|Acids and Bases|
In chemistry, a superbase is an extremely basic compound or caustic substance that has a high affinity for protons. The hydroxide ion is the strongest base possible in aqueous solutions, but bases that exist with much greater strengths than the bases that could exist in aqueous solutions are possible. Such bases are valuable in organic synthesis and are fundamental to physical organic chemistry. Superbases have been described and used since the 1850s. Reactions involving superbases often require special techniques since they are destroyed by water and atmospheric carbon dioxide as well as oxygen. Inert atmosphere techniques and low temperatures minimize these side reactions.
A "superbase" has been defined as an organic compound whose basicity is greater than that of proton sponge, which has a conjugate pKa of 12.1. Strong superbases can be prepared from extending the hydrogen bond network of multiple amino groups substituted on an aromatic core. Superbases are of great interest to practicing organic chemists due to their reactivity as well as good solubility in organic solvents.
IUPAC broadly defines a superbase as a "compound having a very high basicity, such as lithium diisopropylamide." Caubère defines superbases as "bases resulting from a mixing of two (or more) bases leading to new basic species possessing inherent new properties. The term superbase does not mean a base is thermodynamically and/or kinetically stronger than another, instead it means that a basic reagent is created by combining the characteristics of several different bases."
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 sponge (1,8-bis-(dimethylamino)-naphthalene).
Classes of superbases
Organic superbases are almost always charge-neutral, nitrogen-containing species. Despite enormous proton affinity, organosuperbases exhibit low nucleophilicity. 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.
Organometallic compounds of reactive metals can be superbases, including organolithium and organomagnesium (Grignard reagent) compounds. Another type of organometallic 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). 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 carbonyl 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 aggregate of greater reactivity than either reagent alone and with distinctly different properties in comparison to tert-butylpotassium.
Inorganic superbases are typically salt-like compounds with small, highly charged anions, e.g. lithium nitride. Alkali and alkaline earth metal hydrides such as potassium hydride and sodium hydride are superbases. Such species are insoluble 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.
Superbases in organic chemistry
The strongest superbases
The list of the strongest superbases as ranked by their calculated gas phase basicity (proton affinity) is given below.
- Ortho-diethynylbenzene dianion
- Meta-diethynylbenzene dianion
- Para-diethynylbenzene dianion
- Lithium monoxide anion
- Methyl anion
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