Silicon compounds

Hydrogen compounds of silicon (Silanes)

The silicon atom is capable of forming compounds of the general formula $SiX_4$ by reacting with halogens and hydrogen, as well as forming structures with $Si-Si$ bond chains.

Hydrogen compounds form a homologous series, similar to alkanes:

• Silane ($SiH_4$) is the structural analogue of methane ($CH_4$).
• Disilane ($Si_2H_6$) is the analogue of ethane ($C_2H_6$).

Despite structural similarities, the chemical reactivity of silanes is significantly higher than that of the corresponding hydrocarbons. For example, silane can spontaneously ignite upon contact with air. In addition, it reacts extremely vigorously with water, whereas methane interacts only with steam and under very harsh conditions.

Silane hydrolysis reaction: $SiH_4 + 3H_2O = H_2SiO_3 + 2H_2\uparrow$

This reaction is interesting for its redox properties. The process proceeds via a disproportionation mechanism, where hydrogen acts simultaneously as an oxidizing and a reducing agent:

• In the silane molecule ($SiH_4$), the oxidation state of hydrogen is -1.
• In the water molecule ($H_2O$), the oxidation state of hydrogen is +1.
• As a result, molecular hydrogen ($H_2$) is formed with an oxidation state of 0.

Silicon halides and their hydrolysis

Silicon halides also demonstrate high reactivity. Practically all of these compounds undergo complete hydrolysis in an aqueous medium. The only exception in this series is silicon fluoride ($SiF_4$), which behaves differently.

Let us examine the process using the reaction of silicon tetrachloride with water as an example: $SiCl_4 + 4H_2O = H_4SiO_4 + 4HCl$

During this reaction, the initial molecule is completely destroyed with the formation of orthosilicic acid ($H_4SiO_4$) and the release of the corresponding hydrogen halide (in this case, hydrogen chloride).

Reaction mechanism: Silicon vs. Carbon

The chemical activity of silicon halides vastly exceeds that of carbon halides. For example, tetrachloromethane ($CCl_4$) is completely resistant to the action of water.

The reason for silicon's high activity lies in the structure of its electron shell. The silicon atom has vacant $3d$-orbitals at the valence level, due to which it is able to act as an electron pair acceptor.

The mechanism of silicon halide hydrolysis proceeds according to the following scheme:

1. Addition: The water molecule acts as an electron pair donor (via the oxygen atom) and attacks the silicon atom, placing electrons into its vacant $d$-orbital.
2. Cleavage: A hydrogen halide molecule ($HCl$) is split off from the resulting complex.
3. Intermediate product: A particle is formed in which one of the halogen atoms is replaced by a hydroxyl group ($OH$).
4. Cyclical nature: The process of adding new water molecules and cleaving $HCl$ is sequentially repeated until all chlorine atoms are replaced (complete hydrolysis).

Why can't carbon do this? The carbon atom lacks vacant orbitals at the valence level (there is no $2d$ sublevel). Consequently, a water molecule physically cannot transfer its electron pair to it to initiate the reaction. It is precisely this inability to implement the "addition-cleavage" mechanism that explains the complete inertness of $CCl_4$ to hydrolysis.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Redox reactions involving silanes
• Stepwise mechanism of halide hydrolysis
• Properties and structure of structures with $Si-Si$ bonds
• Orthosilicic and metasilicic acids: conditions of formation

Other topics in "Chapter 12. Group IVA Elements"

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