No trouble at the mill for reactions without solvents
1 Aug 2002
A process which has hitherto been used to prepare metal alloys could remove the need for many reactions to be performed in solution, according to researchers at the US Department of Energy's Ames Laboratory.
High-energy ball milling can provide the energy needed to make solid organic crystals react with each other, the team claims.
Ordinarily, organic substances are inactive when they are in their crystalline form - the crystal structure gives them extra stability. Dissolving them removes this stability and gives the molecules the freedom to move around and react, but the environmental and safety problems have forced process engineers to seek ways of reducing their use.
'Most of these solvents pose serious risks to health and the environment,' comments senior scientist Vitalij Pecharsky. 'So if you could produce organic materials without solvents, it would have a great impact on both materials science and chemistry.'
The Ames ball-milling process does without solvents altogether. The organic solids to be reacted are placed in a hardened steel vial containing steel balls, which is shaked vigorously. This transfers energy into the crystals, changing their structure and bringing the different species in the vial into contact so that they can react.
The researchers tested the equipment using a reaction which is commercially important to the fine chemicals and pharmaceutical industries - the Wittig reaction, used to produce unsaturated hydrocarbons by reacting phosphorus ylides with aldehydes or ketones.
Industrially, this is a three-stage process - production of phosphonium salts from organic phosphines and alkyl halides, followed by conversion of the salts into ylides by treating them with strong organic bases, followed by the Wittig step itself. All three stages require the use of solvents. The Ames team performed the entire sequence in two stages, entirely in the solid phase.
First, the researchers ball-milled triphenylphosphine with solid organic bromides to produce solid phosphonium ylide salts. They then milled these crystals with solid aldehydes or ketones and anhydrous potassium carbonate.
'I would never have believed that solvents could be excluded from all these reactions if I hadn't done them myself,' says associate scientist Viktor Balema.
The team has filed a provisional patent application for the process, and is currently investigating more solid-state reactions, this time using transition metal complexes.
