Air-free Technique - Alternatives

Alternatives

Both these techniques require rather expensive equipment and can be time consuming. Where air-free requirements are not stringent, other techniques can be used. For example, using a sacrificial excess of a reagent that reacts with water/oxygen can be used. The sacrificial excess in effect "dries" the reaction by reacting with the water (e.g. in the solvent). However, this method is only suitable where the impurities produced in this reaction are not in turn detrimental to the desired product of the reaction or can be easily removed. Typically, reactions using such a sacrificial excess are only effective when doing reactions on a reasonably large scale such that this by-reaction is negligible compared to the desired product reaction. For example, when preparing Grignard reagents, magnesium (the cheapest reagent) is often used in excess, which reacts to remove trace water, either by reacting directly with water to give magnesium hydroxide or via the in situ formation of the Grignard reagent which in turn reacts with water (e.g. R-Mg-Hal + H₂O → HO-Mg-Hal + R-H). To maintain the resultant "dry" environment it is usually sufficient to connect a guard tube filled with calcium chloride to the reflux condenser to slow moisture re-entering the reaction over time, or connect an inert gas line.

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.