What Do Solvents Do, Anyway?

[ACSGCI]

Contributed by Kendra Leahy, Graduate Student and Ph.D. Candidate, University of Cincinnati

If you asked my research adviser, Professor James Mack, why chemistry is done in solution, you would most likely get a response that started like this: “Well, there was a man named Aristotle, and he said... In the Mack group, graduate students quickly learn: don’t ask questions like that if you don’t have time to hear the answer. But to summarize: chemists got it into their heads early on that we needed a liquid, or solvent, in order to do chemical reactions.

But maybe we should have stepped back a long time ago and asked ourselves, what is the role of this mysterious liquid? Is solvent really necessary to do a chemical reaction?

As long as your definition of a chemical reaction is the same as mine (and Merriam-Webster’s), we’re on the same page. A chemical reaction is a chemical change that occurs when two or more substances combine to form a new substance.  From my research group’s experience with mechanochemistry, you often don’t need the solvent to do the chemical reaction.

If we don’t use solvent, then what do we use? How do we mix reactants? How do we provide energy? In mechanochemistry, yes, you guessed it: we use mechanical energy to cause chemical change. In my group specifically, we use a Spex8000M mixer mill, which we lovingly refer to simply as a ball mill. We take our reactants, solid or liquid, and measure them out into a metal vial. Next, we add a metal ball. Place the cap on the vial, clamp it into the ball mill, let it shake for a while, and voila! A plus B goes to C (or whatever reaction you like).

Each of Prof. Mack’s students has studied a different area of organic chemistry using the ball mill. We study enolates, Diels-Alder reactions, Wittig reactions, oxidations, reductions, cross-coupling reactions, etc.  In all of these reactions, we avoid using solvent to produce chemical change, and this is important.  Many organic solvents, especially nonpolar ones, have human health and environmental concerns that come with their use.  There’s also inherent waste produced in solution chemistry, since the solvent doesn’t end up in the final product.

What’s the next step for greening mechanochemistry? There are many, but one that I specifically study is the isolation of our products. The current state-of-the-art is column chromatography, which is great for separation but terrible for green chemistry. We run reactions without solvent to avoid the drawbacks of solvent, but then we run a column. Column chromatography involves a mixture of solvents, which is more difficult to recycle than a single-solvent system. Furthermore, the choice of solvents is usually dictated by polarity, giving the researcher less freedom to make the greenest choice. Each of these solvents has its own list of human and environmental health issues.  Addressing the problem of chromatography has been a big part of my research for the last four years.

I design reactions so that chromatography is not necessary to obtain pure products. I use only one isolation step: simple gravity filtration. This is made possible by functionalized polymer resins. These functionalized polymer resins stay in the filter paper, separating that functional group from the desired product. I choose a solvent in which my desired product is soluble, and it is easily separated from the rest of the reaction components. That choice is the key: I only need one solvent, and I can choose the greenest option available.

Even though people have been grinding materials together for centuries, the study of using that grinding to cause chemical reactions emerged fairly recently. While no one in the mechanochemical community would argue that mechanochemistry is universally applicable, we would argue that it is a valuable tool to keep in our green chemistry toolbox, and one that needs further exploration.​

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