Physical and chemical properties of alkenes

Ethylene is a colourless, almost odourless gas (with a very faint sweetish smell), slightly lighter than air and sparingly soluble in water; C₂–C₄ alkenes are gases under ordinary conditions, and as the chain grows the members become liquids and then solids. The π-bond of the double bond governs the properties, so addition reactions are typical. Hydrogenation: CH₂=CH₂ + H₂ → CH₃–CH₃ (Ni, heat). Halogenation: CH₂=CH₂ + Br₂ → CH₂Br–CH₂Br; bromine water is decolourized, which is the qualitative test for unsaturation. Addition of hydrogen halides and water follows Markovnikov’s rule: hydrogen adds to the double-bond carbon that already has more hydrogens, and the halogen (or –OH) to the carbon with fewer: CH₂=CH–CH₃ + HBr → CH₃–CHBr–CH₃. Oxidation: with neutral KMnO₄ solution 3CH₂=CH₂ + 2KMnO₄ + 4H₂O → 3CH₂OH–CH₂OH + 2MnO₂ + 2KOH and the purple colour disappears; combustion: C₂H₄ + 3O₂ → 2CO₂ + 2H₂O. In polymerization monomer molecules join to form a polymer: nCH₂=CH₂ → (–CH₂–CH₂–)ₙ polyethylene; propene gives polypropylene, and n is the degree of polymerization.