Asilanol is afunctional group in silicon chemistry with the connectivity Si–O–H. It is related to the hydroxy functional group (C–O–H) found in allalcohols. Silanols are often invoked as intermediates inorganosilicon chemistry and silicate mineralogy.[1] If a silanol contains one or more organic residues, it is anorganosilanol.
The first isolated example of a silanol wasEt3SiOH, reported in 1871 byAlbert Ladenburg. He prepared the “silicol” by hydrolysis ofEt3SiOEt (Et =CH2CH3).[2]
Silanols are generally synthesized byhydrolysis of halosilanes, alkoxysilanes, or aminosilanes. Chlorosilanes are the most common reactants:
The hydrolysis of fluorosilanes requires more forcing reagents, i.e. alkali. The alkoxysilanes (silyl ethers) of the typeR3Si(OR') are slow to hydrolyze. Compared to the silyl ethers, silyl acetates are faster to hydrolyze, with the advantage that the releasedacetic acid is less aggressive. For this reason silyl acetates are sometimes recommended for applications.[3]
An alternative route involvesoxidation of hydrosilanes. A wide range of oxidants have been employed including air,peracids,dioxiranes, andpotassium permanganate (for hinderedsilanes). In the presence of metal catalysts, silanes undergo hydrolysis:[3]
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]
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 fortert-butyl alcohol. The pKa of3−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 asiloxide or a silanolate.
Despite the disparity in acidity, the basicities of alkoxides and siloxides are similar.[3]
Silanols condense to givedisiloxanes:
The conversions of silyl halides, acetates, and ethers to siloxanes proceed via silanols. Thesol-gel process, which entails the conversion of, for example,Si(OEt)4 into hydratedSiO2, proceeds via silanol intermediates.
Silanols exist not only aschemical compounds, but are pervasive on the surface ofsilica and relatedsilicates. Their presence is responsible for the absorption properties of silica gel.[6] Inchromatography, derivatization of accessible silanol groups in a bondedstationary phase withtrimethylsilyl groups is referred to asendcapping. Organosilanols occur as intermediates in industrial processes such as the manufacturing ofsilicones. Moreover, organosilanols occur asmetabolites in the biodegradation of small ringsilicones in mammals.
Some silanediols and silanetriols inhibit hydrolytic enzymes such asthermolysin[7] andacetylcholinesterase.[8]
Literally, silanol refers to a single compound with the formulaH3SiOH (Chemical Abstracts number 14475-38-8). The familySiH4−n(OH)n (n = 1, 2, 3, 4) are highly unstable and are mainly of interest to theoretical chemists. The perhydroxylated silanol, sometimes calledorthosilicic acid, is often discussed in vague terms, but has not been well characterized.
