Chemical properties of alkanes

Methane and its homologues are called saturated (saturated) paraffin hydrocarbons or alkanes. The last name is given to organic substances (they have the general chemical formula CnH2n + 2, reflecting the saturation of molecules whose atoms are bound by simple covalent bonds of carbon-hydrogen or carbon-carbon) in accordance with the international nomenclature of chemical compounds. They have two main sources: crude oil and natural gas. The properties of alkanes in the homologous series with the increase in the number of carbon atoms in the molecule vary regularly.

The first four representatives of the homologous series have historically formed names. The hydrocarbons behind them are denoted by Greek numerals with the ending -an. The relative molecular weight of each subsequent hydrocarbon differs from the previous one by 14 amu. The physical properties of alkanes, such as the melting point (congealing) and boiling point (condensation), density and refractive index, increase with increasing molecular weight. From methane to butane are gases, from pentane to pentadecane-liquid, followed by solids. All paraffins are lighter than water and do not dissolve in it. The alkanes include:

• CH4 is methane;
• C2H6-ethane;
• C3H8 - propane;
• C4H10 is butane;
• C5H12-pentane;
• C6H14-hexane;
• C7H16-heptane;
• C8H18-octane;
• C9H20-nonane;
• C10H22 - decane;
• C11H24-undecane;
• C12H26-dodecane;
• C13H28-tridecane;
• C14H30-tetradecane;
• C15H32-pentadecane;
• C16H34-hexadecane;
• C17H36-heptadecane;
• C18H38-octadecane;
• C19H40-nonadecane;
• C20H42 - eicosane and so on.

The chemical properties of alkanes are characterized by low activity. This is explained by the relative strength of nonpolar C-C and low-polar C-H bonds, as well as the saturation of molecules. All atoms are connected by single σ-bonds, which are difficult to destroy due to their low polarizability. Their rupture can be realized only under certain conditions, with the formation of radicals, which bear the names of the corresponding paraffin compounds with the replacement of the ending. For example, propane - propyl (C3H7-), ethane - ethyl (C2H5-), methane - methyl (CH3-) and so on.

The chemical properties of alkanes indicate the inertness of these compounds. They are not capable of joining reactions. Replacement reactions are typical for them. Oxidation (combustion) of paraffin hydrocarbons occurs only at elevated temperature. They can be oxidized to alcohols, aldehydes and acids. As a result of the registration (the process of thermal decomposition of hydrocarbons) of higher alkanes at a temperature of 450 to 550 ° C, saturated hydrocarbons with a lower molecular mass can be formed. With increasing temperature, thermal decomposition is called pyrolysis.

The chemical properties of alkanes depend not only on the number in the molecule of carbon atoms, but also on the structure. All paraffins can be separated by a normal one (each C atom can be connected to not more than two carbon atoms) and iso-structures (the C atom can be connected to four other C atoms, due to this the molecule has a spatial structure). For example, pentane and 2,2-dimethylpropane have the same molecular weight and chemical formula C5H12, but they will differ in chemical and physical properties: melting point minus 129.7 ° C and minus 16.6 ° C, boiling point 36.1 ° C And 9.5 ° C, respectively. Isomers are more readily involved in chemical reactions than hydrocarbons of normal structure with the same number of C.

The characteristic chemical properties of alkanes are substitution reactions, which include halogenation or sulfonation. As a result of interaction with chlorine of paraffin, chlorine derivatives of methane are formed by the radical mechanism under the influence of temperature or light: chloromethane CH3Cl, dichloromethane CH2Cl2, trichloromethane CHCl3 and carbon tetrachloride CCl4. In the sulfonation of alkanes under UV light, sulfonyl chlorides are obtained: R-H + SO2 + Cl2 → R-SO2-Cl + HCl. These substances are used in the production of surfactants.