The Schlenk line (also vacuum gas manifold) is a commonly used
chemistry apparatus developed by
Wilhelm Schlenk.[1] It consists of a dual
manifold with several ports.[2] One manifold is connected to a source of purified
inert gas, while the other is connected to a
vacuum pump. The inert-gas line is vented through an
oil bubbler, while solvent vapors and gaseous reaction products are prevented from contaminating the vacuum pump by a
liquid-nitrogen or
dry-ice/
acetonecold trap. Special
stopcocks or
Teflon taps allow vacuum or inert gas to be selected without the need for placing the sample on a separate line.[3]
Schlenk lines are useful for safely and successfully manipulating moisture- and
air-sensitive compounds. The
vacuum is also often used to remove the last traces of
solvent from a sample. Vacuum and gas manifolds often have many ports and lines, and with care, it is possible for several
reactions or operations to be run simultaneously.
When the reagents are highly susceptible to
oxidation, traces of oxygen may pose a problem. Then, for the removal of oxygen below the ppm level, the inert gas needs to be purified by passing it through a deoxygenation catalyst.[4] This is usually a column of copper(I) or manganese(II) oxide, which reacts with oxygen traces present in the inert gas.
Techniques
The main techniques associated with the use of a Schlenk line include:
counterflow additions, where air-stable
reagents are added to the reaction vessel against a flow of inert gas;
the use of
syringes and rubber
septa to transfer liquids and solutions;[5]
cannula transfer, where liquids or solutions of air-sensitive reagents are transferred between different vessels stoppered with septa using a long thin tube known as a cannula. Liquid flow is supported by vacuum or inert-gas pressure.[6]
Glassware are usually connected by tightly fitting and greased
ground glass joints. Round bends of
glass tubing with ground glass joints may be used to adjust the orientation of various vessels. Glassware is necessarily purged of outside air by alternating application of vacuum and inert gas. The solvents and reagents that are used are also purged of air and water using various methods.
Filtration under inert conditions poses a special challenge that is usually tackled with specialized glassware. A Schlenk filter consists of sintered glass funnel fitted with joints and stopcocks. By fitting the pre-dried funnel and receiving flask to the reaction flask against a flow of nitrogen, carefully inverting the set-up, and turning on the vacuum appropriately, the filtration may be accomplished with minimal exposure to air.
Dangers
The main dangers associated with the use of a Schlenk line are the risks of an
implosion or
explosion. An implosion can occur due to the use of vacuum and flaws in the glass apparatus.
An explosion can occur due to the common use of liquid
nitrogen in the
cold trap, used to protect the vacuum pump from solvents. If a reasonable amount of
air is allowed to enter the Schlenk line, liquid
oxygen can condense into the cold trap as a pale blue liquid. An explosion may occur due to
reaction of the liquid oxygen with any organic compounds also in the trap.
Gallery
Vacuum/gas manifold setup: 1 inert gas in, 2 inert gas out (to bubbler), 3 vacuum (to cold traps) 4 reaction line, 5 Teflon tap to gas, 6 Teflon tap to vacuum
Vacuum/gas manifold setup: 1 inert gas in, 2 inert gas out (to bubbler), 3 vacuum (to cold traps), 4 reaction line, 5 double oblique stopcock (i.e. a glass tap with 2 separate parallel "channels/lines" that run diagonal to the axis of the tap)
The two reactants for an
aldol reaction are prepared in adjacent flasks, ready for one to be transferred to the other while maintaining air-free conditions
A yellow suspension is filtered through a sintered-glass funnel into another Schlenk flask under air-free conditions
The illustrations from Schlenk's 1913 paper are reproduced on pp. 960-963 of: Schlenk, Wilhelm (1924). "Organometallverbindungen [Organometallic compounds]". In Weyl, Josef (ed.).
Die Methoden der Organischen Chemie [Methods of Organic Chemistry] (in German). Vol. 4 (2nd ed.). Leipzig, Germany: Georg Thieme. pp. 720–978.
^Johansen, Martin B.; Kondrup, Jens C.; Hinge, Mogens; Lindhardt, Anders T. (13 June 2018). "Improved Safety during Transfer of Pyrophoric tert-Butyllithium from Flasks with Protective Seals". Organic Process Research & Development. 22 (7): 903–905.
doi:
10.1021/acs.oprd.8b00151.
S2CID103573742.
^Brown, H. C. "Organic Syntheses via Boranes" John Wiley & Sons, Inc., New York: 1975.
ISBN0-471-11280-1.