Chemical properties of alkanes (saturated aliphatic hydrocarbons)
Alkanes have all their carbon atoms connected by single bonds, making them structurally stable. They mainly undergo the following reactions:
Substitution reactions: Under light, hydrogen atoms in alkanes can be replaced by halogens, producing haloalkanes and hydrogen halides. The substitution reaction between methane and chlorine is a typical example, proceeding stepwise to yield multiple substitution products.
Oxidation reaction (combustion): Alkanes are combustible, and complete combustion produces carbon dioxide and water. The general formula is: C<sub>n</sub>H<sub>2</sub>n + 2 + 3n + 1 2 O<sub>2</sub> = = = (ignition) n<sub>CO</sub> 2 + (n + 1)H<sub>2</sub>O<sub>C</sub> n<sub>CO</sub> 2 + (n+1)H<sub>2</sub> O. Methane burns with a pale blue flame and does not react with acidic potassium permanganate solution at room temperature (no obvious phenomenon).
Crack reaction: At high temperatures, alkanes undergo chain breakage and dehydrogenation to produce alkanes and alkenes with lower carbon numbers. This is a core reaction in petrochemical production. 1. Dehydrogenation Cycling: C6-C8 straight-chain alkanes can undergo dehydrogenation cyclization to form benzene-based aromatic hydrocarbons.
Chemical Properties of Alkenes (containing carbon-carbon double bonds, unsaturated aliphatic hydrocarbons)
Double bond structures are highly reactive and can undergo various types of reactions:
Oxidation Reactions: Combustible, producing a bright flame with black smoke; the double bond can be oxidized by acidic potassium permanganate, causing the acidic potassium permanganate solution to decolorize.
Addition Reactions: Unsaturated carbon atoms can directly combine with other atoms/groups to form new compounds. Typical reactions include:
Addition with bromine: Decolorizes bromine water/bromine in carbon tetrachloride solution. The reaction formula is:
CH₂ = CH₂ + Br₂ → CH₂Br−CH₂ =CH₂ + Br₂ →CH₂Br−CH₂ Br reacts with hydrogen to form alkanes, with hydrogen chloride to form chlorinated hydrocarbons, and with water to form alcohols (an industrial method for producing ethanol).
Addition polymerization: Under the action of a catalyst, double bonds open, resulting in polymerization to form high molecular weight compounds, such as the polymerization of ethylene to form polyethylene:
n CH2 = CH2 = = = catalyst
[− CH2− CH2−]
n nCH2 =CH2
=== catalyst
[−CH2−CH2−]
n
Special addition of dienes: Dienes such as 1,3-butadiene exhibit both 1,2-addition and 1,4-addition products.
Chemical Properties of Alkynes (containing a carbon-carbon triple bond, unsaturated aliphatic hydrocarbons)
The triple bond structure is more reactive than the double bond, and its reaction characteristics are similar to those of alkenes:
Oxidation Reactions: Combustible, producing a very bright flame and dense black smoke; the triple bond can be oxidized by acidic potassium permanganate, causing the acidic potassium permanganate solution to decolorize.
Addition Reactions: Can undergo stepwise addition; 1 mol of triple bond can react with a maximum of 2 mol of additive. For example, acetylene adds to a small amount of bromine water to form CHBr=CHBr, and adds to excess bromine water to form CHBr2−CHBr2−CHBr2; it can also undergo addition reactions with hydrogen, hydrogen chloride, water, etc. Addition polymerization: Under the action of a catalyst, polymers can form high molecular weight compounds. For example, acetylene polymerizes to form polyacetylene:
n CH≡ CH====catalyst
[− CH= CH−]
n nCH≡CH
=== catalyst
[−CH=CH−]
n