Silicon-based organics are a large group of compounds. The second, more common name for them is silicones. The scope of organosilicon compounds is constantly growing. They are used in almost all areas of human activity - from astronautics to medicine. Materials based on them have high technical and consumer qualities.
General concept
Organosilicon compounds are compounds in which there is a bond between silicon and carbon. They may also contain other additional chemical elements (oxygen, halogens, hydrogen, and others). In this regard, this group of substances is distinguished by a wide variety of properties and applications. Unlike other organic compounds, organosilicon compounds have better performance characteristics and higher safety for human he alth both when they are obtained and when using items,made from them.
Their study began in the XIX century. Silicon tetrachloride was the first synthesized substance. In the period from the 20s to the 90s of the same century, many compounds of this kind were obtained: silanes, ethers and substituted esters of orthosilicic acid, alkylchlorosilanes, and others. The similarity of some of the properties of silicon and ordinary organic substances has led to the formation of a false idea that silicon and carbon compounds are completely identical. The Russian chemist D. I. Mendeleev proved that this is not so. He also established that oxygen-silicon compounds have a polymeric structure. This is not typical for organic substances, in which there is a bond between oxygen and carbon.
Classification
Organosilicon compounds occupy an intermediate position between organic and organometallic. Among them, 2 large groups of substances are distinguished: low molecular weight and high molecular weight.
In the first group, silicon hydrogens serve as initial compounds, and the rest are their derivatives. These include the following substances:
- silanes and its homologues (disilane, trisilane, tetrasilane);
- substituted silanes (butylsilane, tert-butylsilane, isobutysilane);
- Ethers of orthosilicic acid (tetramethoxysilane, dimethoxydiethoxysilane);
- haloesters of orthosilicic acid (trimethoxychlorosilane, methoxyethoxydichlorosilane);
- substituted esters of orthosilicic acid (methyltriethoxysilane, methylphenyldiethoxysilane);
- alkyl-(aryl)-halosilanes (phenyltrichlorosilane);
- hydroxyl derivatives of organosilanes(dihydroxydiethylsilane, hydroxymethylethylphenylsilane);
- alkyl-(aryl)-aminosilanes (diaminomethylphenylsilane, methylaminotrimethylsilane);
- alkoxy-(aryloxy)-aminosilanes;
- alkyl-(aryl)-aminohalosilanes;
- alkyl-(aryl)-iminosilanes;
- isocyanates, thioisocyanates and silicon thioethers.
High molecular weight organosilicon compounds
The basis for the classification of macromolecular organic compounds is polymer silicon hydrogen, the structural diagram of which is shown in the figure below.
The following substances belong to this group:
- alkyl-(aryl)-polysilanes;
- organopolyalkyl-(polyaryl)-silanes;
- polyorganosiloxanes;
- polyorganoalkylene-(phenylene)-siloxanes;
- polyorganometallosiloxanes;
- metalloidsilane chain polymers.
Chemical properties
Since these substances are very diverse, it is difficult to establish general patterns that characterize the bond between silicon and carbon.
The most characteristic properties of organosilicon compounds are:
- Resistant to elevated temperature is determined by the type and size of the organic radical or other groups that are associated with the Si atom. Tetrasubstituted silanes have the highest thermal stability. Their decomposition begins at a temperature of 650-700 °C. Polydimethylsiloxylanes are destroyed at a temperature of 300 °C. Tetraethylsilane and hexaethyldisilane decompose upon prolonged heating at a temperature of 350 ° C,in this case, 50% of the ethyl radical is eliminated and ethane is released.
- Chemical resistance to acids, alkalis and alcohols depends on the structure of the radical, which is associated with the silicon atom, and the entire molecule of the substance. So, the bond of carbon with silicon in aliphatic substituted esters is not destroyed when exposed to concentrated sulfuric acid, while in mixed alkyl-(aryl)-substituted esters, under the same conditions, the phenyl group is cleaved. Siloxane bonds also have high strength.
- Organosilicon compounds are relatively resistant to alkalis. Their destruction occurs only in harsh conditions. For example, in polydimethylsiloxanes, the elimination of methyl groups is observed only at temperatures above 200 °C and under pressure (in an autoclave).
Characteristics of macromolecular compounds
There are several types of silicon-based macromolecular substances:
- monofunctional;
- difunctional;
- trifunctional;
- quadrifunctional.
Combining these compounds, you get:
- disiloxane derivatives, which are most often liquid compounds;
- cyclic polymers (oily liquids);
- elastomers (polymers with a linear structure consisting of several tens of thousands of monomers and a large molecular weight);
- polymers with a linear structure, in which end groupsblocked by organic radicals (oils).
Resins with a methyl radical to silicon ratio of 1.2-1.5 are colorless solids.
The following properties are typical for high-molecular organic silicon compounds:
- heat resistance;
- hydrophobicity (resistance to water penetration);
- high dielectric performance;
- maintaining a constant viscosity value over a wide temperature range;
- chemical stability even in the presence of strong oxidants.
Physical properties of silanes
Since these substances are very heterogeneous in structure and composition, we confine ourselves to describing organosilicon compounds of one of the most common groups - silanes.
Monosilane and disilane (SiH4 and Si2H4 respectively) at normal conditions are gases that have an unpleasant odor. In the absence of water and oxygen, they are quite chemically stable.
Tetrasilane and trisilane are volatile toxic liquids. Pentasilane and hexasilane are also toxic and chemically unstable.
These substances dissolve well in alcohols, gasoline, carbon disulfide. The latter type of solutions has a high explosive hazard. The melting point of the above compounds ranges from -90 °C (tetrasilane) to -187 °C (trisilane).
Receive
The addition of radicals to Si proceeds differently and depends on the properties of the starting material and the conditions under which the synthesis occurs. Somecompounds of silicon with organic substances can only be made under harsh conditions, while others react more easily.
Obtaining organosilicon compounds based on silane bonds is carried out by hydrolysis of alkyl (or aryl)-chloroxysilanes (or alkoxysilanes) followed by polycondensation of silanols. A typical reaction is shown in the figure below.
Polycondensation can proceed in three directions: with the formation of linear or cyclic compounds, with obtaining substances of a network or spatial structure. Cyclic polymers have a higher density and viscosity than their linear counterparts.
Synthesis of macromolecular compounds
Organic resins and silicon-based elastomers are produced by the hydrolysis of monomers. The hydrolysis products are subsequently heated and catalysts are added. As a result of chemical transformations, water (or other substances) is released and complex polymers are formed.
Organosilicon compounds containing oxygen are more prone to polymerization than their corresponding carbon-based compounds. Silicon, in contrast, is able to hold 2 or more hydroxyl groups. The possibility of forming cross-linked polymer molecules from cyclic ones mainly depends on the size of the organic radical.
Analysis
Analysis of organosilicon compounds is carried out in several directions:
- Determination of physical constants (melting point, boiling point and other characteristics).
- Qualitative analysis. To detect compounds of this type in varnishes, oils, and resins, the test sample is fused with sodium carbonate, extracted with water, and then treated with ammonium molybdate and benzidine. If organosilicon is present, the sample turns blue. There are other ways to detect.
- Quantitative analysis. For both qualitative and quantitative studies of organosilicon compounds, methods of infrared and emission spectroscopy are used. Other methods are also used - sol-gel analysis, mass spectroscopy, nuclear magnetic resonance.
- Detailed physical and chemical study.
Pre-produce the isolation and purification of the substance. For solid compositions, the separation of compounds is done on the basis of their different solubility, boiling point and crystallization. The isolation of chemically pure organic silicon compounds is often carried out by fractional distillation. The liquid phases are separated using a separating funnel. For mixtures of gases, absorption or liquefaction at low temperatures and fractionation are used.
Application
The scope of organosilicon compounds is very large:
- production of technical fluids (lubricating oils, working fluids for vacuum pumps, petroleum jelly, pastes, emulsions, defoamers and others);
- chemical industry - use as stabilizers, modifiers, catalysts;
- paint and varnish industry - additives for the manufacture of heat-resistant, anti-corrosion coatings for metal, concrete, glass and other materials;
- aerospace engineering - press materials, hydraulic fluids, coolants, anti-icing compounds;
- electrical engineering - production of resins and varnishes, materials for protecting integrated circuits;
- engineering industry - production of rubber products, compounds, lubricants, sealants, adhesives;
- light industry - modifiers of textile fibers, leather, leatherette; defoamers;
- pharmaceutical industry - production of materials for prosthetics, immunostimulants, adaptogens, cosmetics.
The advantages of such substances include the fact that they can be used in a variety of conditions: in tropical and cold climates, at high pressure and in vacuum, at high temperatures and radiation. Anti-corrosion coatings based on them are operated in the temperature range from -60 to +550 °С.
Livestock
The use of organosilicon compounds in animal husbandry is based on the fact that silicon is actively involved in the formation of bones and connective tissues, metabolic processes. This trace element is vital for the growth and development of pets.
As showstudies, the introduction of additives with organosilicon substances into the diet of poultry and livestock contributes to an increase in live weight, a decrease in mortality and feed costs per unit of growth, an increase in the metabolism of nitrogen, calcium, and phosphorus. The use of such drugs in cows also helps in the prevention of obstetric diseases.
Production in Russia
The leading enterprise in the development of organosilicon compounds in Russia is GNIIChTEOS. This is an integrated scientific center that is engaged in the creation of industrial technologies for the manufacture of compounds based on silicon, aluminum, boron, iron and other chemical elements. The specialists of this organization have developed and introduced more than 400 organosilicon materials. The company has a pilot plant for their production.
However, Russia in the global dynamics of the development of the production of organic compounds based on silicon is much inferior to other countries. So, over the past 20 years, the Chinese industry has increased the production of these substances by almost 50 times, and Western Europe - by 2 times. At present, the production of organosilicon compounds in Russia is carried out at KZSK-Silicon, JSC Altaihimprom, at the Redkinsky Pilot Plant, JSC Khimprom (Chuvash Republic), JSC Silan.
