Organosilanols

Structure of trimethylsilanol.

Organosilanols are a group of chemical silicon compounds. More specifically, they are carbosilanes derivatized with a hydroxy group on the silicon atom. Organosilanols are the silicon analogs to alcohols.

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 or by hydrolysis in the alkaline.

$\mathrm {(H_{3}C)_{3}SiCl+\ H_{2}O\longrightarrow (H_{3}C)_{3}SiOH+\ HCl}$

$\mathrm {R_{2}SiCl_{2}+\ 2\ HgO{\xrightarrow[{-80\,{}^{\circ }C}]{toluene}}R_{2}Si(OH)_{2}+\ 2\ Hg^{0}\ \ \ R=organic\ residue}$

$\mathrm {R_{3}SiH+\ H_{2}O\longrightarrow R_{3}SiOH+\ H_{2}\ \ \ R=organic\ residue}$

The hydrolysis of silyl ethers generally proceeds only slowly:

$\mathrm {(H_{5}C_{2})_{3}SiOC_{2}H_{5}+\ H_{2}O\longrightarrow (H_{5}C_{2})_{3}SiOH+\ HOC_{2}H_{5}}$

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.

Classification

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

• organosilane triols, 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.

Literature

• Paul D. Lickiss: The synthesis and structure of organosilanols. In: Advances in Inorganic Chemistry. 42, 1995, S. 147–262, doi:10.1016/S0898-8838(08)60053-7.