Approximately 80% of the experiments conducted in organic laboratories involve a step known as refluxing. Utilizing a reaction solvent, the materials are held in a dissolved state and at a steady temperature by boiling the solvent, capturing the vapor, and then returning it to the flask. Although this method is quite straightforward, it should not be underestimated in terms of its potential hazard and all appropriate safety measures should be taken.
Overview of Reflux
A reflux setup (Fig. 1) facilitates the boiling and condensation of a liquid, with the condensed liquid returning to the original flask. It is similar to a distillation process, with the condenser being vertically positioned. The liquid stays at the boiling point of the solvent (or solution) during the reflux.
Using a reflux apparatus allows for a solution to be conveniently heated, while avoiding the dissipation of solvent that would happen with heating in an open container. Vapors from the solvent are held in the condenser, and the concentration of reactants maintains a constant state over the process. The main goal of refluxing a solution is to heat it in a controlled manner at a stable temperature. For example, if one wanted to heat a solution to 60oC for one hour to conduct a chemical reaction, it would be difficult to keep a warm water bath at that temperature without the right equipment and continuous monitoring. Yet, if methanol was the solvent, the solution could be heated to its boiling point (65oC) and refluxed, meaning it would keep its temperature without regular maintenance. Granted, 65oC is not 60oC and if a precise temperature were essential for the reaction, then special heating hardware would be necessary. Nonetheless, often the boiling point of the solvent is selected as the reaction heat as it is the most practicable option.
1. Place the solution to be refluxed in a round bottom flask and secure it to the ring stand or latticework with an extension clamp and a rubber gasket (Fig.2 a and video). Fill the flask no more than halfway. For some unknown reason, the figures do not include rubber gaskets. If a high temperature boiling (> 150 oC) or flame heating is used, they cannot be used.
2. Add a stir bar or boiling stones to prevent bumping. When refluxing concentrated solutions of sulfuric or phosphoric acid, boiling stones should not be used as they can discolour the solution. For example, the solution remains colourless when a stir bar is utilized with concentrated sulfuric acid (Fig.2 b). In contrast, when a boiling stone is used, the solution darkens while heating (Fig.2 c) and turns the entire solution a deep purple-brown colour (Fig.2 d).
3. Fit rubber hoses onto the condenser (pre-wetting the ends to make them easier to slide on), then suspend the condenser vertically with the round bottomed flask. If using a tall condenser, secure it to the ring stand or latticework (Fig.3 a). Make sure the condenser is connected snugly to the flask. Safety warning: if the connection is not secure, and flammable vapors are released, they may be ignited by the heat source. Do not use a plastic clip to attach the round bottomed flask and condenser, as shown in Fig.3 с. Plastic clips can be fragile (especially when heated) and this setup does not allow for the flask to be safely disconnected from the heat source at the end of the distillation.
Note: The shorter a reflux condenser you need, the higher the boiling point of the solvent (solvent mixture). If your solvent boils at a low temperature (ether), use the longest Liebig reflux condenser.
4. Attach the pipe on the lower arm of the condenser to the water faucet and let the pipe from the upper arm drain into the sink (Fig.3 b). It is essential that water flows into the bottom of the condenser and out of the top (against gravity) to ensure the condenser is effective, that it fills up completely.
5. If numerous refluxes are taking place concurrently (e.g. when many students are performing a reflux simultaneously), the hoses from each reflux setup can be linked up in succession (Fig. 4). To do this, the upper arm of “Setup A” which usually drains to the sink is instead connected to the lower arm of “Setup B.” The upper arm of Setup B then drains to the sink. Connecting equipment in a line reduces the utilization of water, as water streaming out of one condenser enters the next. Multiple reflux setups can be linked in sequence, and the flow of water should be observed to guarantee that all setups are properly cooled.
6. Commence the flow of a regular amount of liquid through the tubes (not excessively powerful so the tubes don’t move drastically due to the high pressure of the water). Double-check that the pieces of glassware are securely attached, and then put the heat source beneath the flask. Activate the stirring plate if using a stir bar.
a) If utilizing a heating mantle, secure it in place with an adjustable platform (e.g. a wire mesh / ring clamp). Leave a few inches underneath the mantle so after the reaction is complete, the mantle can be moved down and the flask can cool. If the heating mantle is not a perfect fit for the size of the round bottomed flask, cover the flask with sand to create a better contact (Fig.5 a).
b) If utilizing a sand bath, submerge the flask in the sand so that sand is at least as tall as the level of liquid in the flask (Fig.5 b).
c) If the setup will eventually be abandoned for an extended period of time (e.g. overnight), affix copper wire over the hose attachments to the condenser to avoid any changes in water pressure causing them to detach.
7. If the heat source is warmed up beforehand (optional), the concoction should start to boil within five minutes. If not, up the temperature. The ideal heating rate is when the solution is boiling vigorously and a “reflux ring” is around one-third of the way up the condenser. A “reflux ring” is the uppermost part where hot vapors are condensing. With some solutions (e.g. an aqueous solution), the reflux ring is clear with droplets in the condenser (Fig.6 a+b). With other solutions (e.g. several organic solvents) the reflux ring is more subtle, but still visible with careful examination (Fig.6 c). Small movement may be seen in the condenser as liquid slides down the sides of the condenser, or background items may appear distorted from the refraction of light through the condensing fluid (in Fig.6 d the stand pole is distorted).
8. If executing a procedure that requires refluxing for a certain duration (e.g. “reflux for one hour”), the period should begin when the substance is not just boiling but actively bubbling in the lower third of the condenser.
9. If the reflux ring ascends to the midpoint or higher of the condenser, the heat should be reduced or else vapor can leave the flask.
10. When the reflux is finalized, switch off the heat source and remove the flask from the heat either by raising the reflux apparatus or lowering the heat source (Fig.7 a).
Do not turn off the water flowing through the condenser until the contents are merely warm to the touch. After a couple of minutes of air cooling, the round bottomed flask can be submerged in a faucet water bath to hasten the cooling process (Fig.7 b).
If you need to prevent atmospheric water vapor from entering your reaction, you must utilize a drying tube and the inlet adapter in the reflux setup (Fig. 8). You can also use them if you require to keep water vapor from any system, not just the reflux setup.
1. If necessary, clean and purify the drying tube. Unless you suspect that the anhydrous drying agent is no longer anhydrous, you don’t need to do a comprehensive cleansing. If the material is caked inside the tube, it is likely ineffective. You should clean and recharge the tube at the start of the procedure. Make sure to use anhydrous calcium chloride or sulfate. It should stay okay for a few uses. If you are lucky, indicating Drierite, a specially prepared anhydrous calcium sulfate, may be blended in with the white Drierite. If the color is blue, the drying agent is useful; if red, the drying agent is no longer dry, and you should discard it (see Desiccants in “Vacuum desiccators”).
2. Put in a loose plug of glass wool or cotton to keep the drying agent from entering the reaction flask.
3. Set up the apparatus as shown, with the drying tube and adapter on top of the condenser.
4. At this point, reagents can be added to the flask and heated with the apparatus. Usually, the apparatus is heated while empty to drive water off the walls of the apparatus.
5. Heat the apparatus, usually empty, on a steam bath, giving the entire setup a quarter-turn every so often to heat it evenly. A burner can be used if there is no threat of fire and if heating is done cautiously. The heavy ground glass joints will crack if heated too much.
6. Give the apparatus time to cool to room temperature. As it cools, air is drawn through the drying tube before it reaches the apparatus. The moisture in the air is trapped by the drying agent.
7. Rapidly put the dry reagents or solvents into the reaction flask, and reassemble the system.
8. Execute the reaction as usual, like a standard reflux.
Addition and reflux
Occasionally, a compound needs to be incorporated into a setup while the reaction is still in progress, typically alongside a reflux. Rather than breaking open the system and exposing oneself to hazardous fumes, an addition funnel is employed. We previously discussed addition funnels when covering separatory funnels, which may have been perplexing.
The Fig.9 a illustrates a true sep funnel. Liquids can be placed in it and blended before extraction. However, it cannot be used to add material to a setup, as it lacks a ground glass joint.
Fig.9 c illustrates a pressure-equalizing addition funnel. It is intended to prevent the vacuum build-up that can occur when a separatory funnel is emptied, by equalizing the pressure between both sides of the liquid being added to the flask. Unfortunately, it is expensive, limited, rare and not suitable for extractions, with the liquid flowing out of the tube onto the floor when the funnel is shaken.
Fig.9 b is a compromise. The pressure-equalizing tube is removed, but the ground glass joint remains, making it suitable for extractions. Caution must be taken to avoid vacuum build-up when adding materials. The stopper should be removed periodically or, alternatively, a drying tube and inlet adapter can be put in place of the stopper, preventing moisture from entering the funnel and creating a vacuum.
How to Set Up
Two methods of setting up an addition and reflux are achievable: a three-neck flask or a Claisen adapter. In both cases, drying tubes may be used to prevent moisture from getting into the reaction.
Boiling Stones (Boiling Chips)
Small chunks of black porous rock (most commonly silicon carbide) referred to as boiling stones (or boiling chips) can be added to a solvent or solution. The trapped air inside them releases bubbles when the liquid is heated, and their high surface area offers nucleation sites for the formation of solvent bubbles. It is best to add them to a chilly liquid, otherwise a vigorous eruption of bubbles may occur. When a liquid is brought to a boil utilizing boiling stones, the bubbles predominantly come from the stones (Fig.11 b).
Boiling stones should not be used in heating concentrated solutions of sulfuric or phosphoric acid, as they may corrupt and taint the solution. After one use, crevices of boiling stones are filled with solvent and they become unable to create bubbles, making them unsuitable for reuse. For instance, Fig.12 shows a Fischer esterification reaction that uses concentrated sulfuric acid. With a stir bar for bump prevention, the solution remains translucent (Fig.12 a). When the same reaction is done with a boiling stone, the solution darkens as it is heated (Fig.12 b) and eventually the entire solution turns a deep purple-brown color (Fig.12 c). Besides polluting the solution, the dark color makes it difficult to manipulate the material with a separatory funnel: two layers are present in Fig.12 d, although it is very hard to see.