Reflux - TECHNIQUE Heating at Reflux Written by Albert T...

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Unformatted text preview: TECHNIQUE: Heating at Reflux Written by Albert T. Sneden, VCU Department of Chemistry with revisions by L.M. Moses There are many experiments in organic chemistry which call for heating the reactants in a solvent at the boiling point of the solvent for an extended period of time. While this could be done in a flask or beaker open to the air, there are several difficulties with such a procedure. All are related to the fact that, as the solvent is heated at its boiling point, the molecules of solvent in the vapor phase will diffuse into the air if there is nothing to retain them in the beaker or flask. If this occurs, the concentration of the reactants in the solvent will change continuously, and this may affect the course of the reaction. Many organic solvents are flammable, and as the solvent vapors diffuse into the air, locally high concentrations of the solvent vapor will occur. If the vapors contact a heat source, an electrical spark, or a flame, they may ignite causing a fire or flash, causing an explosion and fire. In addition, some organic compounds are toxic if inhaled or ingested in high enough concentrations. If large quantities of these vapors are present in the laboratory air as a result of prolonged heating without recovering the solvent vapors, they may cause a toxic reaction in some laboratory workers, either in the short term or the long term. (The knee-jerk, idiotic response to solving these problems is to heat the solution in a closed container. However, this is to be avoided at all costs. A solution being heated increases the vapor pressure above the solution as molecules enter the vapor phase. If the container was totally closed, the pressure inside the container could build up to the point where the walls of the container could no longer hold the pressure and the container would explode. Never heat a material, particularly a liquid, in a totally closed system with no means of relieving the increasing pressure! ) To reduce these problems associated with heating organic solvents for extended periods of time, chemists use the technique known as heating at reflux. Figure 1: Water out Clamp Water In Clamp Heat The solution to be heated is placed in a single-neck, round-bottom flask of appropriate size (the flask should be no more than half full), the flask is clamped to a ring stand, and 2-3 boiling stones (also called boiling chips or Boileezers®) are added. Boiling stones are porous, inert materials that create a steady stream of small air bubbles when heated in a solvent. The streams of bubbles produced by the boiling stones allow the solvent to boil evenly and prevent the solvent from becoming superheated. (Vigorous stirring can also be used to achieve the same effect. This is usually done with a magnetic stirring bar immersed in the solution moved with a magnetic stirrer outside the flask.) If a solution becomes superheated, large air bubbles may be produced spontaneously and suddenly, causing a violent eruption of the solvent known as bumping. Boiling stones are always added prior to applying heat; if they are added after a solution has become superheated, the boiling stone can cause the violent bumping. After the addition of the boiling stones, a reflux condenser is attached to the neck of the flask. The reflux condenser is a condenser with a large diameter water jacket. Water hoses are connected to the hose connections on the condenser with the inlet hose at the bottom and the outlet hose at the top, and water flow is initiated. Make sure that the condenser hoses are the correct size and fit tightly over the hose connections of the condenser so they do not pop off when the water is turned on. If the heating is to go on unattended for a long period, use wire wrapped tightly around the hose over the hose connection to secure the hose. The condenser should also be loosely clamped to the ring stand. (Figure 1) If it is important to minimize moisture in the solution, a drying tube can be placed in the top joint of the reflux condenser. However, the end of the reflux condenser must be open to the atmosphere. Sufficient heat is then applied to the bottom of the flask to cause the solution to boil gently. As the solution boils, the vapors will travel up the condenser toward the top. The cooling water running through the condenser will cool the vapors to the point where they condense and become liquid. The liquid will run down the walls of the condenser and drip back into the flask. At the point where condensation occurs, a ring of liquid will be seen inside the condenser. The heat and water flow should be adjusted so this reflux ring is no more than one-third of the distance to the top of the condenser. When the reflux period is complete, the heat source is removed. The flask and the liquid contained in the flask are allowed to cool to room temperature before the water flow through the condenser is turned off. The apparatus is then disassembled and the liquid from the flask is decanted or filtered into a clean container. ...
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