Non-Aqueous Solvents: Chemistry Water Cannot Support

Why water is limiting

Water self-ionises, and that sets a window. Any acid stronger than the hydronium ion is completely deprotonated in water, and any base stronger than hydroxide is completely protonated. All such species therefore appear equally strong — the levelling effect.

Solvent system definitions

For any self-ionising solvent, an acid increases the concentration of the characteristic cation and a base increases the characteristic anion.

In liquid ammonia, therefore, an ammonium salt behaves as an acid and a metal amide as a base — a neutralisation reaction between them producing the solvent, exactly parallel to acid plus base giving water.

Liquid ammonia

The most important non-aqueous solvent in inorganic chemistry, for two distinct reasons.

It supports very strong bases

Being more basic than water, ammonia does not level bases as severely, so amide and related species can exist in solution. Reactions requiring a base far stronger than hydroxide are therefore run in it.

It dissolves alkali metals to give solvated electrons

Alkali metals dissolve to give deep blue solutions containing metal cations and electrons solvated by ammonia. These solutions are:

• Strongly reducing, which is what makes Birch reduction and dissolving-metal reductions possible.
• Electrically conducting, and at high concentration they become bronze and metallic in appearance.
• Paramagnetic at low concentration, from the unpaired solvated electrons.

The blue colour arises from the solvated electron absorbing in the red region. Explaining the colour, the conductivity and the reducing power from the same species is the expected answer.

Comparing solvents

Permittivity is the property that decides whether a solvent will dissolve ionic compounds at all. Low-permittivity solvents leave ions paired rather than separated, which is why ion pairing is significant in them and negligible in water.

Differentiating solvents

A solvent that does not level a set of acids is described as differentiating for them, since their relative strengths remain distinguishable. Acetic acid, being less basic than water, differentiates between acids that water levels — which is how their relative strengths were originally established.

Choosing a differentiating solvent to compare acid strengths is a standard applied question, and the reasoning is simply that the solvent must be less willing to accept protons than water is.

Frequently asked questions

Why do alkali metal solutions in ammonia conduct electricity?

Because they contain both solvated cations and solvated electrons, and both are mobile charge carriers.

What makes the solutions blue?

The solvated electron absorbs in the red part of the spectrum, so the transmitted light appears blue. The colour is independent of which alkali metal was dissolved, which shows the electron is the absorbing species.

Why can very strong bases exist in ammonia but not water?

Because ammonia is a weaker acid than water, so it does not protonate strong bases as readily. Water levels them by protonating everything stronger than hydroxide.

What decides whether a solvent dissolves an ionic compound?

Chiefly its permittivity, which determines how effectively it screens the attraction between ions, together with its ability to solvate the separated ions.