Transcript Chem 174-Lecture 14a..

Lecture 14a
Drying Solvents
Conventional Drying Agents
• Usually drying agents like anhydrous Na2SO4 or MgSO4
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
A + n H 2O
A*(H2O)n
• They adsorb varying amount of water (n=0.5 moles
(CaSO4) to n=10 moles (Na2SO4))
• 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 oC (2 torr), tetrahydrofuran peroxide: 62 oC (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 (Ph2CO.-Na+), which
is only stable in the absence of oxidants and water
• 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
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 HPO3, H3PO4)
• 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
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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 (Ph3SiCl)
for 24 hours.
Other Solvents II
• Dimethyl sulfoxide
• Contaminated by water
• Reflux over CaH2 and then distillation in vacuo
• Acetone
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• 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
CaH2 and afterwards over P4O10
Acetonitrile
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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.