Chemical reactions that are moisture-sensitive must be carried out in an anhydrous, or water free, environment.
Reagents and reactants can sometimes react with or absorb water from the atmosphere. If this happens, the chemical or physical properties of the reagents can change, and the desired reaction will not take place or lead to a poor yield.
To prevent undesired reactions with water from occurring sensitive reactions are carried out under an inert atmosphere, such as nitrogen or argon, using anhydrous reagents and equipment. Extremely water-sensitive reactions must be carried out inside a glovebox that can maintain an anhydrous environment. This video will demonstrate how to properly dry glassware, solvents, and reagents in order to run an anhydrous reaction.
The chemical makeup of glass causes a film of water to coat the surface that must be removed before preparing an anhydrous reaction. Heat or acetone is often used to remove this layer from clean glassware before use.
Many solvents also absorb water from the environment and must be dried before use. Solvent stills or desiccants are often used to remove water prior to setting up a reaction.
Solvent stills use alkali metals such as sodium to react with water and leave a residual water content of around 10 parts per million.
Desiccants are highly hygroscopic solids, meaning they readily absorb water. Certain desiccants, like sodium sulfate, are used to remove water from small amounts of an organic solvent and must be filtered out before further use.
Molecular sieves are the most commonly used desiccant and are used to dry larger volumes of solvents. They are made from a microporous material composed of sodium and calcium aluminosilicates.
Molecular sieves work by trapping water inside the beads effectively removing it from the solvent. Once used they can be regenerated in an oven.
Finally, there are multiple ways to dry solid reagents. One is by storing it in an oven set 15–20 °C below its melting point. The heat drives water from the reagent leaving behind a dry compound.
If the solid can't be heated or has too low of a melting point it can be dried in a vacuum desiccator. Once dry, the anhydrous reagent can be stored in a bottle under an inert atmosphere inside the desiccator.
Now that you've seen the concepts behind drying the equipment and reagents for anhydrous reactions, let's take a closer look and see how it's done in the laboratory.
To dry glassware in an oven, first gather all the required components for the reaction apparatus. Remove all pieces not made of glass such as the stopcock of a Schlenk flask.
Place the glassware in a drying-oven set to 125 °C and bake for at least 24 h before use.
After 24 h, put on heat protection gloves and remove the glassware from the oven. Assemble the apparatus while the glassware is still hot.
When the glassware is fully assembled and cool, flush the apparatus with an inert gas such as nitrogen. Finally, add back any pieces that were removed prior to drying. The glassware is now ready for the anhydrous reaction.
A faster option than oven drying glassware is to use a Bunsen burner. Certain glassware shouldn't be flame dried, so make sure the setup is safe to flame dry before starting. To begin, set up the full apparatus and remove all components that are not made of glass.
Put on heat resistant gloves, then light the Bunsen burner. Begin flame drying the glassware by heating the bottom of the apparatus. Drive the water out of the setup by moving the flame upward. Continue this process until fogging and steaming stops.
Wait for the apparatus to cool down to about 60 °C, then use heat resistant gloves and add the rest of the apparatus that was removed before flame drying.
To dry solvents using molecular sieves, first add them into a thermo-stable glass container.
The sieves must first be dried for proper operation, so place the container in a 300 to 350 °C oven and bake for 3 – 3 ½ h.
When the beads are dry, use high heat-resistant gloves to remove the container and store it in a drying oven at temperatures above 120 °C. After drying, the molecular sieves may be stored for weeks before use.
When they are needed, remove the beads from the drying oven or desiccator. Work fast and cover the container from this point onward to minimize contact of the beads with atmospheric water.
If removing the beads from an oven allow them to cool down to roughly room temperature.
Weigh out the necessary amount of active beads on a scale. For example, to achieve a 10% mass to volume of beads in a 500 mL bottle of solvent, 50 g of beads are required.
Add the beads to the solvent. For a volatile solvent, such as dichloromethane, leave the lid on top of the bottle but wait a few minutes before fully screwing the lid on to avoid pressure build-up.
Seal the area around the lid by wrapping it with Parafilm to keep moisture out. Store the solvent with the beads for at least 24 h. Afterward, the anhydrous solvent can be used in a reaction.
Alternatively, solvent stills using sodium metal and benzophenone can be used to dry solvents.
Solid reagents are often dissolved in organic solvents. Before removing the liquid and recovering the solid reagent excess water must be removed from the solution.
Obtain a dry container and add the solution. Next, add a drying agent to the container using a spatula. The drying agent will initially clump together, but continue adding until freshly added drying agent no longer clumps and moves freely.
Cover the container with a stopper or aluminum foil and allow the solution to sit for at least 1 h.
To remove the excess drying agent, assemble a vacuum-filtration apparatus with a Büchner funnel and side-arm flask. Add filter paper to the Büchner funnel, then turn on the vacuum.
Slowly decant the organic phase into the Büchner funnel. Avoid transferring the drying agent, as the filter may clog. When most of the liquid has been transferred onto the funnel and drained into the flask below, add the remainder with the drying agent and allow it to sit for a few minutes.
Turn off the vacuum and transfer the filtered solution into a dry round bottom flask. Connect the round-bottom flask to a rotary evaporator and remove all solvent under reduced pressure. The solid or liquid that remains should now be dry.
To dry an already solid reagent place the compound in an open container and determine its weight. Then place it into a drying oven set to a temperature below the melting point of the solid. Allow the reagent to dry for several hours inside the oven.
Remove the container from the oven and place it into a desiccator. Then, allow the sample to cool to room temperature. Reweigh and ensure that the mass is less than before oven drying. Repeat the drying steps until the weight no longer changes. When this happens the reagent is sufficiently dry.
If the reagent does not need to be used immediately, flush the container with an inert gas such as nitrogen and wrap Parafilm around the lid. Place the container inside a desiccator and store until the reagent is needed.
Anhydrous reactions are used in a variety of important organic chemistry syntheses.
A classic example for a reaction that must be done under anhydrous conditions is the Grignard reaction. In the first step of the reaction it is imperative that not even the smallest traces of water be present. In the presence of water the Grignard reagent will preferentially act as a base, resulting in the loss of the nucleophilic activity and form undesirable byproducts.
Many organic syntheses must be performed in extremely dry conditions, like with this example of organic magnet synthesis. The precursor material in this case, sodium metal, is pyrophoric, meaning that it is extremely moisture sensitive and can be flammable or even explosive when in contact with moisture in the air.
Lithium ion batteries are also extremely moisture sensitive and must be assembled in a glove-box or dry room. The negative electrode consists of a lithium compound, while the electrolyte contains a halogenated lithium salt. Since lithium is moisture sensitive any trace water introduced into the battery cell itself would diminish capacity.
You've just watched JoVE's introduction to Preparing Anhydrous Reagents and Equipment. You should now understand how to prepare glassware, solvents, and reagents that are all anhydrous for use in chemical reactions.
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