Inorganic nonaqueous solvent
An inorganic nonaqueous solvent is a solvent other than water, that is not an organic compound. Common examples are liquid ammonia and liquid sulfur dioxide. These solvents are used industrially and in chemical research. These solvents are used for reactions that cannot occur in aqueous solutions.
Acid-base chemistry
The Brönsted theory of acids can be extended to non-aqueous solvants which possess one or more hydrogen atom which can dissociate: such solvant are known as protic solvants.
Strong acids and weak acids
A strong acid is an acid which exists mostly or entirely in its dissociated form, that is to say that the equilibrium
- HA ⇌ H+(solvated) + A−
is far to the right. In water, a strong acid is normally taken to be one with a pKa value of less than zero, e.g. hydrochloric acid.
A weak acid may exist mostly in its undissociated form: this is the case for acetic acid in water.
Limiting acids
The limiting acid in a given solvant is the solvated form of the hydrogen ion. In water, this is usually denoted H3O+ and known as the hydronium ion. An acid which has more of a tendency to donate a hydrogen ion than the limiting acid will be a strong acid in the solvant considered, and will exist mostly or entirely in its dissociated form.
Limiting bases
The limiting base in a given solvant is the ion derived from deprotonation of a solvant molecule. In water, this is the hydroxide ion, OH−. A base which has more affinity for protons than the limiting base cannot exist in solution, as it will react with the solvant.
Liquid ammonia
The limiting acid in liquid ammonia is the ammonium ion, which has a pKa value in water of 9.25. The limiting base is the amide ion, NH2−. This is a stronger base than the hydroxide ion and so cannot exist in aqueous solution. The pKa value of ammonia is estimated to be approximately 34 (c.f. water, 14).
Any acid which is a stronger acid than the ammonium ion will be a strong acid in liquid ammonia. This is the case for acetic acid, which is completely dissociated in liquid ammonia solution. The addition of pure acetic acid and the addition of ammonium acetate have exactly the same effect on a liquid ammonia solution: the increase in its acidity: in practice, the latter is preferred for safety reasons.
Bases can exist in solution in liquid ammonia which cannot exist in aqueous solution: this is the case for any base which is stronger than than the hydroxide ion but weaker than the amide ion. Many carbon anions can be formed in liquid ammonia solution by the action of the amide ion on organic molecules (see sodium amide for examples).
Superacids
A superacid is a medium in which the hydrogen ion is only very weakly solvated. The classic example is a mixture of antimony pentafluoride and liquid hydrogen fluoride:
- SbF5 + HF ⇌ H+ + SbF6−
The limiting base, the hexfluoroantimonate anion SbF6−, is so weakly attracted to the hydrogen ion that virtually any other base will bind more strongly: hence, this mixture can be used to protonate organic molecules which would not be considered bases in other solvants.
Comparisons of acidity and basicity between solvants
There exists a large corpus of data concerning acid strengths in aqueous solution (pKa values), and it is tempting to transfer this to other solvants. Such comparisons are, however, fraught with danger, as they only consider the effect of solvation on the stability of the hydrogen ion, while neglecting its effects on the stability of the other species involved in the equilibrium. Gas phase acidities (normally known as proton affinities) can be measured, and their relative order is often quite different from that of the aqueous acidities of the corresponding acids. Few quantitative studies on acidities in nonaqueous solvants have been carried out, although some qualitative data are available. It appears that most acids which have a pKa value of less than 9 in water are indeed strong acids in liquid ammonia. However, the hydroxide ion is often a much stronger base in nonaqueous solvants (e.g. liquid ammonia, DMSO) than in water.
It should be noted that pH values are at present undefined in nonaqueous solvants, as the definition of pH assumes an aqueous solution.