TRIETHYLAMINE: STRUCTURE, PROPERTIES, USES AND RISKS - CHEMISTRY - 2024

The triethylamine is an organic compound, more specifically an aliphatic amine, whose chemical formula is N (CH ₂ CH ₃) ₃ or NEt ₃. Like other liquid amines, it has an odor similar to that of ammonia mixed with fish; its vapors make it tedious and dangerous to handle this substance without an extractor hood or suitable clothing.

In addition to their respective formulas, this compound is often abbreviated as TEA; however, this can lead to confusion with other amines, such as triethanolamine, N (EtOH) ₃, or tetraethylammonium, a quaternary amine, NEt ₄ ⁺.

Triethylamine skeleton. Source: Mixtures

From its skeleton (top image), it can be said that triethylamine is quite similar to ammonia; while the second has three hydrogens, NH ₃, the first has three ethyl groups, N (CH ₂ CH ₃) ₃. Its synthesis starts from the treatment of liquid ammonia with ethanol, an alkylation occurring.

NEt ₃ can form a hygroscopic salt with HCl: triethylamine hydrochloride, NEt ₃ · HCl. In addition to this, it participates as a homogeneous catalyst in the synthesis of esters and amides, making it a necessary solvent in organic laboratories.

Also, together with other reagents, it allows the oxidation of primary and secondary alcohols to aldehydes and ketones, respectively. Like ammonia it is a base, and therefore it can form organic salts through neutralization reactions.

Structure of triethylamine

Structure of triethylamine. Source: Benjah-bmm27.

The upper image shows the structure of triethylamine with a model of spheres and bars. At the center of the molecule is the nitrogen atom, represented by a bluish sphere; and linked to it, the three ethyl groups, like small branches of black and white spheres.

Although not visible to the naked eye, on the nitrogen atom there is a pair of unshared electrons. These two electrons and the three -CH ₂ CH _{3 groups} undergo electronic repulsions; but on the other hand, they contribute to define the dipole moment of the molecule.

However, such a dipole moment is less than that of, for example, diethylamine, NHEt ₂; and likewise, it lacks any possibility of forming hydrogen bonds.

This is because in triethylamine there is no NH bond, and consequently, lower melting and boiling points are observed compared to other amines that do interact in such a way.

Although there is a slight dipole moment, dispersion forces between the ethyl groups of neighboring NEt ₃ molecules cannot be ruled out. Adding up this effect, it is justified why, although triethylamine is volatile, it boils at around 89ºC, thanks to its relatively large molecular mass.

Properties

Physical appearance

Colorless liquid with unpleasant ammonia and fish odor.

Molar mass

101.193 g / mol.

Boiling point

89 ° C.

Melting point

-115 ° C. Note how weak are the intermolecular forces that bind the triethylamine molecules in their solid.

Density

0.7255 g / mL.

Solubility

It is relatively soluble in water, 5.5 g / 100g at 20ºC. Below 18.7 ºC, according to Pubchem, it is even miscible with it.

In addition to "getting along" with water, it is also soluble in acetone, benzene, ethanol, ether, and paraffin.

Vapor density

3.49 in relation to air.

Vapor pressure

57.07 mmHg at 25 ° C.

Henry's constant

66 μmol / Pa · Kg.

ignition point

-15 ° C.

Autoignition temperature

312 ° C.

Heat capacity

216.43 kJ / mol.

Heat of combustion

10,248 cal / g.

Heat of vaporization

34.84 kJ / mol.

Surface tension

20.22 N / m at 25 ° C.

Refractive index

1,400 at 20 ° C.

Viscosity

0.347 mPa · s at 25 ° C.

Basicity constant

Triethylamine has a pKb equal to 3.25.

Basicity

The basicity for this amine can be expressed by the following chemical equation:

NEt ₃ + HA <=> NHEt ₃ ⁺ + A ^-

Where HA is a weak acidic species. The NHEt ₃ ⁺ A ^- pair makes up what comes to be a tertiary ammonium salt.

The conjugated acid NHEt ₃ ⁺ is more stable than ammonium, NH ₄ ⁺, due to the fact that the three ethyl groups donate part of their electron density to decrease the positive charge on the nitrogen atom; therefore, triethylamine is more basic than ammonia (but less basic than OH ^-).

Applications

Reactions catalyzed by triethylamine. Source: Tachymètre.

The basicity, in this case, the nucleophilicity of triethylamine is used to catalyze the synthesis of ester and amides from a common substrate: an acyl chloride, RCOCl (upper image).

Here the free pair of electrons from nitrogen attacks the carbonyl group, forming an intermediate; which, consecutively, is attacked by an alcohol or an amine to form an ester or amide, respectively.

In the first row of the image the mechanism followed by the reaction to produce the ester is visualized, while the second row corresponds to the amide. Note that in both reactions triethylamine hydrochloride, NEt ₃ · HCl, is produced, from which the catalyst is recovered to start another cycle.

Polymers

The nucleophilicity of triethylamine is also used to add to certain polymers, curing them and giving them greater masses. For example, it is part of the synthesis of polycarbonate resins, polyurethane foams and epoxy resins.

Chromatography

Its remote amphiphilicity and volatility allows its derived salts to be used as reagents in ion exchange chromatography. Another implicit use of triethylamine is that various tertiary amine salts can be obtained from it, such as triethylamine bicarbonate, NHEt ₃ HCO ₃ (or TEAB).

Commercial products

It has been used as an additive in the formulation of cigarettes and tobacco, food preservatives, floor cleaners, flavorings, pesticides, colorants, etc.

Risks

Triethylamine fumes are not only unpleasant but dangerous, as they can irritate the nose, throat, and lungs, leading to pulmonary edema or bronchitis. Also, being denser and heavier than air, they remain at ground level, moving to possible heat sources to later explode.

Containers of this liquid should be as far away from fire because they represent an imminent danger of explosion.

In addition, its contact with species or reagents such as: alkali metals, trichloroacetic acid, nitrates, nitric acid (as it would form nitrosoamines, carcinogenic compounds), strong acids, peroxides and permanganates should be avoided.

It should not touch aluminum, copper, zinc or their alloys, as it has the ability to corrode them.

Regarding physical contact, it can cause allergies and skin rashes if the exposure is acute. In addition to the lungs, it can affect the liver and kidneys. And regarding eye contact, it causes irritation, which could even damage the eyes if they are not treated or cleaned in time.

References

1. Morrison, RT and Boyd, R, N. (1987). Organic Chemistry. 5th Edition. Editorial Addison-Wesley Interamericana.
2. Carey F. (2008). Organic Chemistry. (Sixth edition). Mc Graw Hill.
3. Graham Solomons TW, Craig B. Fryhle. (2011). Organic Chemistry. Amines. (10th edition.). Wiley Plus.
4. Wikipedia. (2019). Triethylamine. Recovered from: en.wikipedia.org
5. Merck. (2019). Triethylamine. Recovered from: sigmaaldrich.com
6. National Center for Biotechnology Information. (2019). Triethylamine. PubChem Database. CID = 8471. Recovered from: pubchem.ncbi.nlm.nih.gov
7. Toxicology Data Network. (sf). Triethylamine. Recovered from: toxnet.nlm.nih.gov
8. New Jersey Department of Health. (2010). Triethylamine. Recovered from: nj.gov