CHEM 231 Lab
Technique Primer
Reflux
The highest temperature attainable in an open
vessel is limited by the boiling point of the reaction
solvent. Thus, for reactions that require high
temperature, it is often necessary to boil the
reaction for extended periods, a technique that
requires special provisions. To keep the reaction
from boiling dry, the contents are placed in a roundbottom flask, to which is attached a West condenser
cooled by tap water; the reaction flask is then
warmed by means of a heating mantle (Figure 1). As
the solvent vapor rises into the condenser it is
cooled and converted back to liquid form, at which
point it flows back into the reaction vessel. This
technique is known as reflux (literally, to flow back).
West
condenser
boiling flask
heating
mantle
Figure 1. Glassware setup for reflux (image from
organicchem.org)
There are a few issues of safety and technique to
keep in mind when conducting a reflux. First, it is
advisable to use a boiling chip whenever heating a
mixture to the boiling point. The process of boiling
requires nucleation sites for bubbles to form (Figure
2). When these sites are lacking, the liquid can
temporarily superheat and then release the energy
sporadically, unpredictably, and sometimes violently.
Boiling chips provide multiple nucleation sites and
thus ensure smooth boiling.
Figure 2. Carbon dioxide bubbles forming around
nucleation sites on a finger (image from Wikiimedia)
Second, a reflux apparatus should never be sealed.
Note in Figure 1 that the top of the West condenser
is open to the atmosphere. This is necessary to
prevent inadvertent overpressure within the
glassware, which can result in an explosion.
Water should flow into the condenser from the
bottom and out through the top. This allows
bubbles within the system to rise to the top and be
forced out. Moreover, the water flow should be just
enough to cool the condenser. High flow rates lead
to pressure in the line, which can cause a hose to
disconnect. This, in turn, creates two safety hazards:
the water from the disconnected hose can flood the
lab; and the reflux apparatus can boil dry and
overheat (in some cases, leading to explosion)
without a water supply to the condenser.
Finally, a few guidelines should be observed with
respect to the heating mantle. Note in Figure 1 that
the heating mantle is elevated (supported by an iron
ring)—this allows for rapid removal of heat should
the reaction exhibit a sudden exotherm. Also note
that the round-bottom flask is securely clamped
about the neck, so that the heating mantle can be
removed without disturbing the rest of the
apparatus. The power cord for the heating mantle
should be connected to a voltage-controlled outlet
(aka rheostat)—plugging the mantle directly into an
unregulated wall outlet will result in rapid overheating. Also take care to keep any cords and hoses
well away from the heating mantle, as the hot
surface can melt through the polymer and create an
electrical and/or thermal hazard.