Alkanes

Alkanes are a family of compounds found in natural gas and petrol. They are made only of carbon and hydrogen atoms joined in chains.

Key ideas

• Hydrocarbons: molecules with only carbon (C) and hydrogen (H).
• Saturated: alkanes have only single covalent bonds (C–C and C–H). No C=C double bonds.
• Bonding: all bonds are single covalent bonds; electrons are shared between atoms.
• General formula: $C_nH_{2n+2}$ for unbranched alkanes.
• Homologous series: members differ by one $CH_2$ unit and show similar properties and trends.

Names and structures (unbranched)

• Methane: $CH_4$
• Ethane: $C_2H_6$ (structural: CH₃–CH₃)
• Propane: $C_3H_8$ (CH₃–CH₂–CH₃)
• Butane: $C_4H_{10}$ (CH₃–CH₂–CH₂–CH₃)

Displayed formulae show every atom and every bond. Structural formulae use groups like CH₃ and CH₂ to make chains clearer.

Physical properties and trends

• Small alkanes are colorless gases; bigger ones are liquids or waxy solids.
• Insoluble in water and less dense than water.
• Boiling point increases as the chain gets longer (more particles attract each other).

Chemical properties

• Generally unreactive because they are saturated and have strong C–C and C–H bonds.
• Combustion (burning): in plenty of oxygen, alkanes burn to carbon dioxide and water, releasing energy. Example: $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$.
• Incomplete combustion: not enough oxygen gives carbon monoxide (CO) or soot (C) and water.
• Substitution with chlorine: a photochemical reaction; ultraviolet light provides activation energy $E_a$. One H is replaced by Cl (monosubstitution). Example: $CH_4 + Cl_2 \xrightarrow{\text{UV}} CH_3Cl + HCl$.

Common misconceptions

• Alkanes do not decolourise aqueous bromine; alkenes do.
• “Saturated” means only single bonds, not “full of hydrogen”.
• Substitution with chlorine needs UV light; it is very slow without it.