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. The first example, triethylsilanol, was reported in 1871 by Albert Ladenburg.
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
Acidity and basicity
Silanols are much more acidic than the corresponding alcohols. For Et3SiOH, the pKa is estimated at 13.6 vs. 19 for tert-butyl alcohol. Because of their greater acidity, silanols can be fully deprotonated in aqueous solution, especially the arylsilanols. Deprotonation of a silanol gives an anion, which can function as a ligand used as support for catalysts.
Despite their enhanced acidity, silanols appear to be nearly as basic as alcohols.
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. The sol-gel process, which entails the conversion of, for example, Si(OEt)4 into hydrated SiO2, proceeds via silanol intermediates.
Literally, silanol refers to a single compound with the formula H3SiOH (Chemical Abstracts number 14475-38-8). Three members of the family SiH4−n(OH)n (n = 1, 2, 3) are highly unstable and are mainly of interest to theoretical chemists. The fourth perhydroxylated silanol, more commonly known as orthosilicic acid, is stable in weakly acidic solutions, and is of interest to oceanographers, and mineralogists.
- Vadapalli Chandrasekhar ,* Ramamoorthy Boomishankar , and Selvarajan Nagendran "Recent Developments in the Synthesis and Structure of Organosilanols" Chem. Rev. 2004, volume 104, pp 5847–5910. doi:10.1021/cr0306135
- 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
- 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