in real time, as well as what is really being produced (Lim, 2015). Now that solvent-free methods for precisely reacting chemicals exist, it is time to move away from the wasteful and harmful approaches of the past, and towards a safer, cleaner, slower design.

Easier said than done, of course, but the pharmaceutical industry has at least become interested in slow chemistry techniques, in the hopes of gaining better control over the rate of decay of drugs in pill form (Lim, 2015). Another strong selling point is the efficacy of mechanochemistry, and the reduced handling of the chemicals— meaning that chemists aren't required to do as much calculating and measuring out of the different volumes and masses of solvents and chemicals involved in the reaction. Since handling is where mistakes usually happen, eliminating solvents is ideal for working with rare, costly materials (Friščič, Mottillo, and Titi, 2020).

The researchers at the forefront of this field know that it's an uphill battle to make chemists lazier. However, they believe that— with the creation of standard protocols for these techniques, with further education and encouragement for chemists interested in this field, and by building our understanding through further research —mechanochemistry can become the new normal (Friščič, Mottillo, and Titi, 2020).

Image by Maddy Mould

References

Friščič, T., Mottillo, C., Titi, H. M. (2020) 'Mechanochemistry from Synthesis', in Angewandte Chemie, 132:1030-1041

Lim, X. (2015) 'The slow-chemistry movement' in Nature, 524:20-21