Alkoxide



  An alkoxide is the catalysts.[1]

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.

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 yield derivatives that are more crystalline than the alkoxides.

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 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 used in place of methanol.

From electrophilic chlorides

      The tetrachloride of titanium reacts with alcohols to give the corresponding tetraalkoxides, concomitant with the evolution of hydrogen chloride:
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.

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 lattice energy of the NaCl, and purification of the product alkoxide is simplified by the fact that NaCl is insoluble in common organic solvents.

By electrochemical processes

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 Zr, 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(OCH3)12, and Re4O6(OC3H7-iso)10.

Properties

Hydrolysis and transesterification

Metal alkoxides hydrolyse with water according to the following equation:

2 LnMOR + H2O → [LnM]2O + 2 ROH

where R is an organic substituent and L is an unspecified ligand (often an alkoxide) A well-studied case is the irreversible hydrolysis of titanium ethoxide:

1/n [Ti(OCH2CH3)4]n + 2 H2O → TiO2 + 4 HOCH2CH3

By controlling the 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).

Formation of oxo-ligands

Many metal alkoxide compounds also feature oxo-ligands in their coordination sphere. Oxo-ligands typically arise via the hydrolysis, often accidentally, and via ether elimination:

2 LnMOR → [LnM]2O + R2O

Additionally, low valent metal alkoxides are susceptible to oxidation by air.

Formation of polynuclear and heterometallic derivatives

Characteristically, transition metal alkoxides and oxides are polynuclear, that is they contain more than one metal. Oxides and alkoxides are sterically undemanding and highly basic ligands that tend to bridge metals.

Upon the isomorphic substitution of metal atoms close in properties crystalline complexes of variable composition are formed. The metal ratio in such compounds can vary over a broad range. For instance, the substitution of rhenium in the complexes Re4O6-y(OCH3)12+y allowed one to obtain complexes Re4-xMoxO6-y(OCH3)12+y in the range of x=[0 to 2.82] and Re4-xWxO6-y(OCH3)12+y in the range of x=[0 to 2].

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

 

References

  1. ^ Bradley, D. C.; Mehrotra, R.; Rothwell, I.; Singh, A. “Alkoxo and Aryloxo Derivatives of Metals” Academic Press, San Diego, 2001. ISBN 0121241408.
  • P.A. Shcheglov, D.V. Drobot. Rhenium Alkoxides (Review). Russian Chemical Bulletin. 2005. V. 54, No. 10. P. 2247-2258. DOI: 10.1007/s11172-006-0106-5
  • N.Ya. Turova. Metal oxoalkoxides. Synthesis, properties and structures (Review). Russian Chemical Reviews. 2004. V. 73, No. 11. P. 1041-1064. DOI: 10.1070/RC2004v073n11ABEH000855
 
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