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Short Answer
The liquid density of pure triethylamine is 0.7255 g/cm³ at 20 °C (0.726 g/mL at 25 °C), making it lighter than water. Triethylamine forms the upper phase during aqueous extractions. Its normal boiling point is 88.8 °C to 89.3 °C at 760 mmHg, its vapor pressure is 51.8 mmHg at 20 °C, and its conjugate acid pKa is 10.75.
Definition and Scope
Triethylamine (chemical formula C6H15N, molar mass 101.19 g/mol, CAS Registry Number 121-44-8) is a tertiary aliphatic amine consisting of three ethyl groups attached to a central nitrogen atom. In chemical synthesis and analytical facilities, triethylamine functions as an organic base, acid scavenger, and chromatographic modifier. Accurate physical constants are vital for rotary evaporation, biphasic liquid-liquid extractions, and mobile-phase buffering in high-performance liquid chromatography.
Because triethylamine combines moderate water solubility, elevated vapor pressure, and strong basicity, bench scientists routinely consult its datasheet properties. Unlike heavy or aromatic bases, triethylamine offers low viscosity and low density, shaping protocols across peptide synthesis and purification suites.
How It Works and How It Is Used in Practice
Triethylamine functions primarily as a non-nucleophilic Brønsted base and Lewis base. With a conjugate acid (triethylammonium, [Et3NH]+) pKa of 10.75 in water at 25 °C, it rapidly neutralizes mineral and organic acids produced during acylations, sulfonylations, and amide couplings. The resulting triethylammonium salts separate cleanly by precipitation or aqueous washing.
In analytical chromatography, triethylamine serves as a mobile-phase additive (0.05% to 0.2% v/v). In reversed-phase HPLC, acidic residual silanols on silica columns cause severe peak tailing for basic analytes. Triethylamine dynamically coats these active silanol sites, restoring symmetrical chromatographic peaks.
In normal-phase column chromatography and thin-layer chromatography, adding 1% to 2% triethylamine deactivates acidic silica sites. This prevents polar basic compounds from streaking or adsorbing irreversibly onto stationary media.
Types and Related Concepts
The table below summarizes evaluated physical constants and thermodynamic properties for pure triethylamine from the National Institute of Standards and Technology (NIST) Chemistry WebBook and NIH PubChem:
| Physical Property | Evaluated Value | Measurement Condition | Primary Reference Source |
|---|---|---|---|
| Liquid Density | 0.7255 g/cm³ (0.726 g/mL) | 20 °C to 25 °C, 1 atm | PubChem CID 8471; TRC / NIST |
| Vapor Density (Relative to Air) | 3.48 to 3.49 (Air = 1) | Standard conditions | PubChem CID 8471; NTP |
| Normal Boiling Point | 88.8 °C to 89.3 °C (362.2 K) | 760 mmHg (101.3 kPa) | NIST Chemistry WebBook SRD 69 |
| Melting / Freezing Point | -114.7 °C (158.45 K) | 1 atm | NIST Chemistry WebBook SRD 69 |
| Vapor Pressure | 51.75 to 53.5 mmHg (6.9 kPa) | 20 °C | PubChem CID 8471; NTP |
| Dynamic Viscosity | 0.347 to 0.363 mPa·s (cP) | 25 °C | PubChem CID 8471 |
| Conjugate Acid pKa | 10.75 (pKb = 3.25) | Water at 25 °C | PubChem CID 8471 |
| Snyder Polarity Index (P’) | 1.9 | Relative test solutes | Snyder Solvent Classification |
| Refractive Index (nD20) | 1.4003 to 1.4010 | Sodium D line, 20 °C | PubChem CID 8471 |
| Flash Point (Closed Cup) | -7 °C (19.4 °F / 20 °F) | Closed cup | PubChem CID 8471; NTP |
| Water Solubility | 68.6 g/L (partial miscibility) | 25 °C | PubChem CID 8471 |
Where It Matters for Labs, Clinics, and Supply Decisions
Physical properties dictate operational choices across bench workflows, equipment settings, and engineering controls:
1. Liquid-Liquid Extraction Phase Layering
Because triethylamine density is 0.7255 g/cm³ at 20 °C, it is significantly lighter than water (1.00 g/cm³). In common organic extraction solvents such as diethyl ether (0.713 g/cm³) or ethyl acetate (0.902 g/cm³), the organic layer remains buoyant as the upper phase. Conversely, chlorinated solvents like dichloromethane (1.326 g/cm³) sink to the bottom. Understanding layer density prevents accidental discarding of target compounds during post-reaction workups.
2. Rotary Evaporation Stripping Parameters
Triethylamine has a boiling point of 88.8 °C and a vapor pressure of 51.8 mmHg at 20 °C. Unlike less volatile bases like diisopropylethylamine (bp 127 °C), triethylamine is stripped readily on standard rotary evaporators. At a bath temperature of 40 °C, a diaphragm vacuum of 100 to 120 mbar removes residual base smoothly without bumping. Similar care in matching vacuum to volatility is required when handling hexane boiling point stripping workflows.
3. Vapor Density and Containment Controls
Although liquid triethylamine floats on water, its vapor density of 3.49 means its gaseous vapors are 3.5 times heavier than air. Escaped vapors sink toward benchtops and floor perimeters. With a low flash point (-7 °C) and a sharp ammoniacal odor noticeable at 0.1 ppm, handling requires a certified chemical fume hood. Complying with guidelines in prudent practices in the laboratory prevents respiratory exposure and eliminates ignition risks.
4. Reagent Purification and Storage
Triethylamine absorbs atmospheric water and carbon dioxide, forming insoluble carbamate salts that deposit as crusts on bottle threads. Synthetic procedures requiring anhydrous conditions demand drying over calcium hydride (CaH2) or potassium hydroxide (KOH) followed by distillation under dry nitrogen. Reagent should be stored over activated molecular sieves in amber bottles.
Common Misunderstandings
Misunderstanding 1: Assuming Triethylamine Density Equals Water Density
Novice researchers sometimes assume organic amines share water’s density of 1.0 g/mL. Because triethylamine density is 0.726 g/mL, 1.0 mL supplies only 0.726 grams (7.17 mmol) of base. Calculating stoichiometry by volume without correcting for density causes a 27% base deficit, leading to incomplete reactions.
Misunderstanding 2: Assuming Liquid Density Matches Vapor Behavior
Chemists occasionally assume that because liquid triethylamine floats on water, its vapors drift upward. In reality, vapor density depends on molecular weight (101.19 g/mol vs air 28.97 g/mol). Triethylamine vapors are 3.5 times heavier than air, settling along benches and floor trenches if vessels are left unsealed.
Misunderstanding 3: Overlooking Lower Critical Solution Temperature (LCST)
Triethylamine displays inverse aqueous solubility. Below its LCST of 18.2 °C, triethylamine and water form extensive hydrogen bonds and become completely miscible. Above 18.2 °C, phase separation occurs into two distinct layers. Washing cold reaction mixtures on ice can cause triethylamine to dissolve entirely into the aqueous wash rather than separating into the organic layer.
Misunderstanding 4: Equating Polarity Index with Basicity
With a Snyder polarity index (P’) of 1.9, triethylamine is classified as a low-polarity solvent. Researchers sometimes assume low polarity implies weak basicity. In truth, triethylamine is a potent base (conjugate pKa 10.75); its low polarity index simply reflects its nonpolar ethyl substituents, which surround the basic nitrogen lone pair.
Frequently Asked Questions
What is the exact density of triethylamine?
Pure liquid triethylamine has a density of 0.7255 g/cm³ at 20 °C and 0.726 g/mL at 25 °C. Being lighter than water, it floats on aqueous layers during extractions.
What is the boiling point of triethylamine?
The normal boiling point of triethylamine is 88.8 °C to 89.3 °C (362.2 K) at 760 mmHg. Under rotary evaporation at 100 mbar, it distills efficiently at 35 °C to 40 °C.
What is the freezing point of triethylamine?
The freezing point of triethylamine is -114.7 °C (158.45 K). It remains fluid during standard low-temperature cooling baths such as dry ice and acetone (-78 °C).
What is the vapor pressure of triethylamine?
Triethylamine has a vapor pressure of 51.75 to 53.5 mmHg at 20 °C and 68.6 mmHg at 25 °C, creating high ambient volatility when bottles are uncapped.
What is the pKa of triethylamine?
The pKa of the triethylammonium conjugate acid ([Et3NH]+) is 10.75 in water at 25 °C (pKb = 3.25), characterizing triethylamine as a moderately strong organic base.
What is the polarity index of triethylamine?
Triethylamine has a Snyder polarity index (P’) of 1.9. It belongs to Snyder Selectivity Group I, behaving as a strong hydrogen-bond acceptor with low overall dipole character.
What is the viscosity of triethylamine?
The dynamic viscosity of triethylamine is 0.347 to 0.363 mPa·s at 25 °C, roughly one-third the viscosity of water, facilitating smooth flow and low syringe resistance.
Sources
- National Center for Biotechnology Information (NCBI). “PubChem Compound Summary for CID 8471, Triethylamine.” PubChem, U.S. National Library of Medicine, 2026. Available at: https://pubchem.ncbi.nlm.nih.gov/compound/8471.
- National Institute of Standards and Technology (NIST). “Triethylamine (CAS Registry Number 121-44-8).” NIST Chemistry WebBook, NIST Standard Reference Database Number 69, edited by P.J. Linstrom and W.G. Mallard, National Institute of Standards and Technology, 2026. Available at: https://webbook.nist.gov/cgi/cbook.cgi?ID=C121448&Units=SI&Mask=4.
- Snyder, L.R. “Classification of the Solvent Properties of Common Liquids.” Journal of Chromatography A, 1974 / Journal of Chromatographic Science, 1978.
