Esecuzione di reazioni al di sotto della temperatura ambiente

Certain chemical reactions must be performed below room temperature for safety or to obtain the desired product.

A cooling bath allows for a system to be maintained at a certain temperature range for the duration of the reaction. This is achieved by placing the reaction flask into the bath, cooling the reaction without ever having direct contact with the reagents.

The bath is typically a well-insulated vessel such as a Dewar flask containing the cryogenic components necessary to reach the desired temperature. In simple setups like this, temperature is not stable, and the bath must be monitored and adjusted throughout the procedure.

This video will explore the different cooling baths regularly used to carry out reactions below room temperature.

During a chemical reaction the species involved must collide for new bonds to form. Raising the temperature increases the internal energy of the system and will cause these species to move more quickly, meaning they will collide more often. As a result, reactions proceed faster at higher temperatures.

However, in some cases, it is desirable to carry out reactions at low temperatures, despite the lowering of the rate of reaction. For example, some reactions are too vigorous, and must be cooled to prevent spilling and pressure build up. Highly exothermic reactions could also rapidly boil over and spurt out if not cooled, creating a safety hazard.

Cooling can be utilized to provide an economic benefit. For example, preventing the boiling off of a solvent or the decomposition of a reagent saves both time and resources.

Cooling is also frequently used to control which product is yielded by a reaction that has competing pathways. In these reactions the pathway with the lower activation energy is generated at lower temperatures, while the pathway with the higher activation energy is preferred at higher temperatures. 

Now that you understand the importance of running reactions below room temperature, let's take a look at how to prepare various types of cooling baths.

Ice-water baths are easy to set up, and are available in every teaching chemistry laboratory. While ice-water itself has a temperature of 0 °C, a melting-point depression can be achieved by the addition of certain salts.

This allows ice-water baths to reach a temperature of -40 °C. The final temperature can be adjusted by increasing or decreasing the concentration of salt additive.

To set up an ice-water bath, begin by weighing the appropriate amounts of ice and salt additive, as outlined in the ice-bath table found in the text protocol.

Next, add the salt to the ice. Pour a small amount of deionized water into the container. Using a stirring rod, mix the bath thoroughly.

Now that the bath has been set up, check with a thermometer to ensure that the desired temperature has been reached. If it has not, add more salt as necessary. When the correct temperature is reached, place the reaction vessel into the ice bath.

Ice-water baths do not retain their temperature long, and need to be adjusted every 20–30 min. To maintain the target temperature, it may be necessary to remove the liquid water and add more ice and salt.

For temperatures down to -78 °C, dry-ice baths are utilized. Dry-ice is solid carbon dioxide, so efficient heat-transfer from it to a reaction vessel requires a solvent. Because dry-ice sublimes at -78 °C, a solvent with a freezing point below that must be used if this temperature is to be reached. Solvents with higher freezing points can be utilized to create warmer dry-ice baths. To prepare a dry-ice bath, begin by putting on cryogenic protection gloves and safety goggles. Never let dry-ice touch bare skin.

For a 1 L bath, obtain about 1/3 of a block of dry-ice and break it into smaller pieces into the container.

Next, slowly add the chosen organic solvent to the dry-ice while stirring with a glass rod. There will be a vigorous fizzing as carbon dioxide gas develops.

Continue to slowly add solvent and stir until most of the dry-ice dissolves, forming a homogenous slurry. This ensures that heat transfer to the reaction flask is as uniform as possible.

Using a cold temperature thermometer or thermocouple, ensure that the bath has reached the desired temperature, then place the reaction vessel into the bath.

Monitor the bath in regular intervals, and add chunks of dry-ice when a rise in the bath temperature is noticed.

Finally, when the desired bath temperature is below what dry-ice can provide, liquid nitrogen is utilized. Liquid nitrogen has a melting point of -196 °C, and solvents are only needed when creating warmer baths.

Due to the extremely low temperatures of liquid nitrogen, a Dewar is the only acceptable vessel.

To prepare a liquid-nitrogen cooling-bath, begin by putting on safety goggles and cryogenic protection gloves. Use care when handling liquid nitrogen, as it can cause frostbite and permanent eye damage.

For a bath with additives, determine the appropriate organic solvent for the desired temperature, as shown in the liquid nitrogen table found in the text. Add the solvent to the Dewar, then slowly add the liquid nitrogen.

Insert a cold-temperature thermometer or thermocouple into the bath to ensure that the desired temperature has been reached. Then, place the reaction vessel into the bath.

For a bath without additives, simply add the appropriate amount of nitrogen to the Dewar to obtain a temperature as low as -196 °C.

Monitor the bath in regular intervals to see if additional nitrogen is needed.

Many different types of reactions across various scientific disciples utilize cooling baths to operate below room temperature.

Mechanical laboratory processes, much like very exothermic reactions, can also create undesirable heat.

In this example bulk barium copper tetrasilicate was prepared through both solid state and melt flux synthesis. Then, these layered materials were exfoliated using sonication techniques.

Sonication uses sound waves to agitate particles. However, because it is a high-energy process, it can create excess heat in a sample.

Therefore, an ice-water bath was used to cool the sample during the one-hour sonication process. Preventing this excess heating ensured the integrity and consistency of product yield.

In this example, a dry-ice bath was used to ensure that diiodomethyllithium was synthesized by deprotonation of diiodomethane.

Reagents were added to a round-bottomed flask containing a stir bar. Then, the round-bottomed flask was placed in a Dewar. Dry-ice and acetone were added to the Dewar, and the entire apparatus was covered to minimize exposure to light. Maintaining low system energy was essential for the stability of the product.

Dry-ice and liquid nitrogen baths are frequently used as cold traps to condense samples. In particular, these cold traps can aid the safe transport of air-sensitive compounds while preventing contamination of equipment. In this example, a liquid nitrogen cold trap was used to condense a volatile and oxidation sensitive sample, for later preparation for mass spectrometrical analysis.

The system was first cleaned and heated, to remove any potential contaminants. The lockable test tube was then submerged in liquid nitrogen, to allow for condensation of the sample through the Schlenk line. The sample was then removed for analysis through mass spectrometry.

You've just watched JoVE's introduction to conducting reactions below room temperature. You should now understand ice-water, dry-ice, and liquid nitrogen cooling baths, and why they are chemically important.

Thanks for watching!