Organic Silicon Chemistry and Its Characteristics
Although silicon and carbon are located in the same main group of the periodic table and have similar chemical properties, silicon is in the third cycle and carbon is in the third cycle.
Two cycles, so there are some differences.
Comparison of properties of silicon and carbon
Project C Si
Atomic radius, pm 91.4 117.6
First ionization energy, kJ/mol 1092 791
Electronegativity 2.5 1.8
Metallic non-metallic quasi metal
Valence bond 4 4
Valence keys 4 6
Bond type Single bond, double bond, triple bond Only single bond
Generation of alkanes
Carbonane CnH2n+2
N can be greater than 1000
Silane SinH2n+2
n<=6
Bond energy combined with low electronegative elements (such as H and C)
C-H 414kJ/mol
C-C 347kJ/mol
Si-H 292.9kJ/mol
Si-C 296
Its bond energy is lower than that of C and similar elements
Bond energy combined with highly electronegative elements (such as O, Cl)
C-O 351kJ/mol
C-Cl 331kJ/mol
Si-O 443 5kJ/mol
Si-Cl 358.6kJ/mol
Its bond energy is higher than that of C combined with similar elements
The thermal cracking of polymers is a homogeneous free radical reaction. Therefore, the thermal stability of polymers is mainly determined by the covalent bond energy between atoms in their molecules; See the following table for data comparison:
Comparison of the bond energies, relative electronegativity differences and ionic properties of the some interatomic covalent bonds
Covalent bond energy, kJ/mol relative electronegativity difference Ionicity,%
C-C 347 0 0
C-H 414 0.4 4
C-N 293 0.5 6
C-Cl 331 0.5 6
C-F 485 1.5 43
C-O 351 1.0 22
Si-Si 177 0 0
Si-C 290 0.7 12
Si-H 292.9 0.3 2
Si-N 435 1.2 30
Si-Cl 358.6 1.2 30
Si-F 541 1.5 43
Si-O 443.5 1.7 51
The Si-O-Si bond plays a shielding role in the thermal oxidation of the hydrocarbon group connected to the silicon atom. The effect varies with the size and nature of the hydrocarbon group. For example, the cracking temperature of ethyl group is lower than that of methyl group; Phenyl and vinyl have better heat resistance than methyl.
Covalent Bond Energy of Si-C in Hydrocarbon Silicon Bond
Covalent bond energy of R-Si bond Si-C, kJ/mol
H3C-Si 314
H5C2-Si 259
Positive H9C4-Si 222
H2C=CH-Si 297
However, it is not enough to only refer to the bond energy data when understanding the chemical behavior of a certain bond, especially in the heterofission reaction. Because the atoms of two different elements have different attraction to electrons, the covalent bonds between atoms of different kinds are always polar.
The polarity can be explained by the difference of the relative electronegativity of the two atoms involved in bonding. The greater the difference of the electronegativity between the two elements, the greater the ionic property of the bond.
The greater the covalent bond energy between atoms, the better the heat resistance, and the less susceptible to thermal cracking; However, the greater the ionic property of the bond, the more vulnerable it is to the attack of nucleophilic and electrophilic reagents to break the bond (heterofission reaction). For example, the polarity of the Si-O bond in the heat-resistant silicone polymer is large, and the ionic property is 51%. Although it can withstand high temperatures, the Si-O-Si bond is easy to break under the attack of nucleophilic or electrophilic reagents, so its stability for chemicals is relatively poor. Its degree is greatly affected by the type, nature and quantity of the groups connected to the silicon atom.
If the connected group is an electron donor (methyl, ethyl, etc.), then the Si-O-Si chain is weakened and the Si-C bond is strengthened. Under the attack of nucleophilic or electrophilic reagents, the Si-O-Si chain is easy to break; On the contrary, if the connected group is an electron acceptor (such as phenyl, chloromethyl, etc.), the Si-O-Si chain will be strengthened and the Si-C bond will be weakened. Under the attack of nucleophilic or electrophilic reagents, the Si-C bond is easy to break. For example, octamethylcyclotetrasiloxane is easy to undergo intermolecular ring opening rearrangement reaction in the presence of acid clay catalyst, while octaphenylcyclotetrasiloxane is completely unaffected; During the hydrolysis of hydrocarbon (aryl) chlorosilane, if the concentration of hydrochloric acid in the water is too high, the phenyl connected to the silicon atom is easy to drop, but the methyl is relatively stable, which indicates this problem.
The electronegativity of silicon is smaller than that of carbon, and the atomic radius is larger than that of carbon. Its properties lie between metals and nonmetals. The electronic layer of silicon atom is highly polarized, so there are many differences between silicon and carbon in chemical properties.
① The C-C bond is stable, which can generate macromolecular organic polymers with the C-C bond as the main chain;
The Si Si bond is unstable, and the number of Si atoms in the compound with Si Si bond cannot exceed 6.
② CH4 is stable in nature; However, SiH4 is easy to hydrolyze, and its hydrolyzability decreases with the gradual substitution of H atom by hydrocarbon group, which is just the opposite of CH4.
③ The reactivity of Si-H bond is higher than that of C-H bond.
④ The Si Cl bond is easier to ionize than the C-C1 bond, which determines the high activity of Si Cl bond in many chemical reactions and its violent reaction to polar reagents. The Si Cl bond is easy to hydrolyze to form SiOH group, which is also easy to be dehydrated and condensed to form stable oligomer or polymer with Si-O-Si bond. This is a typical method for preparing silicone polymer.
The silicone polymer has a Si-O bond as the main chain, and its heat resistance is good. This is due to:
① The bond energy of Si-O bond in organic silicon polymer is higher than that of C-C bond in ordinary organic polymer; The greater the bond energy, the better the thermal stability.
② In the Si-O bond, the relative electronegativity difference between silicon atom and oxygen atom is large, so the polarity of the Si-O bond is large, with a 51% ionization tendency. It has a dipole induction effect on the hydrocarbon group connected to the Si atom, improving the stability of the connected hydrocarbon group to oxidation, which is much higher than the stability of the same group in ordinary organic polymers; That is to say, Si-O-Si bond can shield the oxidation of these hydrocarbon groups.
③ D-p π bond is formed between silicon atom and oxygen atom in silicone polymer, which increases the stability, bond strength and thermal stability of the polymer.
④ The C-C bond of ordinary organic polymers is easy to break into low molecular substances when heated and oxidized; However, after the thermal oxidation of the hydrocarbon group connected to the silicon atom in the silicone polymer, a highly cross-linked and more stable Si-O-Si bond is generated, which can prevent the main chain from breaking and degradation.
⑤ During thermal oxidation, a stable protective layer rich in Si-O-Si bond is formed on the surface of the silicone polymer, which reduces the impact on the interior of the polymer. For example, polydimethylsiloxane is only slightly cracked at 250 ℃, and the main chain of Si-O-Si starts to break at 350 ℃; However, general organic polymers have been completely cracked and lost their performance.
Therefore, silicone polymers have special thermal stability.
Organic silicon products contain Si-O bonds, which are basically the same as inorganic structural units that form silicic acid and silicate; At the same time, it also contains Si-C (hydrocarbon group), which is a polymer between organic and inorganic polymers with some organic properties. Because of this duality, the silicone polymer has not only the characteristics of heat resistance, flame resistance and hardness of general inorganic materials, but also the characteristics of organic polymers such as insulation, thermoplasticity and solubility, so it is called semi inorganic polymer.
