Alkanes are the backbone of crude oil and the fuels made from it. Their bonds are strong and non-polar, so they ignore most reagents, but they burn readily, and that is both their use and their problem.
Alkanes are saturated hydrocarbons, CnH2n+2, with only single bonds and tetrahedral carbons. Their C–C and C–H bonds are strong and almost non-polar, so they are attacked by neither nucleophiles nor electrophiles, which makes them generally unreactive. They come from crude oil by fractional distillation and cracking. They burn: complete combustion gives CO2 and H2O; in limited oxygen, incomplete combustion gives toxic CO and soot. Boiling points rise with chain length and fall with branching.
Fig. 1: boiling points of the straight-chain alkanes
Section 01
Structure and sources.
Build alkanes of increasing length in the simulator, then crack a long one and compare the products.
Why unreactive: C–C (350 kJ mol−1) and C–H (410) are strong; C and H have similar electronegativities, so there are no δ+ or δ− sites to attract reagents.
Fractional distillation separates crude oil into fractions by boiling point.
Cracking breaks long chains into shorter, more useful alkanes and alkenes: C10H22 → C8H18 + C2H4. Thermal cracking uses high temperature and pressure; catalytic cracking uses a zeolite catalyst at lower temperature.
Boiling points: longer chains have more electrons and larger surface contact, so stronger id-id forces; branched isomers boil lower.
Stage 1 · Learn
Check what the sim just showed you
Four quick checks. Each correct answer earns XP and records this skill on your profile.
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Why are alkanes generally unreactive?
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Which is the balanced equation for the complete combustion of propane?
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What is the purpose of cracking?
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Which product of incomplete combustion is toxic because it binds to haemoglobin?
Section 02
Combustion and pollution.
Alkanes are used as fuels because their combustion is very exothermic. What they produce depends on the oxygen supply.
Combustion
Equation
Problem
complete
C3H8 + 5O2 → 3CO2 + 4H2O
CO2, a greenhouse gas
complete (octane)
2C8H18 + 25O2 → 16CO2 + 18H2O
incomplete
2CH4 + 3O2 → 2CO + 4H2O
CO is toxic; soot (C) causes respiratory problems
Other pollutants: NOx from the engine and unburnt hydrocarbons.
Catalytic converter: 2CO + 2NO → 2CO2 + N2; hydrocarbons are oxidised to CO2 and H2O.
Calculation: burning 2.20 g of propane (0.0500 mol) needs 0.250 mol O2 = 6.00 dm3 at room conditions.
Examiner trap
When balancing a combustion equation, balance C first, then H, then O last; if O needs a half, double everything. Alkanes do not decolourise bromine water in the dark: they react with halogens only by free-radical substitution in UV light.
Stage 2 · Exam
Exam-style questions
Unlocks once the four checks above are done. Worth more XP, written to AS Paper 1 and Paper 2 standard.
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Which equation shows a possible cracking reaction?
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Why does pentane (b.p. 36 °C) boil at a higher temperature than 2,2-dimethylpropane (b.p. 10 °C)?
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What volume of oxygen at room conditions is needed to burn 2.20 g of propane (Mr 44.0) completely?
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How is soot formed when a fuel burns?
Skill unlocked
Alkanes, mastered.
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