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SuperbaseFrom Wikipedia, the free encyclopedia

n chemistry, a superbase is an extremely basic compound or substance that has a high affinity for protons. T

hydroxide ion is the strongest base possible in aqueous solutions, but bases exist with much greater strengths

han can exist in water. Such bases are valuable in organic synthesis and are fundamental to physical organic

hemistry. Superbases have been described and used since the 1850s.[1] Reactions involving superbases often

equire special techniques since they are destroyed by water and atmospheric carbon dioxide as well as oxyg

nert atmosphere techniques and low temperatures minimize these side reactions. Superbases also have a

orrosive effect.

Contents

1 Definitions2 Classes of superbases

2.1 Organic

2.2 Organometallic

2.3 Inorganic

3 See also

4 References

Definitions

UPAC defines superbases simply as a "compound having a very high basicity, such as lithium

diisopropylamide."[2] Caubère defines superbases qualitatively but more precisely: "The term superbases sho

only be applied to bases resulting from a mixing of two (or more) bases leading to new basic species possess

nherent new properties. The term superbase does not mean a base is thermodynamically and/or kinetically

tronger than another, instead it means that a basic reagent is created by combining the characteristics of seve

different bases."[3]

Superbases have also been defined semi-quantitatively as any species with a higher absolute proton affinity

APA = 245.3 kcal/mol) and intrinsic gas phase basicity (GB = 239 kcal/mol) than Alder's canonical proton

ponge (1,8-bis-(dimethylamino)-naphthalene).[4]

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Classes of superbases

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

Organic

Organic superbases are almost always neutral, nitrogen-containing species. Despite enormous proton affinityorganosuperbases are prized for their heightened reactivity tempered by low nucleophilicity and relatively m

onditions of use. Increasingly important in organic synthesis, these include the phosphazenes, amidines and

guanidines. Other organic compounds also meet the physicochemical or structural definitions of 'superbase'.

Proton chelators like the aromatic proton sponges and the bispidines are also superbases. Multicyclic

polyamines, like DABCO might also be loosely included in this category.[5]

Organometallic

Organometallic compounds of reactive metals are often superbases, including organolithium and

organomagnesium (Grignard reagent) compounds. Another type of organic superbase has a reactive metalexchanged for a hydrogen on a heteroatom, such as oxygen (unstabilized alkoxides) or nitrogen (metal amide

uch as lithium diisopropylamide). A desirable property in many cases is low nucleophilicity, i.e. a non-

nucleophilic base. Unhindered alkyllithiums, for example, cannot be used with electrophiles such as carbony

groups, because they attack the electrophiles as nucleophiles.

The Schlosser base (or Lochmann-Schlosser base), the combination of n-butyllithium and potassium tert -

butoxide, is a commonly used superbase. n-Butyllithium and potassium tert -butoxide form a mixed aggregat

greater reactivity than either reagent alone and with distinctly different properties in comparison to tert -

butylpotassium.[6]

Inorganic

norganic superbases are typically salt-like compounds with small, highly charged anions, e.g. lithium nitride

Alkali and earth alkali metal hydrides potassium hydride and sodium hydride are superbases. Such species ar

nsoluble in all solvents owing to the strong cation-anion interactions, but the surfaces of these materials are

highly reactive and slurries are useful in synthesis.

See also

SuperacidPhosphazene

References

1. "BBC - h2g2 - History of Chemistry - Acids and Bases". Retrieved 2009-08-30.

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2. IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–

"superacid (http://goldbook.iupac.org/S06135.html)".

3. Caubère, P (1993). "Unimetal super bases". Chemical Reviews 93: 2317–2334. doi:10.1021/cr00022a012.

4. Raczynska, E. D.; Decouzon, M.; Gal, J.-F.; et al. (1998). "Superbases and superacids in the gas phase". Trends in

Organic Chemistry 7: 95–103.

5. Superbases for Organic Synthesis Ed. Ishikawa, T., John Wiley and Sons, Ltd.: West Sussex, UK. 2009.

6. Schlosser, M. (1988). "Superbases for organic synthesis". Pure Appl. Chem. 60 (11): 1627–1634.

doi:10.1351/pac198860111627.

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Categories: Bases

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