Organosilanols

Organic compounds of the form R3–Si–OH
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):

R 2 SiCl 2 + 2 HgO → t o l u e n e − 80 ∘ C R 2 Si ( OH ) 2 + 2 Hg 0 {\displaystyle {\ce {R2SiCl2{}+2HgO->[-80^{\circ }C][toluene]R2Si(OH)2{}+2Hg^{0}}}}

or by hydrolysis in the alkaline:[4]

( H 3 C ) 3 SiCl + H 2 O ⟶ ( H 3 C ) 3 SiOH + HCl {\displaystyle {\ce {(H3C)3SiCl + H2O -> (H3C)3SiOH + HCl}}}
R 3 SiH + H 2 O ⟶ R 3 SiOH + H 2 {\displaystyle {\ce {R3SiH + H2O -> R3SiOH + H2}}}


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

( H 5 C 2 ) 3 SiOC 2 H 5 + H 2 O ⟶ ( H 5 C 2 ) 3 SiOH + HOC 2 H 5 {\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. ^ a b 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
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