Tecniche di chimica organica di base

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:

1. Volumetric Glassware – This includes glassware such as graduated cylinders, volumetric glass pipettes, and volumetric flasks. The purpose of these items is to measure or dispense a precise volume of a substance. A graduated cylinder is generally used to measure volumes in an organic lab. The graduated cylinder has a long cylindrical shape with volume markings on the side. It is essential to use the smallest graduated cylinder possible for the volume needed in order to minimize error. For example, a 25-mL graduated cylinder is adequate to measure 20 mL of reagent, but it is not accurate to measure a smaller volume, like 5 mL. However, do not use a 25-mL graduated cylinder to measure 100 mL by measuring four 25-mL volumes; it is better to use the 100-mL graduated cylinder.
2. Reaction Containment – These specialized pieces can be used to house an organic chemical reaction. Round-bottom flasks are an example of this glassware. Unlike Erlenmeyer flasks, round-bottom flasks provide more surface area and therefore provide more uniform heating or cooling. Round-bottom flasks are used to perform reactions under high heat or vacuum because their round shape is more resistant to cracking. In this category are also specialized glassware known as condensers, which are used to collect gaseous vapors and condense them back into the liquid phase to be collected.
3. Filtration – These pieces of glassware are mainly used to help isolate precipitates out of a reaction mixture. To isolate solid precipitates, a filtration apparatus is required. The simplest setup utilizes a glass funnel attached to an Erlenmeyer flask. Inside the funnel is a piece of filter paper. The reaction mixture containing the solid is poured into the funnel, and gravity will pull the liquid down into the flask, leaving behind the solid. This is known as gravity filtration. However, this setup may require a long period of time to filter the solid. A vacuum filtration setup replaces the Erlenmeyer flask with a vacuum flask with a side arm and the glass funnel with a porcelain filter funnel known as a Büchner funnel. This side arm is attached to a vacuum source. Like before, the reaction mixture is poured into the filter funnel and the filtration is aided by force from the vacuum.
4. Separation – These specialized flasks or funnels are used mainly for liquid-liquid phase extractions, which is a technique that allows for the separation of products by using solvents with different polarities or hydrophobicities. These immiscible liquids will form a heterogeneous solution that separates into two phases. Separatory funnels have a valve known as a stopcock at the bottom, allowing for separation of the phases by pouring them into a new container.
5. Transport – This glassware mostly consists of bottles or vials with a corresponding cap. This glassware is used primarily to transport potentially hazardous material safely.
6. General use – This category is comprised of glassware such as beakers, Erlenmeyer flasks, glass rods, and any other miscellaneous pieces.

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.