Alkoxide
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An alkoxide is the conjugate base of an alcohol and therefore consists of an organic group bonded to a negatively charged oxygen atom. They can be written as RO–, where R is the organic substituent. Alkoxides are strong bases and, when R is not bulky, good nucleophiles and good ligands. . 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 alkoxides are widely used for coatings and as catalysts.
Enolates are unsaturated alkoxide derived by deprotonation of a C-H bond adjacent to a ketone or aldehyde. The nucleophilic center for simple alkoxides is located on the oxygen, whereas the nucleophilic site on enolates is delocalized - both carbon and oxygen are basic.
Phenoxides represent a special class of anions that are closely related to alkoxides, except the organic substitutent is a derivative of benzene. Phenol is significantly more acidic than a typical alcohol, thus phenoxides are correspondingly less basic and less nucleophilic. They are however often easier to handle and afford derivatives that are more crystalline than the alkoxides.
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[edit] Preparation
[edit] 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, and hydrogen is produced as a by-product. A classic case is sodium methoxide produced by the addition of sodium metal to methanol:
- CH3OH + Na → CH3ONa + 1⁄2H2
Other alkali metals can be used in place of sodium, and most alcohols can be using in place of methanol.
[edit] From electrophilic chlorides
The tetrachloride of titanium reacts with alcohols to give the corresponding tetraalkoxides, concomitant with the evolution of hydrogen chloride:
- TiCl4 + 4 (CH3)2CHOH → Ti(OCH(CH3)2}4 + 4 HCl
The reaction can be accelerated by the addition of a base, such as a tertiary amine. Many other metal and main group halides can be used instead of titanium, for example SiCl4, ZrCl4, and PCl3.
[edit] By metathesis reactions
Many alkoxides are prepared by salt-forming reactions from a metal chloride and sodium alkoxide:
- NaOR + MCln → M(OR)n + n NaCl
Such reactions are favored by the high stability of the NaCl, and separation of the product alcoxide is simplified by the fact that NaCl is insoluble in common organic solvents.
[edit] By electrochemical process
Many alkoxides can be prepared by anodic dissolution of the corresponding metals in water-free alcohols in the presence of electroconductive additive. The metals may be Sc, Ga, Y, La, Ln, Si, Ti, Ge, Zr, Hf, Nb, Ta, Mo, W, Fe, Co, Ni, Re, etc. The conductive additive may be lithium chloride, quaternary ammonium halogenide, or other. Some examples of metal alkoxides obtained by this technique: Ti(OC3H7-iso)4, Nb2(OCH3)10, Ta2(OCH3)10, [MoO(OCH3)4]2, Re2O3(OCH3)6, Re4O6(OC3H7-iso)10, etc.
[edit] Properties
Most metal alkoxides hydrolyse irreversibly upon contact with water, according to the following equation:
- Ti(OCH2CH3)4 + 2 H2O → TiO2 + 4 HOCH2CH3
By controlling the stoichiometry of steric properties of the alkoxide, such reactions can be arrested leading to metal-oxy-alkoxide clusters. Other alcohols can be employed in place of water. In this way one alkoxide can be converted to another, a process sometimes called transesterification. Sodium methoxide, for example, is used in the large scale synthesis of bio-diesel, an example of a transesterification reaction. The position of the equilibrium can be controlled by the acidity of the alcohol; for example phenols typically react with alkoxides to release alcohols, giving the corresponding phenoxide. More simply, the trans-esterification can be controlled by selectively evaporating the more volatile component. In this way, ethoxides can be converted to butoxides, since ethanol (b.p. 78 °C) is more volatile than butanol (b.p. 118 °C).
[edit] Illustrative alkoxides
- titanium isopropoxide, used as a catalyst in organic synthesis and a precursor to TiO2.
- aluminium isopropoxide, used as a reagent in organic synthesis.
- tetraethylorthosilicate, used as a precursor to SiO2.
- Potassium tert-butoxide, used as a base for organic elimination reactions.
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