Download Lecture 15b

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:
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A + n H 2O
A*(H2O)n
They adsorb varying amount of water (n=0.5 (CaSO4),
n=2 (K2CO3), n=6 (CaCl2), n=7 (MgSO4), n=10 (Na2SO4)).
Drierite is CaSO4 with 3 % CoCl2 as indicator.
It is blue when it is dry and pink when it is wet.
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”.
Why do we use them if they do not do such a great job?
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.
• 1,2-dimethoxyethane and diglymes are other commonly
used ethers in synthesis are (both display a higher
boiling point than diethyl ether and tetrahydrofuran) due
to their larger molecular weights and higher polarities.
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).
• The formation of hydrogen gas makes the reaction 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
Setup
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.