Lecture 15 b Drying of Solvents Conventional Drying

Lecture 15 b Drying of Solvents

Conventional Drying Agents • Usually drying agents like anhydrous Na 2 SO 4 or Mg. SO 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 (Ca. SO 4), n=7 (Mg. SO 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 -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 other radicals (i. e. , oxygen), of oxidants and protic solvents (i. e. , water, alcohols) • Alternatively Li. Al. H 4 or Ca. H 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: Ca. O or Na/diethyl phthalate • Methanol: fractionated distillation, Na/dimethyl phthalate • Dimethyl sulfoxide • Reflux over Ca. H 2 • Dimethyl formamide • Stirring over anhydrous Mg. SO 4 • Acetone, acetonitrile • First drying over Ca. H 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.