Dean-Stark
Introduction to Dean-Stark
A Dean-Stark trap is a piece of glassware principally used to remove water from a condensation reaction, and is especially used for reversible reactions in order to drive the reaction to completion through the removal of water. The immiscibility of the reaction solvent and water are the key requirement of use for a Dean Stark trap. Two versions of the trap exist, one for use with solvents which are less dense than water, and a second for use with solvents that are more dense than water. The traps are named after Ernest Woodward Dean and David Dewey Stark.
Left: Dean Stark Trap. Right: Reverse Dean Stark Trap (wrapped in lagging to insulate the path).
Historically, Dean Stark traps would predominantly employ benzene as the reaction solvent, although most modern reactions are performed in toluene as a safer alternative, although other solvents such as butanol are sometimes used. The significanlty higher boiling point of toluene compared to benzene means that traps are much more likely to require insulation in order that the glassware leading to the trap can be maintained above the solvent boiling point. Reverse Dean Stark traps often use dichloromethane or chloroform.
Most Dean Stark traps also include graduations to allow the volume of the eliminated product (water) to be measured. This provides a guide as to the progress of the reaction being undertaken, as the theoretical volume of water to be produced can be calculated and compared to the amount in the trap. Some traps also include a tap in order to drain off the liquid from the trap where large volumes are being generated.
The traps can be used for reactions where immisible products other than water are generated, for instance the removal of benzene formed in an aqueous reaction. Here, a reverse Dean Stark trap would be used to capture benzene as the reaction progresses.