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Tetrahydrofuran (THF, C4H8O, CAS 109-99-9) boils at about 66 °C (339 K, 151 °F) at standard atmospheric pressure, 760 mmHg. Reference sources round it to 65 or 66 °C. It freezes near -108.4 °C, is lighter than water, and is volatile and highly flammable. This page is a laboratory solvent datasheet: the numbers, which source gives which value, and what they mean at the bench.
Short answer: THF boiling point in °C, K and °F
NIST lists a normal boiling point of 339 ± 1 K, an average of 16 of 17 reported values. PubChem’s compiled record gives 65-66 °C (151 °F) at 760 mmHg. The apparent disagreement is rounding and unit choice, not different chemistry.
| Source (as compiled) | Reported value | Same value in °C |
|---|---|---|
| NIST Chemistry WebBook (phase change data) | 339 ± 1 K | about 65.9 °C |
| CRC Handbook of Chemistry and Physics, 88th ed. (via PubChem) | 65 °C | 65 °C |
| ILO-WHO International Chemical Safety Card (via PubChem) | 66 °C | 66 °C |
| NIOSH and OSHA (via PubChem) | 151 °F at 760 mmHg | about 66.1 °C |
Conversions for the rounded value: 66 °C = 339.15 K = about 150.8 °F. Quote 66 °C when you need one figure, and cite the source when you need the exact one.
Boiling point depends on pressure
The values above are for 760 mmHg (1 atm). At lower pressure, such as under rotary evaporator vacuum or at altitude, THF boils at a lower temperature; at higher pressure it boils higher. NIST states that its heat-of-vaporization correlation for THF is valid only for 302-339 K, so do not extrapolate outside that range. The enthalpy of vaporization is about 32 kJ/mol (references range from 29.6 to 33.1 kJ/mol; Majer and Svoboda, 1985, give 29.81 kJ/mol at 339.1 K).
Melting point and freezing point of THF
The melting point of THF is about -108.4 °C. PubChem lists the CRC Handbook (88th ed.) at -108.44 °C and other compiled values of -108.3 and -108.5 °C. NIST gives a triple point of 164.76 K, which is -108.4 °C, and fusion temperatures of 164.05-165.1 K from four references. THF therefore remains liquid from about -108 °C to about 66 °C at atmospheric pressure.
Density, vapor pressure and other properties
Tetrahydrofuran density
0.8833 g/cm³ at 25 °C (HSDB citing the CRC Handbook, 88th ed.); ICSC, OSHA and NIOSH list about 0.89. THF is less dense than water, but it is miscible with water at 25 °C (Riddick et al., Organic Solvents, as compiled by PubChem), so it does not form a separate floating layer on its own.
Tetrahydrofuran vapor pressure
The ILO-WHO ICSC gives 19.3 kPa at 20 °C. HSDB (Verschueren) gives 0.173 atm at 20 °C and 0.26 atm at 30 °C; Daubert and Danner give 162 mmHg at 25 °C. The NIOSH/OSHA figure of 132 mmHg carries no stated temperature in the compiled record, so it is not comparable with the others. A vapor pressure near one-fifth of an atmosphere at room temperature means THF evaporates quickly from open vessels.
Tetrahydrofuran viscosity, dielectric constant and refractive index
- Viscosity: 0.53 cP at 20 °C (Flick, Industrial Solvents Handbook, 4th ed.).
- Dielectric constant: 7.4 at 25 °C (Flick, Industrial Solvents Handbook, 4th ed.). The Merck Index records THF as miscible with alcohols, ketones, esters, hydrocarbons and ethers.
- Refractive index: 1.4050 at 20 °C, sodium D line (CRC Handbook, 88th ed.).
- Flash point: 6 °F, about -14 °C (CAMEO Chemicals via PubChem). THF can ignite at normal room temperature.
Is THF a base or an acid? The pKa of tetrahydrofuran
THF is neither an acid nor a base in water in any practical sense. It is a cyclic ether with no acidic hydrogen to give up. The pKa listed in PubChem’s dissociation constants entry (Riddick et al.) is -2.08, a very low value that indicates the ether oxygen takes up a proton only from strong acid. Editorial interpretation: the oxygen lone pairs are why THF behaves as a Lewis base and solvates metal cations in organometallic chemistry.
How is THF formed?
Industrial routes compiled in PubChem’s record include catalytic hydrogenation of maleic anhydride (Merck Index), catalytic hydrogenation of furan with a nickel catalyst, and acid-catalyzed dehydration of 1,4-butanediol (Hawley’s Condensed Chemical Dictionary, 15th ed.). Ullmann’s Encyclopedia describes the Reppe route: acetylene and formaldehyde give 2-butyne-1,4-diol, hydrogenation gives 1,4-butanediol, and acid catalysis above 100 °C closes the ring to THF with loss of water.
Is tetrahydrofuran a carcinogen?
The International Agency for Research on Cancer classified tetrahydrofuran in Group 2B, “possibly carcinogenic to humans”, in IARC Monographs Volume 119 (2019). The PubChem record also notes that California lists it. This is a hazard classification, not a statement about risk at a particular exposure: follow the safety data sheet and your institution’s chemical hygiene plan.
At the bench: what these numbers mean
- Removing THF on a rotary evaporator. Boiling point falls as pressure falls, so THF boils well below 66 °C under vacuum. The 19.3 kPa vapor pressure at 20 °C shows it evaporates readily, so apply vacuum gradually.
- Fire. A flash point near -14 °C plus a vapor pressure near 19 kPa at 20 °C means ignitable vapor can accumulate at ordinary room temperature. Keep ignition sources away and containers closed.
- Peroxides. THF is on the common lists of peroxide-forming solvents, and evaporating a peroxide former concentrates any peroxides present, so do not take it to dryness. See peroxide-forming chemicals and shock-sensitive chemicals in the laboratory.
- Water. Because THF is miscible with water, it does not form a separate layer, so it is a poor choice as the organic phase in an aqueous extraction.
For other solvent reference cards, compare the boiling point of DMF and the boiling point of pentane, and see the hexane and ethyl acetate safety data guides. For solvent choice from a sustainability angle, see the green chemistry lab guide.
Common misunderstandings
- “The boiling point is a single number.” Sources round differently (65 °C, 66 °C, 151 °F, 339 K); all refer to 760 mmHg.
- “It is lighter than water, so it separates from water.” It is less dense than water but miscible with it.
- “A pKa of -2.08 means THF is an acid.” The value describes protonation of the ether oxygen, not loss of a proton.
FAQ
What is the boiling point of tetrahydrofuran in Celsius?
About 66 °C (65 °C in the CRC Handbook, 66 °C in the ICSC) at 760 mmHg.
What is the melting point of THF?
About -108.4 °C (164.76 K triple point per NIST; -108.44 °C per CRC via PubChem).
How is THF formed?
Industrially by hydrogenating maleic anhydride or furan, or by acid-catalyzed cyclization of 1,4-butanediol.
Is THF a base or acid?
Neither in practice: it is a neutral ether and a weak Lewis base, with a listed pKa of -2.08.
Is tetrahydrofuran a carcinogen?
IARC places it in Group 2B, possibly carcinogenic to humans (Monographs Volume 119, 2019).
Sources
- NIST Chemistry WebBook, tetrahydrofuran (CAS 109-99-9), phase change data: webbook.nist.gov (read 2026-11-07).
- PubChem compound record CID 8028, tetrahydrofuran, with compiled CRC Handbook, ILO-WHO ICSC, NIOSH, OSHA, HSDB, Merck Index and IARC entries: pubchem.ncbi.nlm.nih.gov (read 2026-11-07).
This page is general reference information, not a safety data sheet; consult the supplier’s SDS for handling.
