Triethylamine is an organic liquid. Glacial acetic acid is also such a compound. Both are sufficiently inexpensive that they could be used as solvents for organic syntheses. The former is distinctly basic and the latter acidic.
Triethylamine/acetic acid forms a constant high boiling azeotrope bp. 163.0℃ more than 40 Cº above the acetic acid. The azeotropic composition is 69% glacial acetic acid and 31.0% triethylamine by weight. This is a molecular ratio of 3.75: 1.0 acetic acid: triethylamine or 14: 4 in nonfractional units. If we imagine a medium in which the triethylamine and acetic acid molecules neutralize each other in a 1: 1 ratio, this azeotropic composition will be distinctly acidic because it contains a substantial excess of the carboxylic acid. Nevertheless, the excess acetic acid cannot be distilled away. It forms a constant boiling mixture. The upper reaction temperature for this mixture would be its constant boiling azeotrope temperature of 163.0℃.
It is this azeotrope that is a principle subject of US 3,244,761 where at column 9 starting at line 71, it is remarked, “It should be emphasized that even though molar excesses of acids are normally used in the preparation of my selective solvents (of which the triethylamine/acetic acid azeotrope is one) the corrosively of the solvents towards materials vulnerable to acid attack is unexpectedly low, sometimes approaching that of distilled water….The reason for the surprising lack, or minimization, of corrosively in my selective solvents is not understood with certainty.”
More will be related from this patent later.
A different mixture of these two substances has also been described. It is reported that a 1: 1 molar ratio of the two components forms something resembling an ionic liquid. Such a composite would comprise 101 gm of triethylamine with 60 gm of glacial acetic acid. This is 62.73% triethylamine and 37.27% acetic acid; almost the exact opposite of the azeotropic composition. This mixture would be expected to be basic. Upon boiling about three-quarters of its triethylamine would be expected to distill off at its bp of 89.5℃ leaving a composition of the azeotropic mixture.
Both of these aforementioned mixtures may be expected to show useful, and even unusual solvent properties. Both glacial acetic acid and triethylamine as pure fluid compositions, on their own, can be expected to dissolve organic compounds more readily than water. As mixtures with either strongly hydrogen-bonded species or solvent-separated ion pairs (1 : 1 combination of acid and base), an even more comprehensive range may be soluble. Also, because the medium will cause the components to strongly associate with each other, the dielectric strength may permit various inorganic salts also to dissolve appreciably.
We also know what happens when water is added to one of these compositions. The main subject of US 3,244,761 is the use of triethylamine/acetic acid high boiling azeotrope, among other acid-base combinations, for the solvent extraction of polycyclic aromatics from crude hydrocarbon streams. Therein, it is reported that dilution with water causes the triethylamine and the acetic acid to be dissolved in the bulk water enabling any more hydrophilic elements to be recovered. It also reports that mixtures of high boiling azeotrope of triethylamine/acetic acid can be recovered from a mixture with water simply by distillation whereupon the water comes over first at 100℃ and then the azeotrope at 163℃. The evidence for this is in Example VIII of this patent.
This patent also teaches “The solvents of this invention (of which triethylamine/acetic acid is one) in addition to being highly selective towards aromatic hydrocarbons, exhibit high solvent power..”
At another point, ‘still another way in which my process (of solvent extraction) can be carried out is to employ an anti-solvent in conjunction with the amine-acid solvent…The use of such anti-solvents in hydrocarbon extraction processes is well known….[t]ypical anti-solvents for purposes of this invention are kinds of paraffin, such as pentane, heptane, octane, isooctane…”
This latter makes evident that triethylamine/acetic acid is not significantly miscible with such saturated hydrocarbon liquids.
N.B. It is noteworthy that US 3,244,761 also identifies the high boiling azeotrope of triethylamine and acetic acid as a useful ‘separation solvent’ for use in extractive distillation. It could be used for separating compounds neither of which boils above about 160℃ (the bp of the azeotrope is 163℃ ) and where one of the components has an aromatic substructure and the other does not. The more aromatic material will be held back in the still pot. If a composition with a higher upper-temperature limit is required US 3,244,761 suggests a mixture of tributylamine and caproic acid or a combination of dimethylaniline and benzoic acid. Both these mixtures would have bps greater than 450℃.