Why are alkanes unreactive, and how do they burn and react with halogens?
Structure and bonding of alkanes (sigma bonds, tetrahedral carbon), boiling-point trends, complete and incomplete combustion, pollutants, and free-radical substitution with halogens (initiation, propagation, termination).
An OCR H432 module 4 answer on alkanes: sigma bonding and tetrahedral shape, boiling-point trends from London forces, complete and incomplete combustion and pollutants, and the free-radical substitution mechanism with halogens.
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What this topic is asking
OCR specification point 4.1.2 wants you to describe the bonding and shape of alkanes, explain their boiling-point trends and lack of reactivity, write equations for complete and incomplete combustion and name the pollutants formed, and give the free-radical substitution mechanism with halogens in ultraviolet light. Alkanes are the unreactive backbone from which the rest of organic chemistry is built.
Structure and bonding
Boiling-point trends
Boiling points rise as the chain lengthens, because longer molecules have more electrons and a larger surface area of contact, giving stronger London (induced dipole) forces that need more energy to overcome. Branching lowers the boiling point: a more spherical, branched molecule has a smaller contact area, so its London forces are weaker. Alkanes are insoluble in water (non-polar) but mix with non-polar solvents.
Combustion
Burning fossil-fuel alkanes also releases pollutants: (toxic), unburnt hydrocarbons and particulates, plus oxides of nitrogen (formed at high temperature in engines) and sulfur dioxide (from sulfur impurities), which cause photochemical smog and acid rain.
Free-radical substitution
Examples in context
Example 1. Why a faulty gas heater is dangerous. Incomplete combustion of natural gas (mostly methane) in a poorly ventilated room produces carbon monoxide, which binds to haemoglobin more strongly than oxygen and can be fatal. This is the combustion chemistry of alkanes applied to a real safety hazard.
Example 2. Substitution gives a product mixture. The chlorination of methane in industry yields a mixture of chloromethane, dichloromethane, trichloromethane and tetrachloromethane, which must be separated by fractional distillation, a direct consequence of the radical chain continuing to substitute.
Try this
Q1. Write the equation for the complete combustion of butane . [2 marks]
- Cue. (or doubled to avoid the half).
Q2. Explain why alkanes are relatively unreactive. [2 marks]
- Cue. The and bonds are strong and have very low polarity, so there is no electron-rich or electron-poor site to attract a reagent.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR 20185 marksEthane reacts with chlorine in the presence of ultraviolet light to form chloroethane. (a) Name the mechanism. (b) Write equations for the initiation step and the two propagation steps. (c) Write one termination step that forms chloroethane.Show worked answer →
(a) Free-radical substitution (1).
(b) Initiation: (homolytic fission) (1). Propagation 1: (1). Propagation 2: (1).
(c) Termination: (1).
Markers reward the mechanism name, homolytic fission in initiation, both chain-carrying propagation steps that regenerate a radical, and a radical-radical termination giving the named product.
OCR 20203 marksExplain why the boiling points of the straight-chain alkanes increase from methane to hexane, and why 2-methylpropane has a lower boiling point than butane.Show worked answer →
As chain length increases, the number of electrons and the surface contact area increase (1), so the London (induced dipole) forces between molecules are stronger and more energy is needed to separate them, raising the boiling point (1).
A branched alkane such as 2-methylpropane has a smaller surface area of contact than its straight-chain isomer butane, so its London forces are weaker and its boiling point is lower (1).
Markers reward the link between size and number of electrons, the strengthening of London forces, and the branching argument based on reduced surface contact.
Related dot points
- Formulae (empirical, molecular, general, structural, displayed and skeletal), homologous series and functional groups, IUPAC nomenclature, and isomerism (structural and E/Z stereoisomerism with Cahn-Ingold-Prelog priority).
An OCR H432 module 4 answer on the basics of organic chemistry: the six types of formula, homologous series and functional groups, IUPAC nomenclature, and structural and E/Z stereoisomerism using Cahn-Ingold-Prelog priority rules.
- Structure and bonding of alkenes (sigma and pi bonds, trigonal planar carbon), E/Z isomerism, electrophilic addition (hydrogen, halogens, hydrogen halides and steam), Markownikoff's rule, and addition polymerisation.
An OCR H432 module 4 answer on alkenes: pi bonding and trigonal planar shape, electrophilic addition with hydrogen, halogens, hydrogen halides and steam, Markownikoff's rule via the more stable carbocation, and addition polymerisation.
- Polarity of the carbon-halogen bond, nucleophilic substitution (with hydroxide, cyanide and ammonia), the trend in hydrolysis rate with carbon-halogen bond enthalpy, elimination to alkenes, and the role of CFCs in ozone depletion.
An OCR H432 module 4 answer on haloalkanes: the polar carbon-halogen bond, nucleophilic substitution with hydroxide, cyanide and ammonia, the hydrolysis-rate trend with bond enthalpy, elimination to alkenes, and CFCs and ozone depletion.
- Planning multi-step synthesis routes between functional groups using practical techniques (reflux, distillation, purification), and the analytical techniques of infrared spectroscopy (functional-group absorptions) and mass spectrometry (molecular ion and fragmentation).
An OCR H432 module 4 answer on organic synthesis and analysis: building multi-step reaction routes between functional groups, practical techniques, infrared spectroscopy for functional groups, and mass spectrometry for relative molecular mass and fragmentation.
- Enthalpy and standard enthalpy changes, exothermic and endothermic reactions, calorimetry and the q = mcDeltaT equation, average bond enthalpies, and Hess's law including formation and combustion cycles.
An OCR H432 module 3 answer on enthalpy changes: standard enthalpy definitions, calorimetry with q = mcDeltaT, bond enthalpy calculations, and Hess's law cycles for formation and combustion.
Sources & how we know this
- OCR A-Level Chemistry A (H432) specification — OCR (2015)