Drying of solvents

So far, you have usually used solvents without or little pretreatment. However, certain reactions such as a Grignard reaction, an Aldol or a Wittig reaction require anhyrous solvents. It is imperative for the success of the reaction that the solvents used is water free. Significant amounts of water will cause a reaction to cease soon or not to start at all e.g. if you use 95% ethanol instead of absolute ethanol in the aldol experiment. The requirements on purity are much more rigorous in upper division courses where students work with organometallics which are extremely moisture and air sensitive. Trace amounts of impurities can decompose the starting materials (H₂O for TiCl₄), the intermediate (H₂O for RMgBr) or the desired product (O₂ for CoCp₂).

The most common drying agents in the laboratory section will be anhydrous salts of sodium sulfate, magnesium sulfate, calcium chloride, etc. They will be sufficient for most purposes because the compounds that you are handling are not extremely moisture sensitive. However, if you use a solvent like absolute ethanol, make sure that you close the bottle that you took it from right away. A lot of the polar solvents are very hygroscopic (=absorb water from the air).

In the upper division laboratory courses you will be expected to dry the solvents yourself. Here is a summary of the common techniques for the most common solvents:

1. Ethers like tetrahydrofuran, diethylether or dimethoxyethane are frequently used as solvents. They tend to form peroxides, especially when improperly stored (open bottles and in light). These compounds have a higher boiling point than the ethers themselves and are extremely explosive. If you are in doubt about the quality of the ether that you are about to use, test the solvent with a slightly acidic, aqueous solution of potassium iodide. If the organic phase turns yellow or brown, the ether contains significant amount of peroxides that must be removed first. Tetrahydrofuran is much more hygroscopic than the other ethers and should be pre-dried with potassium hydroxide. The actual drying is done with a mixture of sodium metal and benzophenone. If the ether is dry and air-free, the solution turns dark blue due to the appearance of the ketyl radical.

The radical reacts with water, alcohols and oxygen. Alternatively, LiAlH₄ or NaH are used for ethers. In the past, a mixture or potassium and sodium metal was used (K/Na, Na/K), but it can cause explosions because it is extremely reactive and a liquid at room temperature.

2. Hydrocarbons like pentane, hexane, cyclohexane, benzene, toluene are another important class of solvents. These solvents are generally not as hygroscopic as ethers, but often contain unsaturated compounds that have to be removed by washing the solvent with concentrated sulfuric acid. After removing the acid, they can be died over CaH₂ or sodium metal, depending on the quality needed. For most purposes, several hours of reflux, follow by a distillation under inert gas is sufficient. If possible, the use of benzene should be avoided due to its carcinogenic effect.

3. Halogenated solvents (CH₂Cl₂, CHCl₃, CCl₄, C₂Cl₄, C₂Cl₆) can be dried over CaH₂ or P₄O₁₀ after pretreatment with sulfuric acid. Never attempt to dry halogenated solvents with alkali metals or alkali metal hydrides! This will cause violent explosions due to a radical reaction! Whenever possible, these solvents should be replaced with non-halogenated solvents. The combustion of chlorinated solvents generates a high amount of dioxines (Seveso toxin).

4. Alcohols can also be dried with CaH₂. There are several special procedures for various alcohols e.g. ethanol can be dried with sodium metal/phthalic acid diethyl ester.

5. Solvents like acetonitrile, dimethylsulfoxide (DMSO) and acetone can be dried with CaH₂ and then distilled under inert gas. Acetonitrile and acetone can be dried over P₄O₁₀ to remove trace amounts of water. In the presence of larger amounts of water, these compounds form condensation products and have to be pre-dried with Calcium hydride. Other procedures involve the use of anhydrous magnesium sulfate (dimethyl formamide, DMF). The purified solvents should be stored under an inert atmosphere over molecular sieve.