Transcript Lecture 15b - University of California, Los Angeles

Lecture 15b
Drying of Solvents
Conventional Drying Agents
• Usually drying agents like anhydrous Na2SO4 or MgSO4
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
A + n H2O
A*(H2O)n
• They adsorb varying amount of water (n=0.5 (CaSO4),
n=7 (MgSO4), n=10 (Na2SO4))
• 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 (Ph2CO.-Na+), which
is only stable in the absence of other radicals (i.e., oxygen), of oxidants
and protic solvents (i.e., water, alcohols)
• Alternatively LiAlH4 or CaH2 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
HPO3 and H3PO4)
• 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 CaH2
• Dimethyl formamide
• Stirring over anhydrous MgSO4
• Acetone, acetonitrile
• First drying over CaH2 and then over P4O10
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.