Silanol

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.[1] If a silanol contains one or more organic residues, it is an organosilanol.

Structure of trimethylsilanol

Preparation

From alkoxysilanes

The first isolated example of a silanol was Et3SiOH, reported in 1871 by Albert Ladenburg. He prepared the “silicol” by hydrolysis of Et3SiOEt (Et = C2H5).[2]

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.[3]

By oxidation of 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:[3]

R3Si–H + H2O → R3Si–OH + H2

Structure and examples

The Si–O bond distance is typically about 1.65 Å.[3] In the solid state, silanols engage in hydrogen-bonding.[4]

Most silanols have only one OH group, e.g. trimethylsilanol. Also known are some silanediols, e.g., diphenylsilanediol. For sterically bulky substituents, even silanetriols have been prepared.[5][3]

Reactions

Acidity

Silanols are more acidic than the corresponding alcohols. This trend contrasts with the fact that Si is far less electronegative than carbon (1.90 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.[3] 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 silanolate.

Despite the disparity in acidity, the basicities of alkoxides and siloxides are similar.[3]

Condensation and the sol-gel process

Silanols condense to give disiloxanes:

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.

Occurrence

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.[6] In chromatography, derivatization of accessible silanol groups in a bonded stationary phase with trimethylsilyl groups is referred to as endcapping. Organosilanols occur as intermediates in industrial processes such as the manufacturing of silicones. Moreover, organosilanols occur as metabolites in the biodegradation of small ring silicones in mammals.

Trisilanol intermediate in the formation of a cubic silsesquioxane.

Biorelevance

Some silanediols and silanetriols inhibit hydrolytic enzymes such as thermolysin[7] and acetylcholinesterase.[8]

Parent silanols

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.

References
  1. Vadapalli Chandrasekhar, Ramamoorthy Boomishankar, Selvarajan Nagendran: Recent Developments in the Synthesis and Structure of Organosilanols. Chem. Rev. 2004, volume 104, pp 5847–5910. doi:10.1021/cr0306135
  2. A. Ladenburg: On the silicoheptyl series, from Deut. Chem. Ges. Ber., iv, 901 as summarized in "Organic chemistry" J. Chem. Soc., 1872, vol. 25, pp. 133–156. doi:10.1039/JS8722500133
  3. 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
  4. 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
  5. R. Pietschnig and S. Spirk: The Chemistry of Organo Silanetriols. Coord. Chem. Rev. 2016, 87-106. doi:10.1016/j.ccr.2016.03.010
  6. 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
  7. S. M. Sieburth, T. Nittoli, A. M. Mutahi and L. Guo: Silanediols: a new class of potent protease inhibitors, Angew. Chem. Int. Ed. 1998, volume 37, 812-814.
  8. M. Blunder, N. Hurkes, M. List, S. Spirk and R. Pietschnig: Silanetriols as in vitro AChE Inhibitors, Bioorg. Med. Chem. Lett. 2011, volume 21, 363-365.
  • EL Salmawy, M.S., Nakahiro, Y., and Wakamatsu, T. (1993). The role of silanol group in flotation separation of quartz from feldspar using non-ionic surfactants, 18th IMPC, pp. 845–849, The Australian Institute of Mining and Metallurgical Engineering, Sydney, Australia.
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