Which of these liquids is miscible with water? Ethanol, methanol, hexane... Why? I thought since ethanol has a nonpolar tail, it isn't soluble in water.

1 Answer
Apr 30, 2017

Because polarity is a continuum, and solubility lies somewhere on that continuum.......


Ethanol is miscible in water in all concentrations; and so is methanol. On the other hand petroleum ether, hexanes, is completely miscible in ethanol, whereas, believe it or not, methanol is IMMISCIBLE in hexanes. By the same token, should we have an aqueous solution of brine; addition of ethanol would precipitate the brine.

Why? Well, clearly ethanol has polar and non-polar functionality. The hydroxyl group promotes water solubility, whereas the hydrocarbyl tail allows some solubility in a non-polar solvent such as hexanes. The effect of the hydroxyl group is quite startling with respect to volatility. Ethanol has a normal boiling point of #78.4# #""^@C#; but ethane has a normal boiling point of #-89.0# #""^@C#; and propane has a normal boiling point of #-42.0# #""^@C#. Higher alcohols, with longer hydrocarbyl tails, e.g. propanol, butanol, have more limited water solubility.

Throw in some polarity (but not hydrogen bonding), and we find that #"dimethyl ether"# has a normal boiling point of #-21.0# #""^@C#, and #"diethyl ether"# has a normal boiling point of #34.6# #""^@C# (note that while polar, both of these solvents are IMMISCIBLE in water, and this forms the basis of phase extraction of organic compounds from an ethereal phase).

As regards the use of ethanol as a solvent for non-polar solutes, it is one of the most useful solvents in the laboratory, and it is probably the first solvent we would turn to for recrystallization of organic solutes. Organic solutes tend to have some solubility in HOT ethanol (and the solvent has got a good temperature gradient), whereas, upon cooling, the solutes tend to crystallize out - which is arguably the effect of the hydroxyl group (and thus ethanol is a preferred solvent to recrystallize such solutes).

Ethanol is also none too flammable; certainly less so than hexanes; I would happily use a heat gun on an ethanolic solution, whereas I would think twice about using it on a hexanes solution. As an important bonus, ethanol smells nice, and is not too hard on your hands.

Just to add for clarity. The ethanol we use in a lab is often known supplied as industrial methylated spirit. Why? Because when it is supplied it is adulterated with so-called denaturants such as phenol, or methanol (hence #"methylated spirit"#). And if you drink this stuff you go blind. And thus laboratory ethanol, #"IMS"#, ESCAPES the excise and taxes that are normally paid on alcoholic drinks.