A rotary evaporator is a device used to efficiently remove solvents from samples by evaporation under reduced pressure. Rotary evaporation is most commonly and conveniently used to separate low-boiling solvents (such as C6 hydrocarbons) from compounds that are solid at room temperature and pressure. However, if there is a minimal degree of co-evaporation (azeotropic behavior) and there is a sufficient difference in boiling points at selected temperatures and reduced pressures, solvents can also be removed from samples containing liquid compounds.
A rotary evaporator is most commonly used when you want to remove solvents. The liquid will deposit a film over almost the entire surface of the flask. The membrane allows for a larger surface area and liquid evaporates faster. This solvent removal method is most commonly used when you have a large amount of solvent to remove.
Short path distillation is often used to purify compounds that are unstable at high temperatures or to purify small amounts of compounds. The advantage is that the heating temperature under reduced pressure can be much lower than the boiling point of the liquid at standard pressure, and the distillate only has to travel a short distance before condensing. The shorter path ensures that little compound is lost to the side of the device.
Short path distillation is pretty much the crudest type of distillation, and the reason you don't want to use it is because it doesn't have great separation capabilities and is a bit slower than a rotary evaporator. That said, short-path distillation is cheaper and does not require a vacuum pump.
