What THF (CAS 109-99-9) Is
Tetrahydrofuran, C4H8O, molecular weight 72.11, is a five-membered cyclic ether: a saturated furan ring with a single oxygen atom. It is a colourless, mobile liquid with an ether-like odour, fully miscible with water, and it boils at 66 degrees C at 101.3 kPa with a closed cup flash point of minus 14 degrees C. Commercial material is usually 99.5% assay or higher for technical use and 99.9% for anhydrous and reaction grades, where the water specification is the deciding line item.
Properties That Make It a Good Solvent
Three features explain its position in the solvent market. First, it is a polar aprotic solvent: the molecule has a substantial dipole moment of about 1.75 D but no acidic proton, so it dissolves polar organic compounds while remaining inert towards strong bases, organolithium reagents and Grignard reagents that would be destroyed by an alcohol. Second, the oxygen atom carries two lone pairs and acts as a donor, coordinating Lewis acids and metal cations and stabilising reactive intermediates and organometallic complexes. Third, it has an unusually wide liquid range, remaining fluid from about minus 108 degrees C to 66 degrees C, with a low viscosity near 0.48 mPa·s at 25 degrees C and a low ultraviolet cutoff near 212 nm, which makes it useful in spectrophotometric and chromatographic work as well as in synthesis.
Typical Specification
| Property | Typical value | Reference method |
|---|---|---|
| Assay as C4H8O | 99.5 % minimum technical; 99.9 % anhydrous | Gas chromatography |
| Water | 0.03 % maximum | ASTM D1364 (Karl Fischer) |
| Peroxide as H2O2 | 0.005 % maximum | Iodometric titration |
| Inhibitor (BHT) | 100 - 300 ppm in inhibited grade | HPLC |
| Distillation range at 101.3 kPa | 65 - 67 degrees C | ASTM D1078 |
| Density at 20 degrees C | 0.889 g/cm3 | ASTM D4052 |
| Colour | 10 Pt-Co maximum | ASTM D1209 |
| Ultraviolet cutoff | 212 nm | Spectrophotometry |
| Transport classification | UN 2056, Class 3, Packing Group II | UN Model Regulations |
Peroxide Formation: The Storage Issue That Matters Most
On contact with air, THF slowly forms peroxides, especially when it is exposed to light or stored warm. The risk is not the peroxide concentration during normal use but the concentration that accumulates during distillation: as solvent evaporates, the residue concentrates and can detonate on heating or impact. Four rules are standard in plant practice. Store inhibited material, typically with 100 to 300 ppm BHT, in closed steel or amber glass containers under nitrogen, below about 30 degrees C and out of sunlight. Test for peroxide before any distillation or evaporation step, with an iodometric or test-strip method. Never distil to dryness. Rotate stock so that a parcel is used within roughly six months, and requalify older material by test rather than by sight. Where anhydrous THF is required, it should be drawn from sealed containers over molecular sieves with an inert blanket.
Main Industrial Uses
Polymer chemistry dominates consumption: THF is polymerised to polytetramethylene ether glycol, the soft segment of spandex fibres and thermoplastic polyurethane elastomers, and it is a ring-opening precursor route to other polyethers. It is also a widely used reaction solvent for Grignard, organolithium, hydride reduction and polymerisation reactions, a solvent in pharmaceutical and agrochemical manufacture and in lithium battery electrode slurry preparation, a component of PVC solvent cements and printing ink systems, and a carrier in adhesives and coatings where a water-miscible ether is needed. Its miscibility with both water and hydrocarbons makes it valuable in purification by extraction and crystallisation.
Frequently Asked Questions
Q: Why is THF a better solvent than diethyl ether for many reactions?
A: THF has a higher boiling point of 66 degrees C against about 35 degrees C for diethyl ether, so reactions can be run warmer and refluxed more safely, and its donor oxygen coordinates metal cations strongly, which stabilises Grignard and organolithium reagents. Both are flammable, but THF's lower volatility reduces vapour losses.
Q: Is THF miscible with water?
A: Yes, it is fully miscible, which is unusual for an ether and is one reason it is used in water-containing or two-phase systems, in electrode slurry preparation and in purification steps where residual water must be carried through the solvent phase.
Q: How should THF be stored to prevent peroxide formation?
A: In sealed, grounded containers under nitrogen, out of direct sunlight and below about 30 degrees C, with 100 to 300 ppm BHT inhibitor where the application allows it. Test for peroxides before distillation, never evaporate to dryness, and use stock within about six months.
Q: What water content is typical for anhydrous THF?
A: Anhydrous grade is specified at 0.005% to 0.03% water depending on the requirement, and reaction grade at 0.005% or lower where organometallic reagents are involved. Water is measured by Karl Fischer titration to ASTM D1364, and containers are opened only under inert gas.
Q: What is THF made from and what is it used for?
A: It is produced mainly by dehydrating 1,4-butanediol, with other routes from furfural and from butadiene chemistry. The largest single downstream use is polytetramethylene ether glycol for spandex and polyurethane elastomers, followed by reaction solvent, battery and coatings applications.