For some reactions, it is nescessary to maintain the reaction under a flow of a gas rather than using balloon/septa techniques to hold a reaction under a modified atmosphere. There are a number of possible circumstances where this is desirable:

• A gas is produced during the reaction, and a setup is required where the generated gas can be safely vented from the reaction and the reaction remains under the desired gas being introduced to the reaction.

• A gas is consumed during the reaction and it is nescessary to ensure the pressure doesn't reduce during the reaction.

• It is nescessary to 'degas' the reaction solvent, where a gas (typically nitrogen) is bubbled through the solution, and the setup needs to allow this gas flow to be released from the setup.

There are two separate components to these setups, which may be used individually, or more common in combination. These two components are:

• A nitrogen bubbler which usually consists of a ground-glass joint (cone) which can be placed into the socket of a round bottomed flask or condenser. These usually have an attachment for a nitrogen inlet and an oil bubbler outlet. The oil bubbler outlet allows the flow rate of the gas to be observed.

• A Pasteur pipette setup which is used to bubble a gas through the solution in the flask.

This section will explain how to set up a reaction where the solvent is degassed using a Pasteur pipette bubbler with a nitrogen flow, before removing the solvent degassing setup and switching to maintaining the reaction under a nitrogen atmosphere with a bubbler. In this setup, a 100 mL two-neck round bottomed flask (with stirrer bar) and condenser will be assembled, the solvent degassed, and the reaction setup held under nitrogen using the nitrogen flow attached to the bubbler on the top of the condenser.

This is a fairly common approach to using this setup, although other methods might be encountered. It is worth noting that it is uncommon to maintain a Pasteur pipette bubber during a reflux, as the flow of nitrogen is likely to result in the loss of solvent from the reaction. Hence, it is usual to degas the solvent first, then maintain the reaction under a flow of nitrogen from the top of the condenser, which minimises solvent loss from the setup.

A thermometer adapter is used to hold a Pasteur pipette in place, which needs to be adjusted to be a suitable length to reach below the surface of the solvent in the flask. Care needs to be taken to ensure the pipette doesn't get damaged by the stirrer bar in the flask. It is likely some trial and error adjustment will need to be made, and it is sensible to do this prior to adding any solvent or reagents into the reaction setup.

To fit a Pasteur pipette into a thermometer adapter you will need to carry out the following steps. Care should be taken whilst fitting the pipette, as these are particularly fragile, and may break which could result in injury. Care should also be taken when fitting or removing tubing to the pipette. Note: only low pressure gas sources should be used, where the pressure control is adequate for the reaction setup. The nitrogen supply is our teaching labs is suitable, the compressed air supply must not be used as this is at too high a pressure.

1. Dismantle the thermometer adapter. Designs vary, but typically these include a plastic screw-thread cap for the top, a glass cone with screw-thread and a rubber seal.

2. Slide the rubber seal onto the top of the pipette and slide it down the pipette to an appropriate point. This is more or less in line with what will become the top of the thermometer adapter

3. Place the plastic screw-thread onto the top of the pipette and bring it down to the rubber seal.

4. Place the glass cone over the bottom of the pipette and bring up to meet the plastic cap.

5. Carefully screw the glass adapter and cap together to seal the pipette.

6. Check the length of the pipette in your glassware and adjust as appropriate. You should dismantle the adapter to reposition the thread.

7. One the correct length has been obtained, tubing can be connected between the gas supply and the pipette. It may be nescessary to use Parafilm® to wrap around the top of the pipette to increase the diameter of the pipette so that the tubing can be slid over the pipette and form a good seal.

8. Fit the pipette into the round bottomed flask.

At this stage, solvent could be introduced and a gas bubbled through the solvent without a bubbler being fitted. This is sometimes used for reagent gases (eg carbon dioxide, hydrogen or oxygen), but it is more common to fit the bubber first.

A glass bubbler is used to sit on top of the reaction reflux condenser. The bulb of the bubbler needs to be half filled to form a seal, usually with oil. This is usually already present in the bubbler, and you should take care to keep the bubbler standing up to avoid losing oil from the bubbler.

The gas inlet will need to be sealed at this stage, as intially the setup will be purged by degassing the solvent using the pipette which has been assembled in the previous step. This can easily be achevied using a pipette teat which can be placed onto the bubbler.

Once both the pipette and the bubbler are in place, the setup can be used to degas the system. The precise configuration of this may be reaction dependent. E.g. for a setup which does not require dried solvent, the solvent could be added at this stage and degassing being carried out immediately. Otherwise, a purge step will be carried out first, followed by solvent addition, then a degassing process.

Purge

With the setup as shown above, the glassware is ready to be purged with nitrogen. The nitrogen will enter the setup via the Pasteur pipette and exit the system via the oil bubbler. The nitrogen should be turned on gently, observing the bubbling rate in the oil. A gentle continuous bubbling, which doesn't dispace the oil should be observed. It is important the pressure is not increased beyond what the setup is designed to manage. The limitations are the constriction in the pipette, which only allows a low gas flow, and the gas bubbling through the oil.

The setup should be allowed to purge appropriately, then solvent can be introduced to the flask via needle and syringe.

Degassing solvent

The solvent can be added to the setup, e.g. by carefully lifting the condenser slightly to insert a needle into the flask. The pipette connected to the nitrogen supply may need adjustment to ensure that the pipette tip is below the level of the solvent, but not impeding the stirrer bar (note the stirrer bar can easily break the pipette tip).

Removing pipette and transferring gas supply to the bubbler

The

Finishing the process