Chemical properties and uses of alkanes
Because their σ-bonds are strong, alkanes do not react with strong acids, alkalis or KMnO₄ under ordinary conditions. Light or heat makes them halogenate: CH₄ + Cl₂ → CH₃Cl + HCl; with excess chlorine hydrogens are replaced one by one to give CH₂Cl₂, CHCl₃ (chloroform) and CCl₄. In branched alkanes hydrogen is replaced first at the tertiary carbon, then the secondary, and last the primary. Alkanes burn in oxygen to CO₂ and H₂O: CₙH₂ₙ₊₂ + (1.5n + 0.5)O₂ → nCO₂ + (n + 1)H₂O. At 1500 °C methane turns into acetylene: 2CH₄ → C₂H₂ + 3H₂. Breaking C–C bonds of an alkane at high temperature is cracking: C₆H₁₄ → C₂H₆ + C₄H₈ or CH₄ + C₅H₁₀; with a catalyst (catalytic cracking) more branched alkanes result. Dehydrogenation releases H₂: C₂H₆ → C₂H₄ + H₂. Methane is the main component of natural gas, a fuel and a chemical feedstock.
Discuss household gas (stove, water heater): where the smell comes from and what to do if you smell gas (no flame, no switches, open windows, call the emergency gas service). No experiments are done.