Methane and its homologues are called saturated paraffinic hydrocarbons or alkanes. The latter name is given to organic substances (they have the general chemical formula CnH2n + 2, which reflects the saturation of molecules in which atoms are connected by simple covalent carbon β hydrogen or carbon β carbon bonds) assigned in accordance with the international nomenclature of chemical compounds. They have two main sources: crude oil and natural gas. The properties of alkanes in a homologous series with an increase in the number of carbon atoms in a molecule naturally change.
The first four representatives of the homologous series have historical names. The hydrocarbons behind them are designated by Greek numerals with the ending βan. The relative molecular weight of each subsequent hydrocarbon differs from the previous one by 14 amu The physical properties of alkanes, such as melting point (solidification) and boiling point (condensation), density and refractive index, increase with increasing molecular weight. From methane to butane, these are gases, from pentane to pentadecane, liquids, followed by solids. All paraffins are lighter than water and do not dissolve in it. Alkanes include:
- CH4 is methane;
- C2H6 is ethane;
- C3H8 - propane;
- C4H10 - Butane;
- C5H12 pentane;
- C6H14 hexane;
- C7H16 - heptane;
- C8H18 is octane;
- C9H20 - nonane;
- C10H22 - Dean;
- C11H24 - undecane;
- C12H26 - Dodecane;
- C13H28 - Tridecane;
- C14H30 - tetradecane;
- C15H32 - Pentadecane;
- C16H34 - hexadecane;
- C17H36 - heptadecane;
- C18H38 - Octadecane;
- C19H40 - nonadecane;
- C20H42 - eicosan and so on.
The chemical properties of alkanes are characterized by low activity. This is due to the relative strength of nonpolar C β C and lowpolar C β H bonds, as well as the saturation of the molecules. All atoms are connected by single Ο-bonds, which are difficult to break due to their low polarizability. Their rupture can only take place under certain conditions, while radicals are formed that bear the names of the corresponding paraffin compounds with a replacement end. For example, propane - propyl (C3H7β), ethane - ethyl (C2H5β), methane - methyl (CH3β), and so on.
The chemical properties of alkanes indicate the inertness of these compounds. They are not capable of addition reactions. Typical for them are substitution reactions. The oxidation (combustion) of paraffin hydrocarbons occurs only at elevated temperatures. They can be oxidized to alcohols, aldehydes and acids. As a result of cracking (the process of thermal decomposition of hydrocarbons) of higher alkanes at a temperature of 450 to 550 Β° C, saturated hydrocarbons with a lower molecular weight can be formed. With increasing temperature, thermal decomposition is called pyrolysis.
The chemical properties of alkanes depend not only on the number of carbon atoms in the molecule, but also on the structure. All paraffins can be separated normal (each C atom can be connected to no more than two carbon atoms) and iso-structure (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 Β° and minus 16.6 Β° , boiling point 36.1 Β° and 9.5 Β° C, respectively. Isomers enter chemical reactions more easily than normal-structure hydrocarbons with the same number of C atoms.
The characteristic chemical properties of alkanes are substitution reactions, which include halogenation or sulfonation. As a result of interaction with paraffin chlorine by a radical mechanism under the influence of temperature or light, methane chlorine derivatives are formed: chloromethane CH3Cl, dichloromethane CH2Cl2, trichloromethane CHCl3 and carbon tetrachloride CCl4. When sulfonation of alkanes under the influence of UV light, sulfonyl chlorides are obtained: R β H + SO2 + Cl2 β R β SO2 β Cl + HCl. These substances are used in the manufacture of surfactants.