CHEM 3820 / Experiment 1: Schlenk Line

What is the purpose of Schlenk line techniques?

Safely work w/ air- and/or moisture-sensitive chemistry.

What is the experimental glassware connected to? Why?

Connecting glassware to vacuum to remove volatile material or to an inert gas to flush out unwanted chemistry in the glassware.

How does a Schlenk line generally work?

> One long tube connected to vacuum pump, one long tube connected to a source of inert gas.

> Both tubes connected to glassware using three-way stopcocks.

> Glassware to be connected to either tube via stop-cock adjustments.

> Allow control of the environment rxns exposed to.

How can the vacuum pump be safely turned on?

> Apparatus is clean.

> Vacuum side of system clean.

> Trap is clean & free of material condensed in earlier experiments.

> Drain trap into appropriate waste disposal.

> Check connection btwn trap & vacuum if connected.

> Turn on pump.

> Allow pump to draw air from system for several minutes.

> Place Dewar flask on trap and fill Dewar w/ liquid N₂.

> Turn inert gas line on, and observe bubbles flowing thru bubbler → systems go.

How can we safely remove air, e.g. O₂ and CO₂, and replace it w/ an inert gas?

FPT cycle: removing dissolved gases from solvent/solution.

How does each of the steps in the FPT cycle explain how it works?

> Freeze: Sample frozen solid w/ liq N₂ bath, dry ice, or acetone bath, trapping solute, while gases can’t escape.

> Pump: Vacuum applied, removes headspace gas above frozen sample.

> Thaw: Sample melts, trapped dissolved gases escape into headspace.

> Back-fill w/ N₂ to keep O₂ and H₂O out, restore safe pressure inside glassware so it’s not left under vacuum, and sets inert atmosphere.

> Repeat 3x for thorough degassing.

How does the FPT cycle specifically work?

> Setup: Add ~25mL distilled H₂O.

> Setup: Seal flask w/ greased glass stopper & attach side-arm to Schlenk line.

> Freeze: Seal stop-cock & place flask into dry ice/acetone bath.

> Pump: Once liq has been cooled, open stop-cock to vacuum side of Schlenk line & pump for 3–5mins.

> Pump: Ensure liq submerged in bath.

> Thaw: Seal flask again & remove cooling bath, allowing flask to warm slowly.

> Repeat 3x for thorough degassing.

> Back-fill w/ N₂ from N₂ line & disconnect flask from system.

Why does a degassed liquid impact glass walls more sharply?

No gas bubbles to cushion movement → liquid mass slams wall directly.

Where is the cold trap placed in a Schlenk line? What is its purpose?

> Btwn vacuum line & vacuum pump.

> To condense volatile vapours, protecting  the delicate mechanism of the pump from volatile materials in our sample.

What happens if volatiles reach the vacuum pump?

> Volatiles dissolve / contaminate pump oil, corrode internals, and ruin seals.

> Loss of lubrication.

> Vacuum performance.

Why is liquid N₂ used to cool the trap?

At -196 °C, it condenses most volatiles before they reach the pump.

Why is liquid oxygen formation dangerous in the trap?

O₂ condenses at -183 °C; leaks can form liquid O₂, which is highly reactive with organics → explosion risk.

What are the three considerations to keep in mind before turning off the Schlenk apparatus?

> Trap may have condensed volatile substances. Allow trap to protect pump from these.

> Vacuum pump contains large reservoir of pump oil. Do not leave pump connected to vacuum while turned off, so that oil may not be drawn up into system.

> O₂ may have condensed if leaks present in system.

How to safely turn off the vacuum pump when we are done working w/ Schlenk line?

> Ensure glassware is filled w/ inert gas & sealed.

> Ensure everything connected to N₂.

> Ensure nothing connected to vacuum side of system.

> Ensure stop-cocks on Schlenk line are closed to vacuum.

> Turn off pump & remove Dewar flask, checking if there is condensed O₂.

> If no O₂, open release valve to allow vacuum line to fill w/ air.

> Disconnect trap from system to thaw and ensure pump is open to air.

What do you do if there is leftover condensed O₂ in the trap?

> Return Dewar to trap.

> Turn pump on.

> Inform instructor.