Air-free technique

Air-free techniques refer to a range of manipulations in the chemistry laboratory for the handling of compounds that are air-sensitive.

These techniques prevent the compounds from reacting with components of air, usually water and oxygen; less commonly carbon dioxide and nitrogen.

In contrast when using a Schlenk line the purge-and-refill is applied directly to the reaction vessel through a hose or ground glass joint that is connected to the manifold.

By providing a sealed but recirculating atmosphere of the inert gas, the glove box necessitates few other precautions.

Cross contamination of samples due to poor technique is also problematic, especially where a glovebox is shared between workers using differing reagents, volatile ones in particular.

In their more liberal use, gloveboxes are used for the entire synthetic operations including reactions in solvents, work-up, and preparation of samples for spectroscopy.

Certain volatile chemicals such as halogenated compounds and especially strongly coordinating species such as phosphines and thiols can be problematic because they irreversibly poison the copper catalyst.

In the more liberal use of gloveboxes, it is accepted that the copper catalyst will require more frequent replacement but this cost is considered to be an acceptable trade-off for the efficiency of conducting an entire synthesis within a protected environment The other main technique for the preparation and handing of air-sensitive compounds are associated with the use of a Schlenk line.

The main techniques include: Glassware are usually connected via tightly-fitting and greased ground glass joints.

Water is removed by piping the gas through a column of desiccant such as phosphorus pentoxide or molecular sieves.

Thereafter, the stopcock is closed and the solvent is thawed in warm water, allowing trapped bubbles of gas to escape.

Although traditional drying techniques involve distillation from an aggressive desiccant, molecular sieves are far superior.

[5] Aside from being inefficient, sodium as a desiccant (below its melting point) reacts slowly with trace amounts of water.

Drying can also be achieved by the use of in situ desiccants such as molecular sieves, or the use of azeotropic distillation techniques e.g. with a Dean-Stark apparatus.