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{{Short description|Si–OH functional group in silicon chemistry}}
[[File:Me3SIOH.png|thumb|180 px|Structure of [[trimethylsilanol]]]]
A '''silanol''' is a [[functional group]] in silicon chemistry with the connectivity Si&ndash;O&ndash;H. It is related to the hydroxy functional group (C&ndash;O&ndash;H) found in all [[Alcohol (chemistry)|alcohol]]s. Silanols are often invoked as intermediates in [[organosilicon chemistry]] and silicate mineralogy.<ref>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}}</ref> If a silanol contains one or more organic residues, it is an [[organosilanol]].

==Preparation==

===From alkoxysilanes===
The first isolated example of a silanol was {{chem2|Et3SiOH}}, reported in 1871 by [[Albert Ladenburg]]. He prepared the “silicol” by hydrolysis of {{chem2|Et3SiOEt}} (Et = [[ethyl group|{{chem2|CH2CH3}}]]).<ref>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}}</ref>

===From silyl halides and related compounds===
Silanols are generally synthesized by [[hydrolysis]] of halosilanes, alkoxysilanes, or aminosilanes.  Chlorosilanes are the most common reactants:
:R<sub>3</sub>Si&ndash;Cl  +  H<sub>2</sub>O   →   R<sub>3</sub>Si&ndash;OH  +  HCl

The hydrolysis of fluorosilanes requires more forcing reagents, i.e. alkali. The alkoxysilanes ([[silyl ether]]s) of the type {{chem2|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.<ref name=Lickiss/>

===By oxidation of silyl hydrides===
An alternative route involves [[oxidation]] of hydrosilanes.  A wide range of oxidants have been employed including air, [[peracid]]s, [[dioxirane]]s, and [[potassium permanganate]] (for hindered [[silanes]]).  In the presence of metal catalysts, silanes undergo hydrolysis:<ref name=Lickiss/>
:R<sub>3</sub>Si&ndash;H  +  H<sub>2</sub>O   →   R<sub>3</sub>Si&ndash;OH  +  H<sub>2</sub>

==Structure and examples==
The Si&ndash;O bond distance is typically about 1.65&nbsp;[[Angstrom|Å]].<ref name=Lickiss>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}}</ref> In the solid state, silanols engage in hydrogen-bonding.<ref>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)<sub>3</sub>SiOH and HO[(t-BuO)<sub>2</sub>SiO]<sub>2</sub>H'', Appl. Organomet. Chem. 2003, 17, 52–62. {{doi|10.1002/aoc.380}}</ref>

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.<ref>R. Pietschnig and S. Spirk: ''The Chemistry of Organo Silanetriols''. Coord. Chem. Rev. 2016, 87-106. {{doi| 10.1016/j.ccr.2016.03.010 }}</ref><ref name=Lickiss/>

==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 Et<sub>3</sub>SiOH, the p''K''<sub>a</sub> is estimated at 13.6 vs. 19 for [[tert-butyl alcohol|''tert''-butyl alcohol]].  The p''K''<sub>a</sub> of {{chem2|3\sClC6H4)Si(CH3)2OH}} is 11.<ref name=Lickiss/>  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.<ref name=Lickiss/>

===Condensation and the sol-gel process===
Silanols condense to give [[siloxane|disiloxane]]s:

:{{chem2|2 R3SiOH -> R3Si\sO\sSiR3 + 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, {{chem2|Si(OEt)4}} into hydrated {{chem2|SiO2}}, proceeds via silanol intermediates.

==Occurrence==
Silanols exist not only as [[chemical compound]]s, but are pervasive on the surface of [[silica]] and related [[silicate]]s. Their presence is responsible for the absorption properties of silica gel.<ref>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}}</ref>  In [[chromatography]], derivatization of accessible silanol groups in a bonded [[Stationary phase (chemistry)|stationary phase]] with [[trimethylsilyl]] groups is referred to as [[endcapping]]. Organosilanols occur as intermediates in industrial processes such as the manufacturing of [[silicone]]s. Moreover, organosilanols occur as [[metabolite]]s in the biodegradation of small ring [[silicone]]s in mammals.

[[File:Silsesquioxane Partial Cage.svg|thumb|Trisilanol intermediate in the formation of a cubic [[silsesquioxane]]]]

==Biorelevance==
Some silanediols and silanetriols inhibit hydrolytic enzymes such as [[thermolysin]]<ref>[[Scott Sieburth|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.</ref> and [[acetylcholinesterase]].<ref>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.</ref>

==Parent silanols==
Literally, silanol refers to a single compound with the formula {{chem2|H3SiOH}} ([[Chemical Abstracts]] number 14475-38-8). The family {{chem2|SiH_{4–''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 ==

{{Reflist}}

* 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.&nbsp;845–849, The Australian Institute of Mining and Metallurgical Engineering, Sydney, Australia.

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[[Category:Silanols|*]]