Constant boiling azeotropes are potential solvent systems for reactions. Compared to any random solvent mixture, their advantage is that the composition can be consistently prepared with a stable ratio of components so long as the pressure can be held constant. The mixture can be repurified at the end of its use, so long as the other components of the waste reaction mixture are not volatile, simply by distilling the residual solvent mixture.
Azeotropes are typically mixtures of quite unlike solvents so the combinations might be expected to show, in most instances, substantially different properties from any pure liquid solvent.
What might some of these particularly attractive candidates be?
Acetic acid (58.5) Chlorobenzene (41.5) bp 114.7
Acetic acid (38.5) Tetrachloroethylene (61.5) bp 107.4
Acetone (88.5) Carbon Tetrachloride (11.5) bp 56.1
Carbon Disulfide (63.0) Ethyl formate (37.0) bp 39.4
Cyclohexane (72.0) Nitromethane (28.0) bp 70.2
Dibutylamine (49.5) Water (50.5) bp.97.0
Acetic acid/ chlorobenzene and acetic acid/tetrachloroethylene could be interesting solvents for free-radical reactions. both would also be more polar versions of chlorobenzene or tetrachloroethylene that would easily send all their organic solutes into the halogenated layer by simply adding water to the completed reaction mixture.
Acetone/carbon tetrachloride would be a less polar, lower dielectric constant version of acetone or methyl ethyl ketone.
Carbon disulfide/ethyl formate might turn out to be a more convenient fluid for working with compounds particularly soluble in carbon disulfide.
Cyclohexane/nitromethane probably has an upper critical solution temperature within a practically useful range. I the mixture can be cooled to give two separate phases this may be useful in separations.
The dibutyl amine/ water mixture will very likely be useful for dissolving more hydrophobic solutes into an aqueous phase.
It is frequently argued that single-component solvents are to be preferred over solvent mixtures because they are:
(1) easier to purify for reuse, and
(2) easier to acquire in precise, and pure form.
The first of objections is usually a 'red herring' as far as pharmaceutical or fine chemical synthesis is concerned. Solvents used to prepare such products are exceedingly rarely purified for reuse. The reason is simple; the analyses required to demonstrate that the recycled material is equivalent to fresh solvent are too costly and time-consuming.
The supposed second disadvantage simply would not be true for a lower boiling binary solvent azeotrope.