So, when you do real chemistry and are able to use a manifold appropriately you can start to look at some genuinely difficult chemistry requiring dry conditions. The dry (or anhydrous) reaction is quite commonplace in synthetic labs and you will most likely come across it in your career, probably very early on. Once again, I suggest reading APOC as it details the use and setup of a manifold, the ways in which to perform anhydrous reactions and also how to dry solvents. I will hopefully be able to outline a few key steps in how we dry solvents in the lab at present and the few tricks I have picked up along the way.
- So to start you need to select your solvent and identify the ideal way of drying it. A great companion for any lab monkey is Purification of Laboratory Chemicals. This book will also show you ways in which to purify any other chemical in your lab inventory (some are not there but a lot are). So, find your solvent in this book and it is generally ideal to look for the rapid purification and drying methods. In our case we will use THF as an example.
- The next step is to set up your equipment so that the solvent stays dry. First, wash your glassware with acetone to remove any water and also some pure diethyl ether to remove any crud from the acetone. DO THIS IN A HOOD and remove as much solvent from the glassware as possible. Next, I suggest putting your glassware (Distilation apparatus, collecting RBF etc.) in a hot oven overnight to ensure any solvent or water is well and truly gone. When you assemble the equipment the next day, take it out of the oven while hot, assemble and put under vacuum to ensure no condensation occurs (You may have to use another empty RBF to get vacuum ;)). Once dry and cool put the glassware under argon. You should also use a second RBF to use initially to catch the first fractions of the distilation as some impurities or lighter solvents can come through. Next, our book tells us a number of ways to dry a solvent but in our lab we use a still to distill this solvent. However, the use of a distilation bridge can just as easily work when you have no space for a still.
- The use of activated molecular sieves or some other innocuous dried material (e.g. KOH for TEA) can be placed in your dry collection flask to prevent the solvent becoming wet over time. It is also a good idea to grease the farthest part of all joints lightly with silica grease to ensure an airtight seal.
- Now it is time to dry our solvent. The drying agent used is usually sodium metal (either wire or pressed) and benzophenone as an indicator (benzophenone turns blue when complexed with Na in an atmosphere free of oxygen and water). The benzophenone undergoes a reaction with sodium to give non-volatile products that appear blue (due to the nature of the complex) when the solvent is dry and the atmosphere free of oxygen. If the solution is wet the sodium forms NaOH and takes the water out of solution as this salt. As an option another still can be set up to pre-dry the THF using CaH2. This prevents excess consumption of the sodium and benzophenone. N.B. Not a good idea to dry chlorinated solvents with sodium. Always make sure you read the procedure.
- Once you have your solvent you wish to dry, add your drying agent. In the case of sodium it is a good idea to have your solvent under argon and to crush the benzophenone onto the sodium. Try to be quick as the sodium can catch fire in the air, ensure you have a lot of the mineral oil on the sodium so that it is not exposed to air. Place the sodium with crushed benzophenone on it into the solvent and attach this flask to your distilation apparatus.
- Begin your distilation at the required temperature making sure you have a sufficient cooling system in place (i.e. if your solvent boils at lower temperatures then cool it well with ice water, but if it boils high or freezes at low temps then r.t. water is not a problem). Do not boil the still dry, this is dangerous, simply leave enough solvent to cover the drying agent completely.
- Once the distilation is complete, disconnect your collection flask ensuring that it is kept under argon, stopper (using a small amount of grease at the top of the neck) and place to one side.
- Quenching: Scary stuff if you don't do it right. Initially to quench something like CaH or Na you want to use something that will react with the agent but in a controlled way, for these two agents iPrOH (isopropyl alcohol) is a good choice. iPrOH is hindered and will not react quickly with the agent. First, cool the remaining solution on ice. Next slowly add iPrOH (add a decent amount, usually around 50-100mL) and allow to stir for 15min or so. Now you can add EtOH or MeOH (DO NOT ADD WATER) and continue to stir it on ice for a time. Eventually, you can start to add water dropwise to completely quench the solution but be mindful of the temperature of the solution and the development of bubbles. This is a warning that the quench is occuring and usually occurs quite rapidly. Then you can dispose of your waste in an appropriate waste drum and glory in your dry solvent.