Definition and Classification Aliphatic hydrocarbons are hydrocarbons that possess the basic properties of aliphatic compounds. Their molecules contain only carbon and hydrogen elements, and the carbon atoms can be linked in straight chains, branched chains, or rings (alicyclic hydrocarbons). They are named after the fact that they were first extracted from fats.
Based on the type of chemical bonds between carbon atoms, alkanes can be divided into three categories:
Alkanes (including cycloalkanes): Saturated hydrocarbons consisting entirely of carbon-carbon single bonds. The general formula for chain alkanes is:
C<sub>n</sub>H<sub>2n</sub>+2 (n≥1)<sup>CX</sup>n<sup>HX</sup>2n+2</sup>(n≥1);
Alkenes: Unsaturated hydrocarbons containing carbon-carbon double bonds. The general formula for monoalkenes is:
C<sub>n</sub>H<sub>2n</sub> (n≥2)<sup>CX</sup>n<sup>HX</sup>2n<sup>HX</sup>(n≥2);
Alkynes: Unsaturated hydrocarbons containing carbon-carbon triple bonds. The general formula for monoalkynes is:
C<sub>n</sub>H<sub>2n−2</sub> (n≥2)<sup>CX</sup>n<sup>HX</sup>2n−2</sup>(n≥2).
Core Properties and Rules
Physical Properties: As the number of carbon atoms in a molecule increases, the boiling point and melting point gradually increase, and the density gradually increases, but the density of all aliphatic hydrocarbons is less than that of water; at room temperature, hydrocarbons with 1-4 carbon atoms are gaseous (except for neopentane, which is also gaseous), those with 5-16 carbon atoms are liquid, and those with 17 carbon atoms or more are solid.
Chemical Properties: Alkanes are stable and do not react with strong acids, strong bases, or strong oxidizing agents. They can undergo substitution reactions under light (producing a mixture of products), and are combustible and can undergo high-temperature cracking. Alkenes and alkynes (unsaturated aliphatic hydrocarbons) are reactive. In addition to combustion, they can be oxidized by acidic potassium permanganate. Their characteristic reaction is addition reaction (with bromine, hydrogen chloride, water, etc.), and they can also undergo addition polymerization to form high molecular weight compounds.