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Lithium diisopropylamide
From Wikipedia, the free encyclopedia
| Lithium diisopropylamide | |
|---|---|
 |
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| General | |
| Systematic name | Lithium diisopropylamide |
| Other names | LDA |
| Molecular formula | C6H14LiN or LiN(C3H7)2 |
| SMILES | ? |
| Molar mass | 107.1233 g/mol |
| Appearance | ? |
| CAS number | [4111-54-0] |
| Properties | |
| Density and phase | 0.79 g/cm³, ? |
| Solubility in water | Reacts with water |
| Melting point | ?°C (? K) |
| Boiling point | ?°C (? K) |
| Acidity (pKa) | 34 |
| Basicity (pKb) | ? |
| Chiral rotation [α]D | ?° |
| Viscosity | ? cP at ?°C |
| Hazards | |
| MSDS | External MSDS |
| Main hazards | corrosive |
| NFPA 704 | |
| Flash point | ?°C |
| R/S statement | R: ? S: ? |
| RTECS number | ? |
| Related compounds | |
| Other anions | ? |
| Other cations | ? |
| Related ? | ? |
| Related compounds | Superacids |
| Except where noted otherwise, data are given for materials in their standard state (at 25°C, 100 kPa) Infobox disclaimer and references |
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Lithium diisopropylamide is the chemical compound with the formula [(CH3)2CH]2NLi. Generally abbreviated LDA, it is a strong base, used in organic chemistry for the deprotonation of hydrocarbons. The reagent has been widely accepted because it is soluble in non-polar organic solvents and it is non-pyrophoric. LDA is non-nucleophilic, as are other "harpoon bases."
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LDA is formed by treating a cooled (0 to -78 °C) THF solution of diisopropylamine with n-butyllithium. The conjugate acid of diisopropylamine has pKa value of 36; therefore, it is suitable for the deprotonation of most common carbon acids including alcohols and carbonyl compounds (acids, esters, aldehydes and ketones) possessing an alpha carbon with hydrogens. In THF solution, LDA exists primarily as a dimer.[1][2] In THF solution, LDA is proposed to dissociate to afford the active base.
LDA in a solution of various solvents such as THF and ether is commercially available. In practice and for small scale use (less than 50 mmol), it is common (and actually more cost effective) to prepare LDA in situ.
The deprotonations of carbon acids can proceed with either kinetic or thermodynamic reaction control. Kinetic controlled deprotonation requires a base that is sterically hindered. For example in the case of phenylacetone, deprotonation can produce two different enolates. LDA has been shown to deprotonate the methyl group, which is the kinetic course of the deprotonation. A weaker base such as an alkoxide, which reversibly deprotonates the substrate, affords the more thermodynamically stable benzylic enolate. An alternative to the weaker base is to use a strong base which is present at a lower concentration than the ketone. For instance a slurry of sodium hydride in THF or DMF, the base only reacts at the solution-solid interface. It is the case that a ketone molecule might deprotonate at the kinetic site, this enolate will then encounter other ketone molecules. The thermodynamic enolate will form through the exchange of protons, even in an aprotic solvent which does not contain hydronium ions.
It is important to note that LDA can still act as a nucleophile, for instance it can react with tungsten hexacarbonyl as part of the synthesis of a diisopropylaminocarbyne.[citation needed] If given the proper conditions, LDA will act like other any other nucleophile and perform condensation reactions. Other even more hindered amide bases are known, for instance the deprotonation of hexamethyldisilazane (Me3SiNHSiMe3) forms such a base ([(Me3SiNSiMe3]-).
- ^ Williard, P. G.; Salvino, J. M. (1993). "Synthesis, isolation, and structure of an LDA-THF complex". Journal of Organic Chemistry 58 (1): 1-3. DOI:10.1021/jo00053a001.
- ^ (1991) "Crystal structure of lithium diisopropylamide (LDA): an infinite helical arrangement composed of near-linear nitrogen-lithium-nitrogen units with four units per turn of helix". Journal of the American Chemical Society 113 (21). DOI:10.1021/ja00021a066.