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How does a rotary evaporator work

Rotary evaporators are a common feature in most undergraduate laboratories. Their primary purpose is to remove solvent following a reflux, perhaps before crystallization of a reaction product. [Pg.188]

To operate the evaporator, we place the reaction solution in a round-bottomed flask while the pressure inside the evaporator is decreased to about x p. The flask is then rotated. The solvent evaporates more easily at this low pressure than at p. The solvent removed under vacuum is trapped by a condenser and collected for easy re-use, or disposal in an environmentally sensitive way. [Pg.188]

An atmosphere of vapour always resides above a liquid, whether the liquid is pure, part of a mixture, or has solute dissolved within it. We saw on p. 180 how the pressure of this gaseous phase is called its saturation vapour pressure, s.v.p. The s.v.p. increases with increased temperature until, at the boiling point 7)ix ii i, it equals the external pressure above the liquid. Evaporation occurs at temperatures below 7(boii), and only above this temperature will the s.v.p. exceed The applied pressure in a rotary evaporator is less than p, so the s.v.p. of the solvent can exceed the applied pressure (and allow the liquid to boil) at pressures lower than p.  [Pg.188]

We see how decreasing the pressure causes boiling of the solvent at a lower temperature than at its normal boiling temperature, i.e. if the external pressure were p. Such a vacuum distillation is desirable for a preparative organic chemist, because a lower boiling temperature decreases the extent to which the compounds degrade. [Pg.188]

Coffee, for example, itself does not evaporate even at low pressure, since it is a solid. Solids are generally much less volatile than liquids, owing to the stronger interactions between the particles. In consequence, the vapour pressure of a solid is several orders of magnitude smaller than that above a liquid. [Pg.188]


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