Surfing a chemical water world

Reactions in water promise greener, faster and better organic chemistry

A new wave of water-based reactions may transform the world of synthetic organic chemistry. This chemistry is traditionally done in "organic" solvents that often do not mix with water, and the sheer volume of their usage and the associated organic waste created raise significant environmental concerns. Now, however, Bruce Lipshutz of the University of California, Santa Barbara in the US, believes an overlooked liquid could provide a solution. He talks of a "water world" of new chemistry waiting to be discovered and reviews the field in the journal Current Opinion in Green and Sustainable Chemistry.

Organic chemistry is crucial for making some of our most widely used substances, including pharmaceuticals, plastics, and synthetic fibers. However, our reliance on organic solvents as the reaction medium for synthetic organic chemistry is paradoxical, as the organic chemistry of life largely occurs in water. Living cells are tiny bags of chemicals reacting in aqueous (water-based) solutions; yet, in isolation many organic chemicals will not dissolve in water.

What chemists need is a simple way to make organic compounds and water mix. The main method that Lipshutz is developing uses tiny particles called "micelles" that are miscible with water yet contain a water-immiscible interior where organic reactions can proceed (see image). These micelles are self-organizing structures composed of a sphere of "surfactant" molecules that interact with water.

The portion forming the outside of a micelle interacts readily with the surrounding water, due to a polyethylene glycol (PEG) or similar component. The inner portion is composed of components such as vitamin E that are immiscible with water. This creates what Lipshutz calls "nanoreactors". These nanoparticles can take up and concentrate organic chemicals added to the watery mix, promote reactions within them, and release the products.

In addition to micelles, another emerging option is using genetic engineering to create modified enzymes that can catalyze organic reactions in water.

"Our research group is on a worldwide crusade to get organic solvents out of organic chemistry," Lipshutz declares. "We are developing technology that enables faster, cheaper, and better organic synthesis in an environmentally responsible way."

Lipshutz points out that the chemists who contributed to the development of modern synthetic organic chemistry over a hundred years ago had no idea of the environmental problems their chosen path would create. The hazards of organic solvents are also accompanied by other problems, including obtaining the solvents from petroleum reserves, and using scarce transition metal catalysts from the Earth's limited resources, along with the high energy demands of the heating and cooling cycles that conventional approaches often require.

As a leader in this mission to move organic chemistry into water, Lipshutz outlines a list of new rules for chemists to follow. He hopes to literally rewrite the rulebook for practicing organic chemistry. Interest in all the possibilities is now spreading throughout the broader chemistry community. Lipshutz and his colleagues are surfing a wave that seems sure to grow.

Lipshutz, B. H. et al.: "Synthetic chemistry in a water world. New rules ripe for discovery," Current Opinion in Green and Sustainable Chemistry (2018)