Inchemistry, analkoxide is theconjugate base of analcohol and therefore consists of an organic group bonded to a negatively chargedoxygen atom. They are written asRO−, where R is theorganylsubstituent. Alkoxides are strongbases[citation needed] and, when R is notbulky, goodnucleophiles and goodligands. Alkoxides, although generally not stable inprotic solvents such as water, occur widely as intermediates in various reactions, including theWilliamson ether synthesis.[1][2]Transition metal alkoxides are widely used for coatings and ascatalysts.[3][4]
Enolates are unsaturated alkoxides derived by deprotonation of aC−H bond adjacent to aketone oraldehyde. The nucleophilic center for simple alkoxides is located on the oxygen, whereas the nucleophilic site on enolates is delocalized onto both carbon and oxygen sites.Ynolates are also unsaturated alkoxides derived from acetylenic alcohols.
Phenoxides are close relatives of the alkoxides, in which the alkyl group is replaced by a phenyl group.Phenol is more acidic than a typical alcohol; thus, phenoxides are correspondingly less basic and less nucleophilic than alkoxides. They are, however, often easier to handle and yield derivatives that are more crystalline than those of the alkoxides.[citation needed]
Alkali metal alkoxides are often oligomeric or polymeric compounds, especially when the R group is small (Me, Et).[3][page needed] The alkoxide anion is a goodbridging ligand, thus many alkoxides featureM2O orM3O linkages. In solution, the alkali metal derivatives exhibit strong ion-pairing, as expected for the alkali metal derivative of a strongly basic anion.
Alkoxides can be produced by several routes starting from analcohol. Highly reducing metals react directly with alcohols to give the corresponding metal alkoxide. The alcohol serves as anacid, andhydrogen is produced as a by-product. A classic case issodium methoxide produced by the addition ofsodium metal tomethanol:[citation needed]
Otheralkali metals can be used in place of sodium, and most alcohols can be used in place of methanol. Generally, the alcohol is used in excess and left to be used as a solvent in the reaction. Thus, an alcoholic solution of the alkali alkoxide is used. Another similar reaction occurs when an alcohol is reacted with a metal hydride such as NaH. The metal hydride removes the hydrogen atom from the hydroxyl group and forms a negatively charged alkoxide ion.
The alkoxide ion and its salts react with primary alkyl halides in anSN2 reaction to form an ether via theWilliamson ether synthesis.[1][2]
Aliphatic metal alkoxidesdecompose in water as summarized in this idealized equation:
In thetransesterification process, metal alkoxides react withesters to bring about an exchange of alkyl groups between metal alkoxide and ester. With the metal alkoxide complex in focus, the result is the same as for alcoholysis, namely the replacement of alkoxide ligands, but at the same time the alkyl groups of the ester are changed, which can also be the primary goal of the reaction. Sodium methoxide in solution, for example, is commonly used for this purpose, a reaction that is used in the production ofbiodiesel.
Many metal alkoxide compounds also feature oxo-ligands. Oxo-ligands typically arise via the hydrolysis, often accidentally, and via ether elimination:[citation needed]
Many metal alkoxidesthermally decompose in the range ≈100–300 °C.[citation needed] Depending on process conditions, this thermolysis can producenanosized powders of oxide or metallic phases. This approach is a basis of processes of fabrication of functional materials intended for aircraft, space, electronic fields, and chemical industry: individual oxides, their solid solutions, complex oxides, powders of metals and alloys active towards sintering. Decomposition of mixtures of mono- and heterometallic alkoxide derivatives has also been examined. This method represents a prospective approach possessing an advantage of capability of obtaining functional materials with increased phase and chemical homogeneity and controllable grain size (including the preparation of nanosized materials) at relatively low temperature (less than 500–900 °C) as compared with the conventional techniques.[citation needed]
| name | molecular formula | comment |
|---|---|---|
| Tetraethyl orthosilicate | Si(OEt)4 | forsol-gel processing of Si oxides; Si(OMe)4 is avoided for safety reasons |
| Aluminium isopropoxide | Al4(OiPr)12 | reagent forMeerwein–Ponndorf–Verley reduction |
| Potassiumtert-butoxide, | K4(OtBu)4 | basic reagent in alcohol solution for organicelimination reactions |
Sodium methoxide, also calledsodium methylate and sodium methanolate, is a white powder when pure.[6] It is used as aninitiator of ananionic addition polymerization withethylene oxide, forming apolyether with high molecular weight.[citation needed] Both sodium methoxide and its counterpart prepared withpotassium are frequently used as catalysts for commercial-scale production ofbiodiesel. In this process, vegetable oils or animal fats, which chemically are fatty acid triglycerides, aretransesterified with methanol to givefatty acid methyl esters (FAMEs).
Sodium methoxide is produced on an industrial scale and is available from a number of chemical companies.
Potassium methoxide in alcoholic solution is commonly used as a catalyst fortransesterification in theproduction of biodiesel.[7]