superbase

If a superacid is an acid more extreme than concentrated sulfuric acid, a superbase is its mirror image: a base far stronger than the ordinary hydroxide ion — strong enough to rip protons off molecules that almost never give them up, such as the C-H bonds of hydrocarbons. Because water levels any base stronger than OH- (it just makes hydroxide), superbases, like superacids, must be handled in non-aqueous, scrupulously dry conditions; a trace of water destroys them instantly.

Superbases come in a few flavors. The most extreme are simple, very small, very charge-dense anions that desperately want a proton: the hydride ion H-, the amide ion NH2- (from sodium amide, used in liquid ammonia), the oxide ion O2-, the methyl carbanion in methyllithium, and alkyllithiums and Grignard-related organometallics generally. There are also organic superbases prized in synthesis for being strong yet non-nucleophilic: lithium diisopropylamide (LDA), the phosphazene bases (the 'P4' bases of Schwesinger), and amidines and guanidines like DBU and the proton sponges. A useful way to see their strength: the strength of a base mirrors the weakness of its conjugate acid, so a base whose conjugate acid is a feeble proton-donor (a hydrocarbon, pKa near 40-50) is correspondingly ferocious.

Superbases are everyday workhorses of synthetic chemistry, especially for deprotonating weak carbon acids to make reactive carbanions and enolates, exactly because they are strong without attacking the substrate as a nucleophile (that is why bulky LDA is chosen over a small, nucleophilic base like hydroxide or methoxide). They also illustrate the symmetry of acid-base extremes: superacid and superbase chemistry are the two far ends of the same proton-affinity scale, both inaccessible in water for the same reason — the leveling effect.