Cannula transfer of tert-butyllithium on a shared Schlenk line
On a shared Schlenk line, a student plans to transfer 40 mL of 2.5 M tert-butyllithium in pentane (100 mmol) into a 500 mL flask containing 0.90 g of a ketone in 30 mL THF at −78 °C under argon. The written plan says: attach the t-BuLi bottle to the manifold with a long needle, open the bottle to the vacuum line to draw the liquid into a graduated dropping funnel, then open the funnel to the flask. The manifold is also being used by a nearby student to dry glassware; a Dewar of liquid nitrogen is on the bench. The t-BuLi bottle is old, and the septum has been punctured several times. The student wears a lab coat, nitrile gloves, and safety glasses, and intends to quench the residue with water in the fume hood after the addition.
1. Identify the most serious flaws in the transfer plan and describe a safer transfer for 40 mL of 2.5 M t-BuLi, including engineering controls and personal protection.
Read the worked answer
The plan uses vacuum on a pyrophoric organolithium solution. That is the critical error: pentane is volatile, so evacuation can concentrate t-BuLi, draw air into the bottle when the vacuum is released, and leave a dry, pyrophoric film on the needle or bottle neck. The old, multiply punctured septum adds a second ignition path. Use positive argon pressure throughout and transfer with either a dry, argon-purged syringe (50 mL, Luer lock) or a double-tipped cannula with a separate vent needle; never apply vacuum to the bottle. Clamp the bottle and receiving flask, use a blast shield, keep the Dewar and ignition sources away, and have a dry-powder extinguisher nearby. Nitrile gloves are not a barrier for t-BuLi; use appropriate chemical/cryogenic gloves and a face shield. The receiving flask must be vented through an oil bubbler, not a closed manifold.
2. Evaluate the proposed water quench for the residue. Give a stepwise quench that controls gas evolution and heat, and estimate the gas volume if 100 mmol of t-BuLi remains.
Read the worked answer
Direct water is unacceptable. t-BuLi reacts violently with water, and the 100 mmol scale can evolve about 2.2 L of isobutane at STP (about 2.4 L at 25 °C), with local overheating and spray. Cool the flask in an ice bath, keep it behind a shield, and add a less reactive proton source such as dry isopropanol diluted in THF dropwise with stirring; vent the flask through the manifold/bubbler. After gas evolution stops, add saturated NH4Cl slowly, then water, and finally dilute acid if needed. Test the quench for residual pyrophoric material before disposal; keep the quench in the hood and do not seal the flask immediately.