Silicon-based organic matter is a large group of compounds. The second, more common, name 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 those compounds in which there is a bond between silicon and carbon. They may contain other additional chemical elements (oxygen, halogens, hydrogen, and others). In this regard, this group of substances is characterized by a wide variety of properties and scope. Unlike other organic compounds, organosilicon compounds have better operational characteristics and higher safety for human health both when they are received and when using items made from them.
Their study began in the XIX century. The first synthesized substance was silicon tetrachloride. In the period from the 20s to the 90s of the same century, many compounds of this kind were obtained: silanes, esters and substituted esters of orthosilicic acid, alkylchlorosilanes and others. The similarity of part of the properties of silicon and ordinary organic substances led to the formation of a false idea that the silicon and carbon compounds are completely identical. Russian chemist D.I. Mendeleev proved that this is not so. He also found that silicon compounds with oxygen have a polymer structure. This is uncharacteristic for organic substances in which there is a bond of oxygen with 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, silicas are the starting compounds, and the rest are their derivatives. These include the following substances:
- silanes and its homologs (disilane, trisilane, tetrasilane);
- substituted silanes (butylsilane, tert-butylsilane, isobutisilane);
- esters of orthosilicic acid (tetramethoxysilane, dimethoxyethoxysilane);
- orthosilicic acid halides (trimethoxychlorosilane, methoxyethoxydichlorosilane);
- substituted esters of orthosilicic acid (methyltriethoxysilane, methylphenyl diethoxysilane);
- alkyl- (aryl) -halogenosilanes (phenyltrichlorosilane);
- hydroxyl derivatives of organosilanes (dihydroxydiethylsilane, hydroxymethylethylphenylsilane);
- alkyl- (aryl) -aminosilanes (diaminomethylphenylsilane, methylaminotrimethylsilane);
- alkoxy- (aryloxy) aminosilanes;
- alkyl (aryl) aminohalo silanes;
- alkyl (aryl) iminosilanes;
- isocyanates, thioisocyanates and thioesters of silicon.
High molecular weight organosilicon compounds
The classification of high molecular weight organic compounds is based on 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;
- metalloid silane chain polymers.
Chemical properties
Since these substances are very diverse, it is difficult to establish general patterns that characterize the bond of silicon and carbon.
The most characteristic properties of organosilicon compounds are:
- Resistance to elevated temperature is determined by the type and size of the organic radical or other groups that are associated with the Si atom. The tetra-substituted silanes have the highest thermal stability. Their decay begins at a temperature of 650-700 ° C. Polydimethylsiloxylans break down at a temperature of 300 ° C. Tetraethylsilane and hexaethyl disilane disintegrate upon prolonged heating at a temperature of 350 ° C, with the removal of 50% of the ethyl radical and the liberation of ethane.
- 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. Thus, the bond of carbon with silicon in aliphatic substituted ethers is not destroyed by exposure to concentrated sulfuric acid, and in mixed alkyl (aryl) substituted ethers, under the same conditions, the phenyl group is cleaved. Siloxane bonds also have high strength.
- Organosilicon compounds are relatively resistant to alkali. Their destruction occurs only in harsh conditions. For example, in polydimethylsiloxanes, the cleavage of methyl groups is observed only at temperatures above 200 ° C and under pressure (in an autoclave).
Characteristics of high molecular weight compounds
There are several types of macromolecular substances based on silicon:
- monofunctional;
- difunctional;
- trifunctional;
- quadrifunctional.
Combining these compounds, get:
- disiloxane derivatives, which are most often liquid compounds;
- polymers with a cyclic structure (oily liquids);
- elastomers (polymers with a linear structure consisting of several tens of thousands of monomers and high molecular weight);
- polymers with a linear structure in which the end groups are blocked by organic radicals (oils).
Resins in which the ratio of methyl radical to silicon is 1.2-1.5 are colorless solids.
The following properties are characteristic of high molecular weight organic silicon compounds:
- resistance to heat;
- hydrophobicity (preventing the penetration of water);
- high dielectric performance;
- maintaining a constant viscosity over a wide temperature range;
- chemical stability even in the presence of strong oxidizing agents.
Physical properties of silanes
Since these substances are very heterogeneous in structure and composition, we restrict ourselves to the description of organosilicon compounds of one of the most common groups - silanes.
Monosilane and disilane (SiH 4 and Si 2 H 4, respectively) under normal conditions are gases that have an unpleasant odor. In the absence of water and oxygen, they are sufficiently chemically stable.
Tetrasilan and trisilane are volatile toxic liquids. Pentasilan and hexasilane are also toxic and exhibit chemical instability.
These substances are well soluble in alcohols, gasoline, carbon disulfide. The latter type of solution has an increased explosion hazard. The melting point of the above compounds ranges from -90 ° C (tetrasilane) to -187 ° C (trisilane).
Getting
The addition of radicals to Si proceeds in different ways and depends on the properties of the starting material and the conditions under which the synthesis takes place. Some silicon compounds with organic substances can only be made under harsh conditions, while others react more easily.
The preparation of 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 response is shown in the figure below.
Polycondensation can proceed in three directions: with the formation of linear or cyclic compounds, with the receipt of substances of a mesh or spatial structure. Cyclic polymers are characterized by a higher density and viscosity, compared with linear analogues.
Synthesis of high molecular weight compounds
Silicon-based organic resins and elastomers are obtained by 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 materials containing oxygen are more prone to polymerization than their corresponding carbon-based compounds. Silicon, in contrast to it, is capable of holding 2 or more hydroxyl groups. The possibility of the formation of crosslinked polymer molecules from cyclic mainly depends on the size of the organic radical.
Analysis
The 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, then it is exposed to ammonium molybdate and benzidine. If an organosilicon substance is present, the sample turns blue. There are other ways to identify.
- Quantitative analysis. Both qualitative and quantitative studies of organosilicon compounds employ infrared and emission spectroscopy methods. Other methods are also used - sol-gel analysis, mass spectroscopy, nuclear magnetic resonance.
- Detailed physical and chemical research.
Preliminary produce the selection 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 selection of chemically pure organic silicon compounds is often carried out using fractional distillation. The liquid phases are separated using a separatory funnel. For gas mixtures, 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 technology - press materials, hydraulic fluid, coolants, de-icing compounds;
- electrical engineering - the manufacture of resins and varnishes, materials for the protection of integrated circuits;
- machine-building industry - production of rubber products, compounds, lubricants, sealants, adhesives;
- light industry - modifiers of textile fibers, leather, leatherette; foam regulators;
- pharmaceutical industry - the manufacture of materials for prosthetics, immunostimulants, adaptogens, cosmetics.
The advantages of such substances include the fact that they can be used in a wide variety of conditions: in tropical and cold climates, at high pressure and in vacuum, at high temperatures and radiation. Corrosion-resistant coatings based on them are operated in the temperature regime from -60 to +550 ° .
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 domestic animals.
Studies show that the introduction of additives with organosilicon substances in the diet of poultry and livestock contributes to an increase in live weight, a decrease in mortality and feed costs per growth unit, and 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 for the development of organosilicon compounds in Russia is GNIIHTEOS. This is a comprehensive scientific center, which 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 implemented organosilicon materials of more than 400 items. The company has a pilot plant for their production.
However, Russia in the global dynamics of the development of production of organic compounds based on silicon is much inferior to other countries. So, over the past 20 years, China's industry has increased the production of these substances by almost 50 times, and Western Europe - 2 times. At present, the production of organosilicon compounds in Russia is carried out at KZSK-Silicon, Altaikhimprom JSC, at the Redkinsky Pilot Plant, at Khimprom OJSC (Chuvash Republic), and Silan OJSC.