Potassium tert-butoxide

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Potassium tert-butoxide
Skeletal formula of potassium tert-butoxide Ball-and-stick model of the cubane tetramer that potassium tert-butoxide adopts in
Potassium-tert-butoxide-3D-balls-ionic.png
Identifiers
CAS number 865-47-4 YesY
PubChem 70077
ChemSpider 63266 YesY
Jmol-3D images Image 1
Properties
Molecular formula C4H9KO
Molar mass 112.21 g mol−1
Appearance solid
Melting point 256 °C (493 °F; 529 K)
Hazards
MSDS Oxford MSDS
EU classification Harmful (Xn), Corrosive (C)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
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Infobox references

Potassium tert-butoxide is the chemical compound with the formula K+(CH3)3CO. This colourless solid is a strong base (pKa of conjugate acid around 17) which is useful in organic synthesis. It exists as a tetrameric cubane-like cluster. It is often seen written in chemical literature as potassium t-butoxide.

Preparation[edit]

Potassium t-butoxide is commercially available as a solution and as a solid, but it is often generated in situ for laboratory use because samples are so sensitive and older samples are often of poor quality. It is prepared by the reaction of dry tert-butyl alcohol with potassium metal.[1] The solid is obtained by evaporating these solutions followed by heating the solid. The solid can be purified by sublimation at 220 °C and 1 mmHg.

Structure[edit]

Potassium tert-butoxide crystallises from THF/pentane at −20 °C as [tBuOK·tBuOH], which consists of infinite one-dimensional chains linked by hydrogen bonding. Sublimation of [tBuOK·tBuOH] affords the tetramer [tBuOK]4, which adopts a cubane-like structure. Mild Lewis basic solvents such as THF and diethyl ether do not break up the tetrameric structure, which persists in the solid, in solution and even in the gas phase.[2]

Applications[edit]

The tert-butoxide species is itself useful as a strong, non-nucleophilic base in organic chemistry.[3] It is not as strong as amide bases, e.g. lithium diisopropylamide, but stronger than potassium hydroxide. Its steric bulk inhibits the group from participating in nucleophilic addition, such as in a Williamson ether synthesis or an SN2 reaction. Substrates that are deprotonated by potassium t-butoxide include terminal acetylenes and active methylene compounds. It is useful in dehydrohalogenation reactions.

Modifications[edit]

Many modifications have been reported that influence the reactivity of this reagent. The compound adopts a complex cluster structure (the picture to the right is a simplified cartoon), and additives that modify the cluster affect the reactivity of the reagent. For example, DMF, DMSO, hexamethylphosphoramide (HMPA), and 18-crown-6 interact with the potassium center, enhancing the basicity of the butoxide. Schlosser's base, a mixture of the alkoxide and an alkyl lithium compound, is a related but stronger base.[3]

References[edit]

  1. ^ William S. Johnson and William P. Schneider (1963), "β-Carbethoxy-γ,γ-diphenylvinylacetic acid", Org. Synth. ; Coll. Vol. 4: 132 
  2. ^ Chisholm, Malcolm H.; Drake, Simon R.; Naiini, Ahmad A.; Streib, William E. (1991). "Synthesis and X-ray crystal structures of the one-dimensional ribbon chains [MOBut·ButOH] and the cubane species [MOBut]4 (M = K and Rb)". Polyhedron 10 (3): 337–345. doi:10.1016/S0277-5387(00)80154-0. 
  3. ^ a b Drury Caine “Potassium t-Butoxide” in Encyclopedia of Reagents for Organic Synthesis John Wiley & Sons, New York, 2006. doi: 10.1002/047084289X.rp198.pub2. Article Online Posting Date: September 15, 2006