Superbase

In chemistry, a superbase is an extremely strong base. Although there is no official definition for the designation of a superbase, sodium hydroxide is considered a benchmark, much like sulfuric acid is used to define superacids.^[1] The hydroxide ion is considered a good benchmark because it is the strongest base which can exist in aqueous solution.^{[citation needed]}. Superbases have been described and used since the 1850s.^[2]

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

Organometallic compounds of reactive metals are often superbases; including organolithium and organomagnesium (Grignard reagent) compounds. Another type of organic superbase has a reactive metal exchanged for a hydrogen on a heteroatom, such as oxygen (unstabilized alkoxides) or nitrogen (metal amides such as lithium diisopropylamide).^[3]

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.

See also

• Superacid
• Phosphazene

References

1. ^ "Superbase - Education Resources". http://www.thecatalyst.org/resource/2006/04/21/Superbase/. Retrieved 2009-08-30. 
2. ^ "BBC - h2g2 - History of Chemistry - Acids and Bases". http://www.bbc.co.uk/dna/h2g2/alabaster/A708257. Retrieved 2009-08-30. 
3. ^ "IUPAC Gold Book - superacid". http://goldbook.iupac.org/S06135.html. Retrieved 2009-08-30.