Lecture 15b Drying of Solvents

Conventional Drying Agents Usually drying agents like anhydrous Na 2 SO 4 or MgSO 4 are used to dry many 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 (CaSO 4 ), n=7 (MgSO 4 ), n=10 (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 adsorbed by the drying agent and not “consumed”

Moisture Sensitive Compounds Why is a dry solvent important? Grignard reagents Cyclopentadienide Enolates Transition metal halides

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 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-dimethoxy- ethane 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 other radicals (i.e., oxygen), of oxidants and protic solvents (i.e., water, alcohols) 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, hexane, heptane, etc.

Chlorinated Solvents Never use alkali metals or alkali metal hydrides to dry chlorinated solvents because this will lead to violent explosions, sooner rather than later! Drying agents used here are calcium hydride (converted to Ca(OH) 2 ) or phosphorous pentoxide (converted to HPO 3 and H 3 PO 4 ) Reflux and distilled under inert gas The same reagents can be used for hydrocarbon solvents i.e., hexane, toluene, etc.

Other Solvents Alcohols Ethanol: CaO or Na/diethyl phthalate Methanol: fractionated distillation, Na/dimethyl phthalate Dimethyl sulfoxide Reflux over CaH 2 Dimethyl formamide Stirring over anhydrous MgSO 4 Acetone, acetonitrile First drying over CaH 2 and then over P 4 O 10

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 because 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 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.