Laboratory procedures in organic chemistry typically utilize standard glassware equipment and techniques. Beakers and Erlenmeyer flasks are often used for simple mixing and holding solvents. Round-bottom flasks with either a short or a long neck are used for reactions under high heat or vacuum. This is because the round shape is more resistant to cracking under these conditions.
Heating reactions is a common lab procedure. However, a Bunsen burner is not generally used for organic compounds because they are flammable. When heating a reaction in a beaker or Erlenmeyer flask, a hot plate with a magnetic stirrer is typically used. When heating a reaction in a round-bottom flask, a water bath or heating mantle is used to accommodate the shape of the flask.
Many reactions must be heated to a certain temperature for a specific amount of time to proceed. These reactions are performed under reflux. A reflux setup uses a round-bottom flask containing a solvent whose boiling point overlaps with the optimal temperature of the reaction. A condenser is fitted onto the flask, and cold water enters at the bottom arm and exits out the top arm. As the mixture is heated and stirred, the solvent evaporates and then condenses back into the flask, preserving the reaction volume.
Another common technique used in the organic chemistry lab is filtration, which is a method to separate solids from liquids. Gravity filtration is the simplest filtration technique used to separate granular insoluble solids. This setup consists of an Erlenmeyer flask, a conical funnel, and a folded piece of filter paper. The mixture is poured into the funnel, and the liquid passes through into the flask, called the filtrate. The solids remain on the filter paper.
A faster filtration technique is vacuum filtration. Here, a Büchner funnel is connected to a filter flask using a rubber adaptor. The filter flask looks like an Erlenmeyer flask, but it has a side arm that connects the flask to the vacuum via vacuum tubing. Then, a flat piece of filter paper is placed inside of the funnel and wet slightly before the vacuum is slowly turned on. The liquid is sucked through the filter paper into the flask, while the solid is retained on the filter paper.
In this lab, you will perform a simple reaction to form benzoic acid, enabling you to practice simple organic chemistry techniques, like weighing solids, measuring volume, and performing vacuum filtration. You'll then practice the reflex technique using tetrahydrofuran.
Glassware in Organic Chemistry
There are standard pieces of glassware that are used in the organic chemistry lab. Beakers and Erlenmeyer flasks are typically used for simple mixing or to hold solvents but they should not be used to measure volume, unless only an approximate volume is needed. Glassware can be divided into the following categories:
Weighing Organic Reagents
Organic reagents can come in multiple forms, and it is important to practice good lab technique when obtaining these reagents for use in experiments. Always review a reagent's MSDS to determine potential hazards when working with it. Solid reagents should be measured out using weighing boats or weighing paper and the appropriate type of balance.
If a precise measurement is required, use an analytical balance. If a large amount of reagent is to be used, use a top-loading balance. Place a weighing boat or weighing paper on the balance and push the “tare” button. The tare feature will null the mass of the weighing paper. Always use spatulas to transfer solid reagents from the supply to the weighing boat. Never reuse the same spatula for different reagents as this can contaminate the stock reagents. And never return excess reagent to the stock bottle. Instead, discard it in the appropriate way as instructed.
When transferring the solid to the appropriate vessel, use a small funnel to carefully pour the solid inside. If any solid remains, use a small amount of the solvent to be used on the weighing boat and transfer it into the flask.
Organic liquids can be measured by using volumetric glassware, such as a graduated cylinder. Calculate the volume of the liquid using its density, then tare the reaction flask on the balance. Use a pipet or graduated cylinder to transfer the volume directly to the reaction flask. Follow the guidelines for the specific type of glassware. Never reuse the same piece of glassware for multiple reagents and never return excess reagent to the liquid stock bottles. For liquids, c
Heating Organic Reactions
Sometimes heat is required to enable an organic chemical reaction to occur. In the general lab setting, heat is commonly applied using a Bunsen burner with a direct gas flame. In organic chemistry labs, an open flame from a Bunsen burner can create a dangerous situation. Organic reagents, particularly solvents, are highly combustible and some form vapors with relative ease. For that reason, Bunsen burners are not used in organic chemistry labs.
Instead, heating baths, hotplates, or mantles are used to provide an indirect source of heat. Hotplates with magnetic stirring functionality are used to heat beakers and Erlenmeyer flasks. Heating mantles are designed to safely heat a round-bottom flask with varying volumes. Water baths are used when the temperature of the reaction does not need to exceed 100 °C. A reaction in glassware is immersed in a water bath that is heated by a hot plate. The temperature is modulated to the appropriate range. If the required temperature needs to exceed 100 °C but not 250 °C, a silicone mantle can be used. If the temperature must exceed 250 °C, a sand bath can be used.
Many reactions require being heated to a certain temperature for long periods of time in order for it to proceed. However, if a reaction is heated for a long period of time, the solvent may evaporate, causing a loss of reaction solution. Instead, a reflux setup is often used, which uses a round-bottom flask containing a solvent. The boiling point of the solvent overlaps with the optimal temperature of the reaction. The round-bottom flask is clamped to a stand, and a condenser is fitted onto the flask. Cold water flows through the condenser from the bottom arm to the top arm while the mixture is heated and stirred. As the mixture is heated, the solvent evaporates and then condenses back into the flask, preserving the reaction volume.
