A silanol is a functional group in silicon chemistry with the connectivity Si–O–H. It is related to the hydroxy functional group (C–O–H) found in all alcohols. Silanols are often invoked as intermediates in organosilicon chemistry and silicate mineralogy.
From silyl halides
Silanols are generally synthesized by hydrolysis of halosilanes, alkoxysilanes, or aminosilanes. Chlorosilanes are the most common reactants:
- R3Si–Cl + H2O → R3Si–OH + HCl
The hydrolysis of fluorosilanes requires more forcing reagents, i.e. alkali. The alkoxysilanes (silyl ethers) of the type R3Si(OR') are slow to hydrolyze. Compared to the silyl ethers, silyl acetates are faster to hydrolyze, with the advantage that the released acetic acid is less aggressive. For this reason silyl acetates are sometimes recommended for applications.
From silyl hydrides
An alternative route involves oxidation of hydrosilanes. A wide range of oxidants have been employed including air, peracids, dioxiranes, and potassium permanganate (for hindered silanes). In the presence of metal catalysts, silanes undergo hydrolysis:
- R3Si–H + H2O → R3Si–OH + H2
Structure and examples
Silanols are more acidic than the corresponding alcohols. This trend contrast with the fact that Si is far less electronegative than carbon, 1.9 vs 2.55, respectively. For Et3SiOH, the pKa is estimated at 13.6 vs. 19 for tert-butyl alcohol. The pKa of (3-ClC6H4)Si(CH3)2OH is 11. Because of their greater acidity, silanols can be fully deprotonated in aqueous solution, especially the arylsilanols. The conjugate base is called a siloxide or a silanoate. Although silanols are significantly more acidic than the corresponding alcohol, the basicities of the two series are similar.
Despite their enhanced acidity, silanols appear to be nearly as basic as alcohols.
Condensation and the sol-gel process
Silanols condense to give siloxanes:
- 2 R3SiOH → R3Si-O-SiR3 + H2O
The conversions of silyl halides, acetates, and ethers to siloxanes proceed via silanols. The sol-gel process, which entails the conversion of, for example, Si(OEt)4 into hydrated SiO2, proceeds via silanol intermediates.
Silanols exist not only as chemical compounds, but are pervasive on the surface of silica and related silicates. Their presence is responsible for the absorption properties of silica gel. In chromatography, derivitization of accessible silanol groups in a bonded stationary phase with trimethylsilyl groups is referred to as endcapping.
Literally, silanol refers to a single compound with the formula H3SiOH (Chemical Abstracts number 14475-38-8). The family SiH4−n(OH)n (n = 1, 2, 3, 4) are highly unstable and are mainly of interest to theoretical chemists. The perhydroxylated silanol, sometimes called orthosilicic acid, is often discussed in vague terms, but has not been well characterized.
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- Paul D. Lickiss: The Synthesis and Structure of Organosilanols, Advances in Inorganic Chemistry Volume 42, 1995, Pages 147–262 doi:10.1016/S0898-8838(08)60053-7
- Beckmann, J.; Dakternieks, D.; Duthie, A.; Larchin, M. L.; Tiekink, E. R. T.: Tert-butoxysilanols as model compounds for labile key intermediates of the sol-gel process: crystal and molecular structures of (t-BuO)3SiOH and HO[(t-BuO)2SiO]2H, Appl. Organomet. Chem. 2003, 17, 52–62. doi:10.1002/aoc.380
- Nawrocki, Jacek: The silanol group and its role in liquid chromatography, Journal of Chromatography A 1997, volume 779, 29–72. doi:10.1016/S0021-9673(97)00479-2