Superbase


Superbase

In chemistry, a superbase is an extremely strong base. There is no commonly accepted definition for what qualifies as a superbase, but most chemists would accept sodium hydroxide as a 'benchmark' base just as sulfuric acid is a 'benchmark' acid (see superacid). The hydroxide ion is a good benchmark because it is the strongest base that can exist in a water solution. Stronger bases are neutralized by water acting as an acid, to produce a corresponding hydroxide (and protonated superbase). Another use that can define superbase is stoichiometric α-deprotonation of a carbonyl compound into an enolate, something that cannot be done by "regular bases". Despite this, the term still doesn't have a standard chemical definition, so for example Proton Sponge may be called "superbase".

There are three main classes of superbases: organic, organometallic, and inorganic.

Organometallic compounds of reactive metals are usually superbases, for example organolithium and organomagnesiums (Grignard reagents). Another type of organic superbase has a reactive metal exchanged for a hydrogen on a heteroatom, such as oxygen (unstabilized alkoxides) or nitrogen (lithium diisopropylamide).

Reactions involving superbases are usually water-sensitive, conducted under an inert atmosphere and at a low temperature. A desirable property in many cases is low nucleophilic reactivity, i.e. a non-nucleophilic base. Unhindered alkyllithiums, for example, cannot be used with electrophiles such as carbonyl groups, because they attack the electrophiles as nucleophiles.

In organic synthesis, the Schlosser base (or Lochmann-Schlosser base), i.e. the combination of "tert"-butyllithium and potassium tert-butoxide, is a commonly used superbase. "tert"-Butyllithium undergoes a cation exchange with potassium tert-butoxide giving tert-butyl potassium and lithium tert-butoxide, an exchange driven by lithium's affinity for the alkoxide oxygen. Replacement of the lithium cation with potassium causes the tert-butyl anion to acquire greater ionic character and thus greater basicity.

Inorganic superbases are typically salts with highly charged, small negative ions, e.g. lithium nitride, which has extreme negative charge density and so is highly attracted to acids, like the aqueous hydronium ion. Alkali and earth alkali metal hydrides (sodium hydride, calcium hydride) are superbases.

ee also

*Superacid
*Phosphazene


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