Carbon-carbon single bonds, and the rest of the valence bonds are combined with hydrogen connect all the carbon atoms in the molecule. They are divided into two types: cycloalkanes and paraffins. The general formula of paraffin is CnH2n+2, which is the simplest type of organic compound. The main sources of alkanes are oil and natural gas, which are important chemical raw materials and energy materials.
Definition:
Compounds composed of only two elements, hydrocarbons and hydrogen, are called hydrocarbons, or hydrocarbons for short. According to the different molecular skeletons of hydrocarbons, hydrocarbons can be divided into two categories: chain hydrocarbons (aliphatic hydrocarbons) and cyclic hydrocarbons (alicyclic hydrocarbons). Chain hydrocarbons can be divided into saturated hydrocarbons and unsaturated hydrocarbons. The overall structure is mostly organic compounds composed of carbon and hydrogen atoms with carbon-carbon single bonds and carbon-hydrogen single bonds. Saturation means that the combination of carbon atoms and other atoms in the molecule has reached the maximum.
In addition, alkanes are a type of saturated hydrocarbons, and saturated hydrocarbons include cycloalkanes and chain alkanes.
Physical properties
1. When the number of carbon atoms is less than or equal to 4, alkanes are gaseous at room temperature, and other alkanes are solid or liquid at room temperature (neopentane is gaseous at room temperature).
2. It is insoluble in water and easily soluble in organic solvents.
3. As the number of carbon atoms increases, the boiling point gradually increases.
4. As the number of carbon atoms increases, the relative density gradually increases. The density of alkanes is generally less than that of water.
Micro structure
Alkane is not a plane structure drawn by the structural formula, but a three-dimensional shape. All carbon atoms are sp3 hybridized, and each atom is connected by a σ bond. The bond angle is close to 109°28', and the average bond length of the CC bond The average bond length of the CH bond is 154 pm, and the average bond length of the CH bond is 109 pm. Since the electron cloud of the σ bond is distributed axisymmetrically along the bond axis, the two bonding atoms can “freely” rotate around the bond axis.
Chemical formula
Starting from methane, each additional carbon atom adds two hydrogen atoms, so the general formula of alkanes is CnH2n+2, n represents the number of carbon atoms (n=1,2,3,···), theoretically n can be very large, but known alkanes n is about 100 or less. [4] A series of compounds with the same molecular formula and structural characteristics are in the same series, the homologous difference of alkanes is CH2, and alkanes with different numbers of C atoms are homologous to each other. The homologues in the same series have similar structures and similar chemical properties, and their physical properties change regularly with the increase of carbon atoms.
Physical properties
Odor
Low boiling point alkanes are colorless liquids with special odor; high boiling point alkanes are viscous oily liquids, odorless.
State of matter
The physical properties of alkanes show regular changes as the number of carbon atoms in the molecule increases.
At room temperature 25°, alkanes containing 1 to 4 carbon atoms are gases.
Alkanes containing 5 to 16 carbon atoms are liquid. However, in fact, alkanes containing 10 to 19 carbon atoms can be solid at normal temperatures.
Normal alkanes containing more than 18 carbon atoms are solid, but the melting point of normal alkanes containing 60 carbon atoms (melting point 99°C) does not exceed 100°C. Alkanes are non-polar molecules with zero dipole moments, but the distribution of charges in the molecules is not very uniform, and instantaneous dipole moments can be generated during movement. Interaction force (dispersive force). In addition, there are van der Waals forces between molecules. These intermolecular forces are one or two orders of magnitude smaller than chemical bonds. The energy required to overcome these forces is also low. Therefore, the melting point and boiling point of general organic compounds rarely exceed 300°C.
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