Setting up a reflux

Equipment

To carry out a reflux you will require:

• A round-bottomed flask and stirrer bar

• A condenser and water tubing

• Heating apparatus eg a stirrer-hotplate, lab jack and heating block

• Two clamps and bosses, that will be mounted one above the other on a clamp stand

Positioning

The first thing to consider when setting up a reflux is the positioning of where you plan to set up the apparatus. You will need to have a clamp stand, near both a water supply and sink to run the condenser. Additionally, it is important to consider the position of items such as flammable solvents or waste that may be near the apparatus, as the hot plate could act as an ignition source for any solvent vapours, leading to an uncontrolled fire.

Choice of glassware

The next consideration is which glassware to use. A reflux should be conducted in a round-bottomed flask (RBF), which are available in different sizes and configurations.

• Flask size – Round-bottomed flasks come in a variety of sizes (10 cm³, 25 cm³, 50 cm³, 100 cm³, 250 cm³ etc.) so it is important to select a flask that is large enough to contain the reagents and solvent, without being more than half full.

• Joint size – Round-bottomed flasks typically have a neck with a ground glass joint, these can also come in different sizes, so the size of the neck of the flask needs to match the joint of the condenser.

• Flask configuration – round-bottomed flasks can be designed to have a single neck, or two necks, or even three necks. Flasks in multi-neck configurations can be useful when a reaction requires the addition of reagents after it has begun.

Clamping the round bottomed flask

Once the correct glassware has been chosen, the apparatus can then be assembled. First, with a stirrer bar in the round-bottomed flask, it should be clamped securely and upright, with the clamp around the ground glass joint. This is the main support point for the for the apparatus, so it needs to be secure.

Hotplate and heating block

Underneath the round bottomed flask, a heat source will need to be introduced. The most commonly used is an electric stirrer hotplate with an aluminium heating block to transfer the heat. These are usually set up on a lab jack which enables the heat to be easily removed, without the need to adjust clamping of glassware.

Hotplate

The stirrer hotplate should be positioned centrally underneath the round bottomed flask. As the hotplate includes a magnet to cause the stirrer bar to rotate, positioning the hotplate centrally ensures that the stirring of the stirrer bar will be most effective.

Heating block

Heating blocks come in a range of sizes to match the different flask sizes. It is important that the heating block is of the correct size for the flask, so that there is a good contact between the flask and block, which allows for effective heat transfer. The heating block should be lifted into position using the lab jack (see 'Raising the lab jack into position').

Lab jack

For a stirrer-hotplate set-up, a raised lab jack should be placed under the heating apparatus as it allows the heat source to be removed quickly by lowering it away from the reaction flask. When setting up a lab jack under a reaction, you should ensure that the lab jack is set sufficiently high to allow the lab jack to be lowered enough to remove the heating from the reaction.

Raising the lab jack into position

At this stage, the glassware should be securely with a heating block, hotplate and lab jack positioned underneath the setup. There should be sufficient room to raise and lower the heat to and from the reaction. With the heating block centred on the hotplate, the setup should be positioned underneath the middle of the round bottomed flask, and the lab jack slowly raised to bring the heating block into contact with the round bottomed flask. The heating block needs to have a good contact with the heating block, so that there is effective heat transfer to the reaction, but the glassware must not be placed under any stress by over raising the lab jack. When it is in place, there should be free movement in the heating block around the flask, and there should be no movement of the flask in a vertical direction when the lab jack was raised. Glassware placed under stress is potentially dangerous, and can cause glassware to crack, particularly whilst being heated. However, if heating blocks are not in contact, an effective reflux will be difficult to achieve due to the poor heat transfer.

Addition of reagents

Next, the reagents should be added into the flask. Typically, solid reagents can be weighed and added using a weighing boat, and liquid reagents and solvents can be dispensed into a measuring cylinder, and added to the flask using a glass funnel.

Fitting the condenser

Once all the reagents have been added, the condenser can be placed into the ground glass joint of the round-bottomed flask. It should sit vertically in the flask.

Condenser clamping

A clamp can be placed lightly around the condenser, just to act as extra support, but should not be clamped tightly, as this can stress or break the ground glass joint.

Greasing of glassware

If the condenser requires greasing, this is the point at which the grease should be added. The use of grease should generally be avoided, as it can cause impurities in the reaction mixture, however in some cases it is necessary. The most common reason for using grease is if strongly basic chemicals (such as NaOH) are being used, as these can fuse the joints together. 

Condenser tubing

Thin-walled rubber tubing is used to take the water from the tap to the condenser, and then out of the condenser and into the sink. The water should go in the bottom of the condenser and out of the top. The tubing should be attached carefully, to ensure it doesn’t leak, as this can cause a mess in the workspace, as well as potentially becoming a serious hazard if it drips into equipment (eg. hotplate, oil bath). 

There are two main types of attachment, either glass inlets, where the tubing is attached directly to the glass, or screw thread condensers. These require tubing with plastic joints. The tubing should be first attached to the joint, and then screwed onto the condenser. Take extra care that the joints are screwed on correctly, as this is a point where leaks can occur.

The tubing should then be arranged so it is not in the way of the experimental work, for example they should not be hanging right next to the heat source. The tubes can be hooked behind clamps if necessary, and the water outlet tube can be secured in the sink by attaching on a clamp boss.

Checking the setup

At this point, it is generally a good idea to double check that all the apparatus is set up correctly, and all the clamps are appropriately tightened, because once the system has been heated, it can be harder to make adjustments to the setup.

Beginning heating and stirring

Water flow

Once the apparatus is fully set up, the water flow can be turned on. The optimal flow rate will vary between reactions, however in general a slow flow of water is adequate. Fast flows of water are wasteful, and can cause further problems as they pressurise joints.

Stirring

The stirring can then be turned on. The appropriate rate of stirring can vary between reactions, but generally the stirring should agitate and mix the reagents, and keep them suspended/dissolved in the solvent, without causing the liquid to rise too high in the flask or splash out of the flask.

Heating

After the stirring is in place, the heating can be switched on. The amount of heating may vary between reactions and is mostly dependent on the solvent. Higher boiling point solvents will require more heating to achieve reflux in a reasonable timeframe. Some heat sources (eg. Hotplates, isomantles) have a control to set the heat input, which may be indicated as a target temperature (°C) or as an arbitrary power rating. With a target temperature, the corresponding sensor generally detects the temperature of the heating element, which may be significantly higher than the temperature of the reaction vessel - for this reason, it is prudent to set the control to exceed (but not by too much) the boiling point of the solvent, in order to achieve reflux in a manageable time.

Acheiving reflux

As the reaction heats up, the solvent will approach its boiling point, and begin to bubble. Solvent vapour will begin to visibly condense inside the condenser, and drip back into the flask. This condensation should only be in the lower half of the condenser; if it rises higher, this indicates that too much heat is being supplied. Continuing to do this might lead to some of the solvent escaping the condenser, which is undesirable and potentially dangerous. Heat supply can be reduced slightly by moderating the heating controls, or reduced significantly by removing the heat source (i.e. lowering the lab-jack in the case of a stirrer-hotplate set-up). Once reflux is occurring as described, it can be safely left for the duration of the reaction.

Allowing to cool

Once the reaction has finished, the heat source should be switched off. When using hotplates, the heating mantle should be lowered away from the reaction flask to help the reaction cool down quicker, and to prevent the flask from fusing to the heating block as it cools.