Lecture 14a Drying Solvents

Conventional Drying Agents Usually drying agents like anhydrous Na 2 SO 4 or MgSO 4 are used to dry organic solutions They remove the majority of the water but not all of it because the drying process is an equilibrium reaction They adsorb varying amount of water (n=0.5 moles (CaSO 4 ) to n=10 moles (Na 2 SO 4 )) Their efficiency is measured by intensity, capacity and velocity can greatly vary from one solvent to the other Problem: The water is just absorbed by the drying agent and not “consumed”

Moisture Sensitive Compounds Why is a dry solvent important? Grignard reagents Cyclopentadienides Transition metal halides Hydrolysis

Ethers I Ethers are very commonly used solvents because of their ability to dissolve a broad variety of compounds Many ethers are hygroscopic due to their polarity and their ability to form hydrogen bonds with water Most ethers react with oxygen in air in the presence of light to form explosive peroxides, which have higher boiling points that the ethers themselves (diethyl ether peroxide: 40 o C (2 torr), tetrahydrofuran peroxide: 62 o C (2 torr)) Diethyl ether and tetrahydrofuran are often inhibited with BHT (3,5-di-tert.-butyl-4-hydroxytoluene), which is also used as anti-oxidant in cosmetics, pharmaceuticals, etc. Other ethers used in synthetic work are 1,2-dimethoxyethane and diglyme (both display a higher boiling point than diethyl ether and tetrahydrofuran)

Ethers II Purification Step 1: Test for peroxides with KI-starch paper (turns dark blue) or acidic KI-solution (turn yellow-brown) in the presence of peroxides Step 2: Removal of water and peroxides by treatment with sodium/benzophenone (color change from beige to dark blue) Due to the formation of hydrogen gas the reaction because irreversible The dark blue color is due to a ketyl radical anion (Ph 2 CO.- Na + ), which is only stable in the absence of oxidants and water Alternatively LiAlH 4 or CaH 2 can be used as drying agents for less rigorous applications This approach can also be used for many hydrocarbons i.e., toluene

Chlorinated Solvents Never, ever use alkali metals (i.e., Na, K, K/Na) or alkali metal hydrides (i.e., NaH) to dry chlorinated solvents since this will in most cases lead to violent explosions, sooner or later! Drying agents to dry chlorinated solvents are calcium hydride (converted to Ca(OH) 2 ) or phosphorous pentoxide (converted to HPO 3, H 3 PO 4 ) They wet solvent is refluxed for several hours and then distilled under inert gas The same reagents can be used for hydrocarbon solvents i.e., hexane, toluene, etc.

Other Solvents I Water Dissolved salts may be removed by distillation or ion exchange. It can be degassed purging it with an inert gas for an extended time. Alternatively, several freeze-pump-thaw cycles can help to remove dissolved gases (i.e., oxygen). Alcohols Alcohols are mainly contaminated with varying amounts of water Ethanol: CaO or Na/diethyl phthalate Methanol: fractionated distillation, Na/dimethyl phthalate N,N-Dimethylformamide Dimethyl formamide (DMF) is contaminated by dimethylamine Anhydrous magnesium sulfate is used to remove the majority of the water (final concentration: ~ 0.01 M) followed by a vacuum distillation. For higher quality, the pre-dried solvent can stored over BaO before it is distilled over alumina (50 g/L). The pre-dried solvent can be refluxed with triphenylchlorosilane (Ph 3 SiCl) for 24 hours.

Other Solvents II Dimethyl sulfoxide Contaminated by water Reflux over CaH 2 and then distillation in vacuo Acetone Acetone is contaminated by aldehydes (i.e., acetaldehyde), which can be removed by treatment with silver nitrate or potassium permanganate For less rigorous applications, drying over anhydrous calcium sulfate or potassium carbonate provides good results For more sensitive application, the pre-dried solvent can be refluxed over CaH 2 and afterwards over P 4 O 10 Acetonitrile Acetonitrile is contaminated with acetamide, ammonia and ammonium acetate. Often times, it is pre-dried with calcium hydride and then refluxed over phosphorus pentoxide. If the pre-drying step is skipped, the formation of an orange polymer will be observed during the drying process.

Summary Removal of water and other compounds is important to maintain the quality of the reagents, optimize yields and reduce undesirable side reactions Obtaining very pure solvents can be an arduous task in some cases since the purification usually involves many steps and extended reflux in most cases The purified solvents are often stored under inert gas and over a molecular sieve to keep them dry for some time (Note that the molecular sieve has to have the correct porosity (i.e., 4 Å) and also has to be properly activated prior to its use!) Maintaining the solvent purification systems is also very important to avoid unpleasant surprises i.e., disintegrating flasks, explosion due to the build-up of peroxides, etc.