Organic synthesis by electrospray

17 November 2012 - Orgoish

prep electrospray muller 2012  Graham Cooks has a fondness for electrospraying. Basic principle: eject a solution through a nozzle to which is applied a large voltage, liquid exiting the nozzle adopts a cone-shape and from its tip highly charged microdroplets are emitted from which solvent is rapidly evaporated. Convenient in electrospray ionization for generating charged macromolecules in MS. Stuff that Cooks like throw at a mass spectrometer through an electrospray includes milk and urine (DOI), blood (DOI) and living plants (DOI). In a novel exploit Cooks is electrospraying organic reactions (DOI).

More specifically the water solution to be electrosprayed contains indanone, 4-chlorobenzaldehyde and potassium hydroxide and the reaction taking place is a Claisen-Schmidt condensation. Compared to a standard bulk reaction the electrospray reaction is much faster due to rapid solvent evaporation. Taking into account droplet speed (200 m/s) and distance travelled to collecting unit (5 cm) the reaction is near completion (> 92%) in a matter of milliseconds.

Off course the reaction scale is minute, at the milligram scale, and the number of side-products has increased. Not easily understood from the article is the effect on reaction time of the negatively charged reaction intermediates in this reaction. The sign of the voltage does not seem to matter much. Another issue to tackle is reaction scale, if 4 nozzles can produce a milligram under a minute, how expensive will 4000 nozzles be when producing a gram per minute?