Chemistry › Alkenes and alcohols
Alkenes and alcohols
Unit C18 · 4 lessons.
Ethene and ethanol are one reaction apart in each direction. The double bond undergoes electrophilic addition, explained by curly arrows and by Markovnikov's rule, and alkenes make addition polymers. Alcohols are made by hydration of ethene and by fermentation, classified as primary, secondary or tertiary, oxidised, and dehydrated back to alkenes.
Two explanations are marked closely. The major product of addition to an unsymmetrical alkene comes from the more stable carbocation, tertiary before secondary before primary, and the answer needs that stability, not only the rule. And the product of oxidising a primary alcohol depends on the apparatus: distil the aldehyde off as it forms, or heat under reflux to reach the carboxylic acid.
The papers that set it are AQA Papers 2 and 3, OCR A Papers 2 and 3, Edexcel Papers 2 and 3 and Cambridge Papers 1, 2 and 4.
Specification sections: AQA 3.3.4.1, 3.3.4.2, 3.3.4.3, 3.3.5.1, 3.3.5.2, 3.3.5.3 · OCR A 4.1.3, 4.2.1 · Edexcel 6.14 to 6.39, 17.11 · CAIE 13.3, 14.2, 20.1, 35.3, 16.1, 15.1
Notes
Illustrated notes for each topic in this unit, with worked examples and a checklist of objectives.
- Alkenes: structure, electrophilic addition and Markovnikov's rule
The C=C double bond as a σ bond and a π bond, the planar shape and restricted rotation, why alkenes attract electrophiles, the electrophilic addition mechanism with hydrogen bromide, with bromine and its induced dipole, and with concentrated sulfuric acid followed by water, primary, secondary and tertiary carbocations and the inductive effect, major and minor products and Markovnikov's rule, the bromine water test, the industrial hydration of ethene with steam, and for other boards sp² hybridisation, hydrogenation over nickel, and oxidation by cold dilute and hot concentrated acidified potassium manganate(VII).
- Addition polymers
Addition polymerisation of alkenes, monomers and repeating units drawn with brackets and n, finding the monomer from a section of polymer, poly(ethene), poly(propene), poly(chloroethene) with and without plasticiser, PTFE and poly(phenylethene), their properties and uses and the intermolecular forces behind them, why polyalkenes are unreactive and non-biodegradable, and landfill, incineration, sorting by density, recycling, and biodegradable and photodegradable alternatives, and why polymerisation is exothermic.
- Alcohols: hydration, fermentation and classification
Alcohols and the hydroxyl group, primary, secondary and tertiary alcohols, boiling points and solubility from hydrogen bonding, ethanol by fermentation of glucose and the reasons for its conditions, ethanol by hydration of ethene with steam and a phosphoric acid catalyst and the mechanism, fermentation and the hydration of ethene compared on rate, purity, cost and renewability, the masses of raw material each route needs, biofuels, why bioethanol is described as carbon neutral and why it is not quite, and for Cambridge the reaction of alcohols with sodium and why ethanol is a weaker acid than water.
- Oxidation and elimination of alcohols
Oxidation of primary, secondary and tertiary alcohols with acidified potassium dichromate(VI), the orange to green colour change, equations using [O], aldehydes by distillation and carboxylic acids by heating under reflux and the apparatus for each, required practical 5 in full, tests for aldehydes, ketones and carboxylic acids, the acid-catalysed elimination of water to give alkenes and its mechanism, the three alkenes from butan-2-ol, boiling temperature as a check on purity, Edexcel core practical 5 oxidising ethanol, and for other boards the conversion of alcohols into halogenoalkanes with hydrogen halides and phosphorus halides, including Edexcel core practical 6, 2-chloro-2-methylpropane from 2-methylpropan-2-ol and hydrochloric acid.
Key definitions for this unit
Practical work
The practical capabilities this unit's work belongs to, each with its techniques, its calculations and the errors that cost marks.
Worked in full, from the apparatus to the evaluation:
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