Question 1

Why is degassing important in organic chemistry?

Accepted Answer

Degassing removes dissolved gases like oxygen and carbon dioxide from liquids, which is critical because these gases can cause unwanted reactions with sensitive reagents, interfere with spectroscopic measurements, or create bubble formation that disrupts experiments. Many organic synthesis techniques require degassed solvents to proceed successfully.

Question 2

How does Henry's Law explain freeze-pump-thaw degassing?

Accepted Answer

Henry's Law states that the mole fraction of dissolved gas is directly proportional to the gas's partial pressure above the liquid. Freeze-pump-thaw cycling exploits this principle by lowering pressure in the headspace, which decreases gas solubility and releases dissolved gases as visible bubbles from the frozen solvent.

Question 3

What equipment is needed to perform freeze-pump-thaw degassing?

Accepted Answer

Freeze-pump-thaw degassing requires a Schlenk line, which is a dual glass manifold with multiple ports that provides both vacuum and inert gas capabilities. You also need a clean, dry Schlenk flask, flexible tubing, a Dewar containing liquid nitrogen or dry ice for freezing, and a warm water bath for thawing the solvent.

Question 4

What precautions should be taken when filling a Schlenk flask with solvent?

Accepted Answer

Fill the flask with no more than 50 percent of its volume because many solvents expand upon freezing, which could cause the flask to shatter. Before use, inspect the flask carefully for cracks or fractures that might lead to breakage during the degassing process under vacuum.

Question 5

What happens during each freeze-pump-thaw cycle?

Accepted Answer

Each cycle involves freezing the solvent in liquid nitrogen, applying vacuum to evacuate the headspace and lower gas solubility, sealing the flask, then thawing in a warm water bath to release dissolved gases as bubbles. The process repeats until no more gas bubbles evolve, indicating complete degassing.

Question 6

How is a degassed solution protected after the freeze-pump-thaw process?

Accepted Answer

After degassing cycles are complete, the Schlenk flask is sealed under inert gas by opening the inert gas valve on the Schlenk line and then opening the flask stopcock to expose the solvent to an inert atmosphere. The flask and line valves are then closed, keeping the solution isolated from air and ready for use.

Question 7

What are common applications of degassing in chemical research?

Accepted Answer

Degassing is essential for organic synthesis where oxygen is detrimental, such as cadmium selenide nanocrystal synthesis. It is also used in studies like the Miller-Urey experiment, which requires a primordial atmosphere free of modern gases, and in preparing microfluidic devices where bubble-free polymer solutions are critical.

Degassing Liquids with Freeze-Pump-Thaw Cycling - Assessment Quiz