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SMASHING steel balls together is the latest idea in the drive to make chemical synthesis more environmentally friendly. The collisions provide the right conditions to drive reactions between organic compounds without the need for harmful solvents.

Organic chemicals often have a stable crystal structure that locks up their molecules, making them unreactive. To persuade the molecules of different chemicals to mix and react, they must normally be dissolved in a solvent such as benzene or toluene. But the solvents can鈥檛 be reused and are often toxic or harmful to the environment.

Researchers have investigated alternatives such as microwaves and hot pressurised water, but none has been cheap or efficient enough to persuade industry to give up solvents. Now chemists in the US have come up with a mechanical approach. Viktor Balema and his colleagues at the government鈥檚 Ames Laboratory in Iowa took their inspiration from ball milling, a process already used in industry to produce and modify metal alloys by shaking them in vials of ball bearings.

When they tried the technique with organic chemicals, the researchers noticed small chemical changes taking place in their samples. So they decided to check whether it was possible to use the balls to make completely new compounds. They chose several well-known reactions to test this out, including the Wittig reaction, which produces unsaturated hydrocarbons. These are used as building blocks in the preparation of many organic materials and pharmaceuticals.

The researchers shook the reactants and the small steel balls together for several hours. Then they measured what happened using a variety of techniques, including X-ray diffraction and nuclear magnetic resonance. The reactions all proceeded very efficiently, in some cases giving even higher yields than the solvents. 鈥淚 would never have believed that solvents could be excluded from all these reactions if I hadn鈥檛 done it myself,鈥� says Balema.

The researchers are not sure of the exact mechanism but they think the collisions break down the crystal structure of the chemicals, allowing the molecules to mix and combine into new compounds.

Balema says scaling up the method should be straightforward and estimates that opting for steel balls over solvent should make organic reactions 5 to 6 times cheaper. But Ed Kordoski of the Synthetic Organic Chemical Manufacturers Association in Washington DC says that although the yields seem impressive, companies won鈥檛 be keen to invest in new equipment. He also points out that adopting a new process is never simple. If a company changes the way it makes a drug, for example, it has to reapply for the licence for that drug, in case it affects the structure of the compound or levels of by-products. 鈥淭he practicality is not as good as they think it is,鈥� he says.

Lawrence Scott, a chemist from Boston College, says simply proving that the idea works should stimulate further research in the area. 鈥淚t will be fun to see how much other chemistry will be possible under such conditions,鈥� he says. And Balema remains optimistic. 鈥淚 am quite sure that after some additional work, solvent-free mechano-chemistry will be of great interest to industry,鈥� he says.