Learn first: Basics of organic chemistry and isomerism
Alkanes (paraffins) are saturated hydrocarbons: every carbon–carbon bond in the molecule is single (a σ-bond) and each carbon atom is sp³-hybridised. Alkanes make up most of natural gas and crude oil; they are the main fuels and a key feedstock for the chemical industry. Chemically they are "unreactive" (the name paraffin means "little affinity"), yet free-radical substitution, combustion, cracking and formula-finding calculations appear on the exam every year. This topic is the foundation of organic chemistry: naming, isomerism and reaction mechanisms are first met here.
Alkanes form a homologous series: each member differs from the previous one by a CH₂ group (the homologous difference).
General formula CₙH₂ₙ₊₂; molar mass M = 14n + 2
The first ten members: CH₄ methane, C₂H₆ ethane, C₃H₈ propane, C₄H₁₀ butane, C₅H₁₂ pentane, C₆H₁₄ hexane, C₇H₁₆ heptane, C₈H₁₈ octane, C₉H₂₀ nonane, C₁₀H₂₂ decane. From the fifth member on, names come from Greek numerals.
Because carbon is sp³-hybridised, its four bonds point to the corners of a tetrahedron with a bond angle of 109°28′ (about 109.5°). So a long chain is not straight but zigzag. The C–C bond length is 0.154 nm. Free rotation about a σ-bond is possible, so a molecule can take different conformations.
Carbon atoms in a chain are primary (one neighbouring C), secondary (two), tertiary (three) or quaternary (four). This matters in reactions: a hydrogen on a tertiary carbon is replaced most easily.
Cycloalkanes CₙH₂ₙ are cyclic saturated hydrocarbons (cyclopropane, cyclobutane, cyclopentane, cyclohexane). They are functional-group isomers of alkenes. The rings of cyclopropane and cyclobutane are strained, so they undergo addition (cyclopropane plus hydrogen gives propane), whereas cyclohexane is as stable as an ordinary alkane.
Alkanes show only structural (chain) isomerism, starting with C₄H₁₀: butane and 2-methylpropane (isobutane). C₅H₁₂ has 3 isomers (pentane, 2-methylbutane, 2,2-dimethylpropane), C₆H₁₄ has 5 and C₇H₁₆ has 9.
IUPAC naming steps:
Removing one hydrogen from an alkane gives an alkyl group: CH₃– methyl, C₂H₅– ethyl, C₃H₇– propyl, C₄H₉– butyl. For example, CH₃–CH(CH₃)–CH₂–CH₃ is 2-methylbutane.
Physical properties: C₁–C₄ are gases, C₅–C₁₅ liquids and higher alkanes solids (paraffin wax). Alkanes are non-polar, so they are practically insoluble in water and less dense than it. Boiling point rises with molar mass because intermolecular (dispersion) forces grow. Branching makes a molecule more spherical, reduces the contact area and lowers the boiling point: pentane boils at 36 °C, 2,2-dimethylpropane at about 10 °C.
Alkanes contain only strong, weakly polar σ-bonds, so they do not react with acids, alkalis or ordinary oxidising agents. They do not decolourise bromine water or KMnO₄ solution, which is how they are told apart from unsaturated hydrocarbons.
Free-radical substitution (halogenation) takes place in light (hν) or on heating. It is a chain mechanism: initiation (Cl₂ → 2Cl·), propagation (Cl· + CH₄ → CH₃· + HCl, CH₃· + Cl₂ → CH₃Cl + Cl·) and termination (radicals combining).
CH₄ + Cl₂ → CH₃Cl + HCl → CH₂Cl₂ → CHCl₃ → CCl₄ (one HCl at each step)
Hydrogen is replaced most easily on a tertiary carbon, then secondary, and with most difficulty on a primary carbon. Bromination is especially selective.
Combustion is the most important practical reaction and releases a lot of heat:
CₙH₂ₙ₊₂ + (3n+1)/2 O₂ → nCO₂ + (n+1)H₂O
With too little oxygen, CO or soot (C) forms. An alkane always gives more moles of water than of CO₂: n(H₂O) − n(CO₂) = n(alkane).
Nitration (Konovalov reaction, dilute HNO₃, heat): CH₄ + HNO₃ → CH₃NO₂ + H₂O.
High-temperature changes:
Industrially, alkanes are separated from natural gas and by distilling crude oil. Laboratory methods:
Wurtz reaction: 2CH₃Cl + 2Na → C₂H₆ + 2NaCl (the chain doubles)
Dumas reaction: CH₃COONa + NaOH → CH₄ + Na₂CO₃ (fusion; the chain loses one C)
From a carbide: Al₄C₃ + 12H₂O → 4Al(OH)₃ + 3CH₄
Hydrogenation of alkenes: C₂H₄ + H₂ → C₂H₆ (Ni, heat)
A Wurtz reaction with two different haloalkanes gives a mixture of three alkanes: CH₃Cl + C₂H₅Cl → ethane, propane, butane. In the Dumas reaction a carboxylate salt gives an alkane with one carbon fewer: C₂H₅COONa gives ethane.
Complete combustion of 1 mol of an alkane used 8 mol of oxygen. Identify the alkane.
The mass fraction of carbon in an alkane is 83.33 %. Find its formula.
Burning an alkane gave 194.4 g of a mixture of carbon oxides, with 10 times more CO than CO₂ (by moles). The alkane : O₂ mole ratio is 1.1 : 4. Identify the alkane.
Methane is household gas and the raw material for synthesis gas, hydrogen, methanol and ethyne. Propane–butane mixtures are sold as liquefied gas in cylinders. Petrol, kerosene and diesel are mixtures of liquid alkanes; petroleum jelly and paraffin wax are higher alkanes. Chlorinated derivatives (CH₂Cl₂, CHCl₃, CCl₄) are used as solvents. Cracking and dehydrogenation of alkanes are the starting point for alkenes, alkynes and arenes, so this topic underpins "Alkenes", "Alkynes" and "Oil and gas".