Drying solvents

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Having properly functioning solvents is an absolute necessity for fine home chemistry work, especially for those interested in organic chemistry. Removal of water and other impurities from these which is often required for store-bought or homemade solvents, can be accomplished by many means, depending on the solvent.

Drying with an anhydrous salt

Probably the most commonly used method for removing water from a solvent is by using the anhydrous form of a salt as desiccant. In this process, which is useful for both polar and nonpolar solvents, involves adding the salt (such as anhydrous magnesium sulfate, sodium sulfate, calcium sulfate, calcium chloride or compounds that react with water forming precipitates such as calcium oxide, calcium hydride) directly to the solvent, followed by one or more careful distillations to avoid drawing water out of the hydrated salt. If water is present, a finely powdered anhydrous salt tends to "clump up" upon absorbing the water. Desiccants used for this process typically need to be checked beforehand to ensure that they will not react with the solvent in any way. Though it isn't a salt, sodium metal is often used in the form of shavings in the same manner with nonpolar organic solvents due to its extreme reactivity towards water. Sodium should only be used in very clean solvents with a LOW water content, otherwise the risk is run of explosions and fire.

Salting out

Salting out is a technique that takes advantage of one solvent's reduced solubility in a solution of some compound relative to its solubility in pure water. This is mostly used for polar solvents that are miscible or highly soluble in water, especially when normal distillation produces an azeotrope. This technique is often used prior to distilling with a salt, as it cannot remove all of the water, but is a convenient way to remove most of it without having to use any anhydrous compounds, and it is one of the only ways to break an azeotrope outside of vacuum distillation. Examples of salting out include the separation of ethanol and water using potassium carbonate, the removal of isopropanol from water using sodium chloride, sodium hydroxide, or a mix of the two.

Molecular sieves

Main article: Molecular sieve

Molecular sieves are precise tools for the removal of water or other liquid components of a mixture. Using precisely sized pores in a material such as silica, clay, or alumina, they selectively trap molecules of a certain size by adsorption. While they may not be particularly cheap and can be difficult to re-dry after their use, molecular sieves have the advantage of being able to remove a significant amount of water from a solvent without introducing any of their own impurities. A disadvantage of molecular sieves, however, is the long amount of time they must be given to complete the water-removal process, often in excess of 24 hours.

Solvents and their principal method(s) of water removal

This section includes a commonly used method for drying a solvent that is sufficient enough for that solvent's typical use, either for solvation or as a reagent. This is not an exhaustive list by any means, and other methods can most likely be used.

Solvent Method
Acetic acid
  • Anhydrous copper(II) sulfate
  • Phosphorous pentoxide
  • Fractionally distillation over 5% acetic anhydride and 2% CrO3.
Acetic anhydride
  • Drying with phosphorous pentoxide then fractionally distill over potassium carbonate
  • Sodium wire (some sources)
Acetone
Acetonitrile
  • Drying with calcium chloride, followed by either distillation or decantation; Calcium chloride can also be used to keep the solvent dry
  • Drying with anhydrous potassium carbonate, then 3A molecular sieves, followed by distillation
  • Drying with phosphorous pentoxide (5% w/v), followed by distillation (tip: avoid this method if using in reactions with acid sensitive substrates)
Aniline
  • Distillation over potassium hydroxide
  • Drying over calcium hydride, followed by distillation.
  • Drying with barium oxide, decantation or filtration, followed by distillation over fresh barium oxide.
Anisole
  • Drying with calcium chloride/sulfate, another drying with sodium wire, followed by distillation over fresh sodium wire
Benzene
  • Drying with anhydrous sodium sulfate, followed by either decantation or distillation.
  • Drying with phosphorous pentoxide, sodium, lithium aluminum hydride, calcium hydride, 4A molecular sieves, calcium sulfate, followed by decantation or filtration
  • Drying with alumina, calcium hydride or 4A molecular sieves, followed by distillation.
  • Store over molecular sieves or sodium wire.
Butanol
  • Drying with magnesium sulfate, calcium oxide or potassium carbonate, followed by distillation over magnesium/iodine, sodium or calcium metal
  • Drying with 3A moloecular sieves (powder)
sec-Butanol
  • Drying over potassium carbonate or calcium sulfate, followed by fractional distillation.
  • Distillation over calcium oxide, then over magnesium.
  • Distillation over calcium hydride.
tert-Butanol
  • Drying with calcium hydride (5% w/v), followed by distillation.
  • Drying with calcium oxide, potassium carbonate, calcium sulfate or magnesium sulfate, followed by fractional distillation.
  • Distillation over magnesium/iodine or calcium, sodium, potassium metal.
  • Storage over 3A molecular sieves.
Chloroform
  • Distillation over anhydrous magnesium sulfate
  • Drying with phosphorus pentoxide followed by distillation.
Dichloromethane
Diethyl ether
  • Dried using anhydrous calcium chloride, molecular sieves, followed by distillation
  • For higher purity, sodium metal and benzophenone are used.
Dimethyl sulfoxide
  • Distillation in partial vacuum over molecular sieves
  • Calcium hydride can also be used, though it's somewhat less effective.
Dimethylformamide
  • Drying with barium oxide or molecular sieves, followed by partial vacuum distillation.
Ethanol
  • Distillation of the 95% azeotrope followed by salting out with anhydrous potassium carbonate and a second distillation over anhydrous calcium or magnesium sulfate
  • Direct drying can be done with magnesium metal, followed by distillation or filtration.
Ethyl acetate
  • Drying with anhydrous sodium sulfate, ethanol removal can be facilitated by salting out using potassium carbonate, followed by distillation of the ethyl acetate layer over anhydrous magnesium sulfate.
Isopropanol
  • Salting out to 91% using sodium chloride, followed by salting out to 100% using sodium hydroxide. Once sodium hydroxide has been added and the two layers separated, the entire mixture should be cooled as low as possible to precipitate/freeze sodium hydroxide, which will hurt glassware in a distillation. The isopropanol layer can be poured off the frozen sodium hydroxide solution and distilled over anhydrous magnesium sulfate, preferably with a small amount of sulfuric acid present to mitigate glassware damage from hot sodium hydroxide.
Methanol
  • Salting out with potassium carbonate or simple distillation to remove from water with or without a desiccant
  • Direct drying can be done with magnesium metal turnings, followed by distillation or filtration.
Methyl ethyl ketone
  • Drying with calcium sulfate, magnesium sulfate, sodium sulfate or potassium carbonate.
  • Drying over boron trioxide
Tetrahydrofuran
  • Distillation over molecular sieves
  • Adding metallic potassium under inert conditions for complete removal of water, followed by distillation.
Toluene
  • Adding sodium metal with benzophenone, followed by distillation
  • Azeotropic distillation with benzene, molecular sieves.
Xylene
  • Drying with molecular sieves followed by distillation or decantation
  • Distillation over sodium metal.

Compatibility of desiccants with solvents

While most desiccants can be used safely to dry most common solvents, some cannot be used as they will either react with the said solvent or dissolve in it.

Desiccant Compatible Incompatible Notes
Alkali metals Alkanes, arenes, ethers Acetone, alcohols, halogenated solvents, DMSO, nitromethane Reacts violently with halogenated solvents, less so with alcohols and DMSO; air sensitive
Alkali metal hydroxide Amines and pyridines Acids, base-sensitive solvents, nitromethane Reacts with acids releasing water
Alkaline earth metals Alcohols, alkanes, arenes, ethers Acetone, alcohols, halogenated solvents, DMSO Reacts violently with halogenated solvents, less so with alcohols and DMSO; Reaction with alcohols give their respective alkoxides, that can be regenerated back to alcohol by adding water; air sensitive (except magnesium)
Alkaline earth metal oxides Alcohols, alkanes, arenes, basic solvents, ethers, halogenated solvents Acetone, esters, dipolar aprotic solvents May react with alcohols in excess, will cause aldol condensation with ketones; not useful with dipolar aprotic solvents
Alumina Alcohols, alkanes, arenes, esters, ethers, halogenated solvents Acetone, acids Reaction with acetone and acids
Boron trioxide Acetone, acetonitrile, esters, ethers, halogenated solvents Alcohols, basic solvents Reacts with alcohols and basic solvents
Calcium chloride Alkyl and aryl halides, esters, ethers, halogenated solvents Acetone, acids, alcohols, aldehydes, amines, carbonyl compounds Reaction of CaCl2 with acetone forms an addition compound
Calcium hydride Alcohols, alkanes, amines, DMF, ethers, HMPA, pyridines Esters Reacts with esters; air sensitive
Calcium sulfate Acetone, alcohols, aldehydes, halogenated solvents, ketones and pretty much all solvents Inert Drying may be strongly exothermic
Cement (Portland) Alcohols, alkanes, arenes Acids, esters Reacts with acids, esters; after hydration results in a very hard mass
LiAlH4 Alkanes, arenes Alcohols, esters, halogenated solvents Reaction with alcohols, esters, halogenated solvents
LiBH4 Alkanes Alcohols, halogenated solvents, esters Reaction with alcohols, halogenated solvents, esters
Lithium chloride Alkanes, arenes, halogenated solvents Butanol, propanol, methylformamide, hydrazine Dissolution in alcohols, methylformamide, hydrazine
Magnesium sulfate Acetone, alcohols, aldehydes, alkanes, arenes, esters, ethers, halogenated solvents, ketones, pretty much all solvents Inert May cause small traces of aldol condensation with acetone
Molecular sieves Alcohols, ethers Acetone, acids Will cause aldol condensation of acetone; reacts with acids
Phosphorus pentoxide Halogenated solvents Alcohols, amines, organic acids and carbonyl compounds; HMPA, DMSO, acetone Reaction; decomposition
Potassium carbonate Acetone, alcohols, aldehydes, halogenated solvents, various ketones, methyl ethyl ketone Acids Reaction with acids releases carbon dioxide and water
Silica gel Alkanes, arenes, esters, ethers Acetone May cause self-condensation with acetone
Sodium sulfate Acids, alcohols, esters, ethers, halogenated solvents Acetone, ketones Will cause some aldol condensation with ketones

References

Relevant Sciencemadness threads