Alkanes
Section: 11. Organic Chemistry | Syllabus: Cambridge AS Level Physics 9702
Alkanes Alkanes are saturated hydrocarbons containing only single covalent bonds between carbon atoms. They are the simplest type of organic compounds and form the basis for understanding organic chemistry.
Alkane A saturated hydrocarbon containing only carbon and hydrogen atoms joined by single covalent bonds. Called "saturated" because they contain the maximum possible number of hydrogen atoms for each carbon atom.
General Formula The general formula for alkanes is: C n H 2n+2 where n = number of carbon atoms Example For methane (n=1): C₁H₂(₁)+₂ = CH₄ For ethane (n=2): C₂H₂(₂)+₂ = C₂H₆ Homologous Series Alkanes form a homologous series - a family of compounds with similar characteristics.
Homologous Series A family of organic compounds with the same functional group, same general formula, similar chemical properties, and a gradual change in physical properties. The same general formula (C n H 2n+2 ) Similar chemical properties A gradual change in physical properties Each member differs from the next by CH₂ (one carbon and two hydrogen atoms) The First Ten Alkanes The first ten members of the alkane family demonstrate the pattern of adding one CH₂ unit for each successive member.
Name Molecular Formula Structural Formula State at Room Temp Methane CH₄ CH₄ Gas Ethane C₂H₆ CH₃-CH₃ Gas Propane C₃H₈ CH₃-CH₂-CH₃ Gas Butane C₄H₁₀ CH₃-CH₂-CH₂-CH₃ Gas Pentane C₅H₁₂ CH₃-CH₂-CH₂-CH₂-CH₃ Liquid Hexane C₆H₁₄ CH₃-(CH₂)₄-CH₃ Liquid Heptane C₇H₁₆ CH₃-(CH₂)₅-CH₃ Liquid Octane C₈H₁₈ CH₃-(CH₂)₆-CH₃ Liquid Nonane C₉H₂₀ CH₃-(CH₂)₇-CH₃ Liquid Decane C₁₀H₂₂ CH₃-(CH₂)₈-CH₃ Liquid Pattern The first four alkanes (methane through butane) are gases at room temperature.
From pentane onwards, alkanes are liquids at room temperature. Naming Alkanes Alkane names are based on Greek prefixes indicating the number of carbon atoms, followed by the suffix -ane . Prefixes for Carbon Numbers Meth- = 1 carbon Eth- = 2 carbons Prop- = 3 carbons But- = 4 carbons Pent- = 5 carbons Hex- = 6 carbons Hept- = 7 carbons Oct- = 8 carbons Non- = 9 carbons Dec- = 10 carbons Key Point All alkane names end with -ane .
This suffix identifies the compound as a saturated hydrocarbon. Physical Properties of Alkanes As the carbon chain length increases down the homologous series, several physical properties change in predictable ways.
Boiling Point Increases as chain length increases Reason: Larger molecules have stronger intermolecular forces (London dispersion forces) More energy is needed to overcome these forces Melting Point Increases as chain length increases Same reason as boiling point - stronger intermolecular forces Viscosity (Thickness) Increases as chain length increases Longer molecules get tangled more easily Makes the liquid flow less easily (more viscous) Flammability Decreases as chain length increases Smaller molecules vaporize more easily They mix better with oxygen and ignite more readily Important All these property changes are due to the increasing strength of intermolecular forces (van der Waals forces) as the molecular size increases.
Chemical Properties of Alkanes Alkanes undergo several important chemical reactions, with combustion being the most significant for their use as fuels. Combustion (Burning) Alkanes burn in oxygen to release energy, making them excellent fuels.
Complete Combustion (plenty of oxygen) Produces carbon dioxide and water Releases lots of energy (exothermic reaction) Clean blue flame Example for methane: CH₄ + 2O₂ → CO₂ + 2H₂O General equation: Alkane + Oxygen → Carbon dioxide + Water (+ energy) Incomplete Combustion (limited oxygen) Produces carbon monoxide (CO) and/or carbon (soot) and water Less energy released than complete combustion Yellow/orange smoky flame Carbon monoxide is toxic - can cause death Examples for methane: 2CH₄ + 3O₂ → 2CO + 4H₂O (produces CO) CH₄ + O₂ → C + 2H₂O (produces carbon/soot) Safety Warning Incomplete combustion is dangerous because carbon monoxide is a colorless, odorless, toxic gas that binds to hemoglobin and prevents oxygen transport in the blood.
Reactivity Alkanes are relatively unreactive They contain only strong C-C and C-H single bonds They don't react with most reagents like acids, alkalis, or oxidizing agents Main reaction is combustion Substitution Reactions Alkanes can undergo substitution reactions with halogens in the presence of UV light.
A hydrogen atom is replaced by a halogen atom Requires UV light or sunlight Forms a halogenoalkane and hydrogen halide Example: Methane with chlorine CH₄ + Cl₂ → CH₃Cl + HCl (in UV light) This can continue to form: CH₃Cl + Cl₂ → CH₂Cl₂ + HCl CH₂Cl₂ + Cl₂ → CHCl₃ + HCl CHCl₃ + Cl₂ → CCl₄ + HCl Uses of Alkanes Alkanes are among the most important organic compounds due to their use as fuels and chemical feedstocks.
Fuels - burning for energy in heating, cooking, electricity generation, vehicles Methane (natural gas) - domestic heating and cooking Propane and butane - camping gas, portable heaters Petrol (gasoline) - mixture of alkane…
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