tetrahydrofuran

Tetrahydrofuran
IUPAC name	Oxacyclopentane
Other names	THF, tetrahydrofuran, 1,4-epoxybutane, butylene oxide, cyclotetramethylene oxide, oxacyclopentane, diethylene oxide, oxolane, furanidine, hydrofuran, tetra-methylene oxide
Identifiers
CAS number	[109-99-9]
RTECS number	LU5950000
SMILES	C1CCCO1
Properties
Molecular formula	C4H8O
Molar mass	72.11 g/mol
Appearance	colorless liquid
Density	0.8892 g/cm3 (g/mL) @ 20 °C, liquid
Melting point	-108.4 °C (164.75 K)
Boiling point	66 °C (339.15 K)
Solubility in water	Miscible
Viscosity	0.48 cP at 25 °C
Structure
Molecular shape	envelope
Dipole moment	1.63 D (gas)
Hazards
MSDS	External MSDS
EU classification	Flammable (F) Irritant (Xi)
R-phrases	R11, R19, R36/37
S-phrases	S16, S29, S33
NFPA 704	3 2 0
Flash point	-14 °C
Related compounds
Related heterocycles	Furan Pyrrolidine Dioxane
Related compounds	Diethyl ether
Supplementary data page
Structure and properties	n, εr, etc.
Thermodynamic data	Phase behaviour Solid, liquid, gas
Spectral data	UV, IR, NMR, MS
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox references

Tetrahydrofuran (THF) is a colorless, water-miscible organic liquid with low-viscosity at standard temperature and pressure. It is a heterocyclic compound with a chemical formula C₄H₈O, and is the fully hydrogenated analog of the aromatic organic compound furan. It is one of the most polar of the organic functional class of ethers, and has a relatively low freeze point, and so is a commonly used modern organic chemical laboratory solvent across a range of temperatures. THF has a smell similar to its chemical cousin, diethyl ether, but is a much less potent anesthetic than diethyl ether, and is quite dangerous if inhaled or ingested (see Precautions).

Production

About 200 million kilograms of tetrahydrofuran are produced annually.^[1] The most widely used industrial process involves the acid-catalyzed dehydration of 1,4-butanediol. The butanediol is derived from carbonylation of acetylene followed by hydrogenation. Du Pont developed a process for producing THF by oxidizing n-butane to crude maleic anhydride followed by catalytic hydrogenation.^[2] A third major industrial route entails hydroformylation of allyl alcohol followed by hydrogenation to the butanediol.

THF can also be synthesized by catalytic hydrogenation of furan derived from pentose. Although this method involves renewable resources, it is not widely practiced.^[3]

Solvent properties

THF is an aprotic solvent with a dielectric constant of 7.6. Hence, it is a moderately polar solvent, and can dissolve a wide range of nonpolar and polar chemical compounds.^[4] THF is water-miscible, and can form solid clathrate hydrate structures with water at low temperatures.^[5]

THF can sometimes substitute for diethyl ether in chemical reactions and procedures when a moderately higher-boiling solvent ethereal is required and its water miscibility is not an issue. Both solvents are classified as ethers (THF being a cyclic one and diethyl ether acyclic) and so each have an oxygen atom with two carbon substituents, and neither have acidic C-H bonds. The oxygen atom of either solvent can coordinate to electron-deficient centers such as Li⁺, Mg²⁺, and boranes, forming adducts. Hence, like diethyl ether, THF can be used in hydroboration reactions to synthesize primary alcohols, and as a solvent for organometallic reactions such as organolithium and Grignard reactions,^[6] in both cases because of their oxygen atom's ability to coordinate to the metal ion of the reagent (e.g., the magnesium of the Grignard reagent). Though similar, there are actually different geometries surrounding the carbon atoms bound to the oxygen in THF versus diethyl ether—unlike the rotationally "free" diethyl ether chains, the carbons in THF are "pulled back" and constrained in their ring—leading to the general understanding that THF behaves as a strongly coordinating solvent/ligand, and diethyl ether as a weakly coordinating one.^[7] Thus, while diethyl ether remains the solvent of choice for some reactions (e.g., Grignard reactions), THF fills that role in many others where strong coordination is desirable, and the precise properties of ethereal solvents such as these (alone and in mixtures and at various temperatures) allows for fine-tuning modern chemical reactions.

THF is often used in polymer science. For example, it can be used to dissolve rubber prior to determining its molecular mass using gel permeation chromatography. THF dissolves PVC as well and is the main ingredient in PVC adhesives. It can be used to liquefy old PVC cement, and is often used industrially to degrease metal parts.

2-MethylTHF

2-Methyltetrahydrofuran (2MeTHF) is a THF alternative that is being promoted as being more ecologically friendly.^[8] While generally still somewhat more expensive at the "front end", it may provide for greater overall process economy. 2MeTHF has solvating properties that are intermediate between diethyl ether and THF, has limited water-miscibility, and forms an azeotrope with water on distillation. Its lower melting point makes it useful for lower temperature reactions, and its higher boiling point allows procedures under reflux at higher temperatures (relative to THF).

Reactions

THF can be polymerized by strong acids to give a linear polymer called poly(tetramethylene ether) glycol (PTMEG), CAS Registry Number [25190-06-1], also known as PTMO, polytetramethylene oxide. The primary use of this polymer is to make elastomeric polyurethane fibers like Spandex.^[9]

Precautions

THF is considered a relatively nontoxic solvent, with the median lethal dose (LD₅₀) comparable to that for acetone. Reflecting its remarkable solvent properties, it penetrates the skin causing rapid dehydration. It is highly flammable.

The greatest danger posed by THF follows from its tendency to form highly-explosive peroxides on storage in air. To minimize this problem, commercial samples of THF are often inhibited with BHT. THF should not be distilled to dryness, because the explosive peroxides concentrate in the residue.

See also

• monomer
• polytetrahydrofuran
• Trapp mixture

References

1. ^ "Ethers, by Lawrence Karas and W. J. Piel". Kirk‑Othmer Encyclopedia of Chemical Technology. John Wiley & Sons, Inc. 2004. 
2. ^ Merck Index of Chemicals and Drugs, 9th ed.
3. ^ Morrison, Robert Thornton; Boyd, Robert Neilson: Organic Chemistry, 2nd ed., Allyn and Bacon 1972, p. 569
4. ^ http://www.cem.msu.edu/~reusch/VirtualText/enrgtop.htm#top4
5. ^ http://gashydrate.fileave.com/NMR-MRI%20study%20of%20clathrate%20hydrate%20mechanisms.pdf
6. ^ Elschenbroich, C.; Salzer, A. ”Organometallics : A Concise Introduction” (2nd Ed) (1992) Wiley-VCH: Weinheim. ISBN 3-527-28165-7
7. ^ E.g., B.L. Lucht, D.B. Collum "Lithium Hexamethyldisilazide: A View of Lithium Ion Solvation through a Glass-Bottom Boat" Accounts of Chemical Research, 1999, volume 32, 1035–1042, and references therein.
8. ^ http://www.sigmaaldrich.com/etc/medialib/docs/Sigma/Brochure/greener_solvent_alternatives.Par.0001.File.tmp/greener_solvent_alternatives.pdf
9. ^ "Polyethers, Tetrahydrofuran and Oxetane Polymers by Gerfried Pruckmayr, P. Dreyfuss, M. P. Dreyfuss". Kirk‑Othmer Encyclopedia of Chemical Technology. John Wiley & Sons, Inc. 1996. 

• Loudon, G. Mark. Organic Chemistry 4th ed. New York: Oxford University Press. 2002. pg 318.

External links

• International Chemical Safety Card 0578
• NIOSH Pocket Guide to Chemical Hazards
• THF usage
• THF info
• U.S. OSHA info on THF
• "2-Methyltetrahydrofuran, An alternative to Tetrahydrofuran and Dichloromethane". Sigma-Aldrich. http://www.sigmaaldrich.com/Area_of_Interest/Research_Essentials/Solvents/Key_Resources/Me_THF.html. Retrieved on 2007-05-23. 
• "Tetrahydrafuran (THF) Storage and Handling". BASF. http://www.basf.com/diols/pdfs/thf_brochure.pdf. Retrieved on 2007-05-24.