Electrical 'switch' used to control chemical reactions

In order to meet society’s needs for novel pharmaceuticals, cleaner fuels and more, chemists have to develop synthesis methods to obtain new products that do not exist in their natural state. Researchers at the University of Geneva (UNIGE), in collaboration with Cardiff University, have now discovered how to use an external electric field to control and accelerate a chemical reaction, with their results published in the journal Science Advances.

In chemistry, creating complex organic chemical compounds from simpler reagents is called organic synthesis. Through successive reactions, chemists assemble small molecules to ultimately form the desired products. Organic synthesis is crucial to the manufacture of drugs, polymers, agrochemicals, pigments and fragrances. These successive steps are extremely precise and delicate to control. To limit the required resources, the yield of each reaction step should be optimal. Achieving better control and simpler operation of these reactions remains a major challenge.

“Any molecular transformation results from electrons — negatively charged elementary particles — moving from one place in a molecule to another,” said UNIGE Professor Stefan Matile, who led the new study. Electrons can be influenced by an external electric field; it is therefore theoretically possible to electrically control chemical reactions. But although simple in principle and promising in terms of impact, this approach has come up against several limitations, and its few implementations have performed poorly.

With their teams, Matile and his Cardiff counterpart, Professor Thomas Wirth, have succeeded in activating an organic chemical reaction with a simple electric field. To do this, they designed an electrochemical microfluidic reactor. Their results clearly show the dependence between the state of progress of the chemical reaction and the intensity of the applied electric field. This device enables a chemical reaction to be activated simply by flipping a switch.

“This type of reactor takes the form of a small box in which the reaction mixture can circulate between two electrodes producing the electric field,” said Ángeles Gutiérrez López, a PhD student in Matile’s group. “The electrodes are 5 cm x 5 cm square plates placed as close together as possible. They are separated by a quarter-millimetre-thick sheet. This sheet contains the flow channel for circulating the molecules between the electrodes.”

The electrodes are coated with carbon nanotubes. As they flow through the reactor, the reactants interact weakly with the carbon nanotubes, exposing them to the electric field. This induces electronic polarisation in the molecule, activating the chemical transformation.

To create the desired chemical bonds with a high yield, chemists usually implement complex, multi-step strategies involving numerous intermediates. These strategies require important resources and energy. The new electrical device proposed by Matile and Wirth could simplify these strategies and thus reduce the carbon impact of chemical syntheses, with the device having the advantage of being easy to control.

“Our ‘reactor’ is in some ways like the particle accelerator at CERN in Geneva, but instead of accelerating subatomic particles, it accelerates electrons during molecular transformations,” Matile said. Fundamental advances are still needed to unlock the device’s full potential; however, this method could be applied to organic chemistry in the not-too-distant future, making the production of drugs, new fuels or new plastics greener and more controllable.

Image caption: The device takes the form of a small box in which the reaction medium circulates between two electrodes producing the electric field. Image ©Stefan Matile.