Alkoxide

An alkoxide is the conjugate base of an alcoholand therefore consists of an organic group bonded to a negatively charged oxygen atom. They are 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. An alkoxide is the conjugate base of an alcoholand therefore consists of an organic group bonded to a negatively charged oxygen atom. They are 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 alkoxides 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 onto both carbon and oxygen sites. Phenoxides are close relatives of the alkoxides, in which the alkyl group is replaced by a derivative of benzene. 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. 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, thus many alkoxides feature M2O or M3O 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 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: Other alkali metals 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. Titanium tetrachloride reacts with alcohols to give the corresponding tetraalkoxides, concomitant with the evolution of hydrogen chloride: 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.