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chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a natural science that covers the elements that make up matter to the compounds made of atoms, molecules and ions: their composition, structure, proper ...
, an alkoxide is the
conjugate base A conjugate acid, within the Brønsted–Lowry acid–base theory, is a chemical compound formed when an acid donates a proton () to a base—in other words, it is a base with a hydrogen ion added to it, as in the reverse reaction it loses a ...
of an alcohol and therefore consists of an organic group bonded to a negatively charged
oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements ...
atom. They are written as , where R is the organic
substituent A substituent is one or a group of atoms that replaces (one or more) atoms, thereby becoming a moiety in the resultant (new) molecule. (In organic chemistry and biochemistry, the terms ''substituent'' and ''functional group'', as well as '' side ...
. Alkoxides are strong bases and, when R is not bulky, good
nucleophile In chemistry, a nucleophile is a chemical species that forms bonds by donating an electron pair. All molecules and ions with a free pair of electrons or at least one pi bond can act as nucleophiles. Because nucleophiles donate electrons, they ar ...
s and good
ligand In coordination chemistry, a ligand is an ion or molecule (functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's elect ...
s. Alkoxides, although generally not stable in protic solvents such as water, occur widely as intermediates in various reactions, including the Williamson ether synthesis.
Transition metal In chemistry, a transition metal (or transition element) is a chemical element in the d-block of the periodic table (groups 3 to 12), though the elements of group 12 (and less often group 3) are sometimes excluded. They are the elements that can ...
alkoxides are widely used for coatings and as
catalyst Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...
s. Enolates are unsaturated alkoxides derived by deprotonation of a bond adjacent to a
ketone In organic chemistry, a ketone is a functional group with the structure R–C(=O)–R', where R and R' can be a variety of carbon-containing substituents. Ketones contain a carbonyl group –C(=O)– (which contains a carbon-oxygen double b ...
or
aldehyde In organic chemistry, an aldehyde () is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred to as an aldehyde but can also be classified as a formyl gro ...
. 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 derivative of
benzene Benzene is an organic chemical compound with the molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar ring with one hydrogen atom attached to each. Because it contains only carbon and hydrogen atoms ...
.
Phenol Phenol (also called carbolic acid) is an aromatic organic compound with the molecular formula . It is a white crystalline solid that is volatile. The molecule consists of a phenyl group () bonded to a hydroxy group (). Mildly acidic, it r ...
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.


Structure

Alkali metal alkoxides are often oligomeric or polymeric compounds, especially when the R group is small (Me, Et). The alkoxide anion is a good
bridging ligand In coordination chemistry, a bridging ligand is a ligand that connects two or more atoms, usually metal ions. The ligand may be atomic or polyatomic. Virtually all complex organic compounds can serve as bridging ligands, so the term is usually ...
, thus many alkoxides feature or linkages. In solution, the alkali metal derivatives exhibit strong ion-pairing, as expected for the alkali metal derivative of a strongly basic anion.


Preparation


From reducing metals

Alkoxides can be produced by several routes starting from an alcohol. Highly reducing metals react directly with alcohols to give the corresponding metal alkoxide. The alcohol serves as an
acid In computer science, ACID ( atomicity, consistency, isolation, durability) is a set of properties of database transactions intended to guarantee data validity despite errors, power failures, and other mishaps. In the context of databases, a se ...
, and
hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-to ...
is produced as a by-product. A classic case is sodium methoxide produced by the addition of
sodium Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable ...
metal to
methanol Methanol (also called methyl alcohol and wood spirit, amongst other names) is an organic chemical and the simplest aliphatic alcohol, with the formula C H3 O H (a methyl group linked to a hydroxyl group, often abbreviated as MeOH). It is ...
: :2 CH3OH + 2Na -> 2CH3ONa + H2 Other
alkali metal The alkali metals consist of the chemical elements lithium (Li), sodium (Na), potassium (K),The symbols Na and K for sodium and potassium are derived from their Latin names, ''natrium'' and ''kalium''; these are still the origins of the names ...
s can be used in place of sodium, and most alcohols can be used in place of methanol. 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.


Properties


Reactions with alkyl halides

The alkoxide ion and its salts react with primary alkyl halides in an reaction to form an ether via the Williamson Ether Synthesis.


Hydrolysis and transesterification

Aliphatic metal alkoxides decompose in water as summarized in this idealized equation: : In the transesterification process, metal alkoxides react with esters 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, for example, is commonly used for this purpose, a reaction that is used in the production of
biodiesel Biodiesel is a form of diesel fuel derived from plants or animals and consisting of long-chain fatty acid esters. It is typically made by chemically reacting lipids such as animal fat ( tallow), soybean oil, or some other vegetable oi ...
.


Formation of oxo-alkoxides

Many metal alkoxide compounds also feature oxo-
ligands In coordination chemistry, a ligand is an ion or molecule (functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's electr ...
. Oxo-ligands typically arise via the hydrolysis, often accidentally, and via ether elimination: :


Thermal stability

Many metal alkoxides thermally decompose in the range ~100–300 °C. Depending on process conditions, this thermolysis can afford nanosized 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.


Illustrative alkoxides


Further reading

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References

{{Coordination complexes Functional groups Coordination chemistry