Organosilanols

Structure of trimethylsilanol.

In organosilicon chemistry, organosilanols are a group of chemical compounds derived from silicon. More specifically, they are carbosilanes derived with a hydroxy group (−OH) on the silicon atom.^[1] Organosilanols are the silicon analogs to alcohols. Silanols are more acidic and more basic than their alcohol counterparts and therefore show a rich structural chemistry characterized by hydrogen bonding networks which are particularly well studied for silanetriols.^[2][3]

Preparation

Organosilanols can be obtained by hydrolysis of organohalosilanes, such as chlorotrimethylsilane. They can also be prepared by the oxidation of organosilanes with oxidizing agents (R = organic residue):

${\displaystyle {\ce {R2SiCl2{}+2HgO->[-80^{\circ }C][toluene]R2Si(OH)2{}+2Hg^{0}}}}$

or by hydrolysis in the alkaline:^[4]

${\displaystyle {\ce {(H3C)3SiCl + H2O -> (H3C)3SiOH + HCl}}}$

${\displaystyle {\ce {R3SiH + H2O -> R3SiOH + H2}}}$

The hydrolysis of silyl ethers generally proceeds only slowly:^[4]

${\displaystyle {\ce {(H5C2)3SiOC2H5 + H2O -> (H5C2)3SiOH + HOC2H5}}}$

Hydrolysis of organosilanes is a first-order reaction. The hydrolysis rate of the Si-H bond depends on the type and number of organic residues. Thus, the hydrolysis rate of trialkylsilanes is significantly slower than that of triarylsilanes. This can be explained by a stronger increase in electron density on the silicon atom by the alkyl groups. Correspondingly, the reaction rate of the tri-n-alkylsilanes decreases in the series of ethyl, propyl, butyl groups. Trialkylsilanes with n-alkyl residues react by a factor of 10 faster than the analogous silanes with branched alkyl residues.^[5]

Classification

Depending on the substitution pattern of the silicon atom, a further distinction can be made. Organosilanols are classified as:

• organosilanetriols, when three hydroxy groups and an organic residue are bound to a silicon atom, e. g. methylsilanetriol, phenylsilanetriol
• organosilandiols, when two hydroxy groups and two organic residues are bound to a silicon atom, e. g. dimethylsilanediol, diphenylsilanediol
• organosilanols, when one hydroxy group and three organic residues are bound to a silicon atom, e. g. trimethylsilanol, triethylsilanol or triphenylsilanol.

References

1. IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "silanols". doi:10.1351/goldbook.S05664
2. Paul D. Lickiss (1995), "The synthesis and structure of organosilanols", Advances in Inorganic Chemistry, vol. 42, pp. 147–262, doi:10.1016/S0898-8838(08)60053-7
3. Rudolf Pietschnig, Stefan Spirk (2016), "The Chemistry of Organo Silanetriols", Coordination Chemistry Reviews, vol. 323, pp. 87–106, doi:10.1016/j.ccr.2016.03.010
4. Barry Arkles. "Silanes" (pdf). Gelest. p. 50. Retrieved 2016-12-10.
5. G. Schott, C. Harzdorf (October 1960), "Silane. I Alkalische Solvolyse von Triorganosilanen", Zeitschrift für anorganische und allgemeine Chemie (in German), vol. 306, no. 3–4, pp. 180–190, doi:10.1002/zaac.19603060306