Chemistry › Alkanes and halogenoalkanes
Alkanes and halogenoalkanes
Unit C17 · 4 lessons.
Crude oil is separated by fractional distillation and cracked into smaller, more useful molecules. Alkanes burn to release energy and pollutants, and free-radical substitution puts a halogen onto the chain. The halogenoalkanes then react with nucleophiles by substitution and with strong bases by elimination, and CFCs show the same radical chemistry destroying ozone.
The conditions decide the product. Hydroxide ions in water mostly substitute, giving an alcohol; hydroxide ions in ethanol, heated, mostly eliminate, giving an alkene, so the solvent and the temperature are part of the answer. The rate of hydrolysis follows the carbon–halogen bond enthalpy, fastest for C–I, and not the polarity of the bond.
AQA Papers 2 and 3, OCR A Papers 2 and 3, Edexcel Papers 1, 2 and 3 and Cambridge Papers 1 and 2 examine it, and each paper's page under past papers lists what else it covers.
Specification sections: AQA 3.3.2.1, 3.3.2.2, 3.3.2.3, 3.3.2.4, 3.3.3.1, 3.3.3.2, 3.3.3.3 · OCR A 4.1.2, 4.2.1, 4.2.2, 6.2.1, 6.2.4 · Edexcel 2.20, 6.8 to 6.36, 17.5 · CAIE 12.1, 14.1, 15.1, 19.2
Notes
Illustrated notes for each topic in this unit, with worked examples and a checklist of objectives.
- Alkanes: fractional distillation, cracking and combustion
Alkanes as saturated hydrocarbons with the general formula CₙH₂ₙ₊₂, boiling point against chain length and branching, fractional distillation of crude oil, thermal and catalytic cracking, reforming, complete and incomplete combustion, the pollutants CO, NOₓ, SO₂, soot and unburnt hydrocarbons, photochemical smog and PAN, NO₂ as a catalyst in acid rain, catalytic converters, flue-gas desulfurisation and the greenhouse effect.
- Free-radical substitution
The reaction of methane with chlorine in ultraviolet light, radicals and homolytic fission, the initiation, propagation and termination steps written as equations and with fishhook arrows, why the reaction is a chain, the bond enthalpies that make it run, why fluorine explodes and iodine does not react, further substitution, the isomer mixture from propane and the selectivity of chlorine against bromine, atom economy, and the same chain in the ozone layer.
- Halogenoalkanes: nucleophilic substitution and rates of hydrolysis
The polar carbon-halogen bond and its bond enthalpy, nucleophiles, nucleophilic substitution of primary halogenoalkanes by hydroxide ions, cyanide ions and ammonia with their conditions and curly-arrow mechanisms, naming nitriles and amines, why excess ammonia is used, and the rate of hydrolysis of chloro-, bromo- and iodoalkanes compared with aqueous silver nitrate in ethanol, making halogenoalkanes from alcohols, hydrolysing nitriles to carboxylic acids, primary against tertiary hydrolysis, SN1 and SN2, and optical activity as evidence for the mechanism.
- Elimination from halogenoalkanes, and CFCs and the ozone layer
Elimination of HX from a halogenoalkane with ethanolic potassium hydroxide, the mechanism with hydroxide acting as a base, the competition between substitution and elimination, the alkenes formed from 2-bromobutane, the ozone layer and how it absorbs ultraviolet light, CFCs and the chlorine radical cycle, chlorine reservoirs and the Antarctic spring, the research that identified CFCs as the cause, the Montreal Protocol and HFCs as replacements.
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