Elimination — AQA A-Level Chemistry
Test yourself on Elimination with AQA A-Level practice questions.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge
Elimination explained
Hydroxide ion can behave in two ways towards 2-bromopropane, and both happen at the same time.
Read the full explanation
As a nucleophile it attacks the δ⁺ carbon and displaces bromide, giving propan-2-ol by nucleophilic substitution. As a base it removes a hydrogen from a carbon next to the C–Br carbon; those bonding electrons swing in to form a C=C bond while bromide leaves, giving propene by elimination. Conditions shift the balance rather than switching one route off: warm aqueous potassium hydroxide favours substitution, while hot potassium hydroxide dissolved in ethanol favours elimination. Exam answers must name both mechanisms correctly and say which role the hydroxide is playing in each, since the same reagent is doing two different jobs.
Alkenes can be formed from alcohols by acid-catalysed elimination reactions.
Heating an alcohol with an acid catalyst removes water across two adjacent carbons and leaves a double bond: ethanol gives ethene, CH₃CH₂OH → CH₂=CH₂ + H₂O. Concentrated sulfuric or phosphoric acid is used, or the vapour is passed over hot aluminium oxide. The acid is a catalyst, regenerated at the end, so it does not belong on either side of the overall equation. Because the hydrogen can be taken from either neighbouring carbon, an unsymmetrical alcohol gives more than one alkene: butan-2-ol gives but-1-ene and both E and Z forms of but-2-ene. The reaction is classed as elimination, and specifically as dehydration, because the fragment lost is a whole water molecule.
Alkenes produced by this method can be used to produce addition polymers without using monomers derived from crude oil.
Ethene is normally cracked from crude oil fractions, but there is another route. Sugar from a crop is fermented to ethanol, the ethanol is dehydrated over an acid catalyst to ethene, and that ethene is polymerised to poly(ethene). The carbon in the polymer then comes from carbon dioxide fixed by a plant in the last growing season rather than from a finite fossil resource, so the process conserves crude oil and is renewable in principle. The polymer itself is chemically identical to poly(ethene) from oil, so it is no more biodegradable. The route also has real costs: land, fertiliser and water for the crop, and a large energy input to distil and dehydrate the ethanol.
Students should be able to: outline the mechanism for the elimination of water from alcohols.
Dehydration runs in three arrow-drawing steps. A lone pair on the alcohol's oxygen attacks H⁺ from the acid, giving a protonated alcohol; this turns the poor leaving group, OH⁻, into a good one, water. Next, the C–O bond breaks heterolytically, the arrow running from that bond to the oxygen, so a water molecule leaves and a carbocation remains. Finally a base, water or the hydrogensulfate ion, removes a hydrogen from a carbon adjacent to the positive carbon; the arrow runs from that C–H bond into the C–C bond, making the π bond of the alkene, and the H⁺ is handed back so the catalyst is regenerated. Choosing which adjacent hydrogen to remove decides which alkene you draw.
Required practical 5 Distillation of a product from a reaction.
This practical isolates a product by exploiting a difference in boiling point. Heat the reaction mixture in a flask containing anti-bumping granules, using a water bath or heating mantle rather than a flame because organic liquids are flammable. The thermometer bulb sits level with the side arm, where the vapour leaves, so it reads the temperature of the vapour being condensed rather than of the liquid. Cooling water enters the condenser at the lower end and leaves at the upper end so the jacket stays full. Collect only the fraction distilling over the expected boiling range. Afterwards the distillate is usually washed in a separating funnel, releasing pressure often if carbon dioxide is produced, then dried with anhydrous calcium chloride.
Your focus
- Name both mechanisms operating when 2-bromopropane is warmed with potassium hydroxide and give the product of each.
- Explain how changing from aqueous to ethanolic solvent alters the ratio of alkene to alcohol.
- Draw both mechanisms side by side, labelling hydroxide as nucleophile in one and base in the other.
Show all 15 objectives
- Write the equation for the acid-catalysed dehydration of a named alcohol, showing water as a product.
- Deduce every alkene that can be formed from an unsymmetrical alcohol, including E and Z isomers.
- Explain why the acid used is described as a catalyst and does not appear in the overall equation.
- Describe the three-stage route from plant sugar to poly(ethene), naming the reagents and conditions at each stage.
- Explain why using this route does not make the resulting poly(ethene) biodegradable.
- Discuss one benefit and one drawback of making addition polymers from crop-derived ethanol.
- Draw the three-step elimination mechanism for the dehydration of a named alcohol.
- Identify which curly arrow is responsible for forming the carbon–carbon double bond.
- Explain why the alcohol is protonated first, referring to the quality of the leaving group.
- Label a distillation apparatus, placing the thermometer and the condenser water flow correctly.
- Explain why the distillate is collected only over a narrow temperature range.
- Justify each step of the work-up, including washing with sodium carbonate and drying with an anhydrous salt.
Elimination exam tips
Marking Points
- one mark for naming elimination as the route to the alkene and nucleophilic substitution as the route to the alcohol
- one mark for stating that hydroxide acts as a base in one reaction and as a nucleophile in the other
- one mark for the conditions that favour each route, aqueous and warm against ethanolic and hot
- one mark for the three curly arrows in the elimination, including the arrow from the C–H bond into the C–C bond
- one mark for naming concentrated sulfuric or phosphoric acid as catalyst, with heating
- one mark for the balanced equation showing the alkene and water
- one mark for naming the reaction type as elimination, or dehydration
- one mark for identifying all the possible alkenes from an unsymmetrical alcohol, including E and Z isomers
- one mark for the route, sugar to ethanol by fermentation, ethanol to ethene by dehydration, ethene to poly(ethene) by addition polymerisation
- one mark for saying the monomer comes from a renewable plant source rather than crude oil
- one mark for a sustainability advantage, such as conserving a finite resource
- one mark for a limitation, such as land use or the energy needed for distillation and dehydration
- one mark for the arrow from a lone pair on the alcohol oxygen to H⁺
- one mark for the arrow from the C–O bond to the oxygen, showing water leaving
- one mark for the arrow from the correct C–H bond into the correct C–C bond
- one mark for naming the mechanism as elimination
- one mark for showing H⁺ released at the end, regenerating the catalyst
- one mark for the thermometer bulb level with the side arm where the vapour leaves
- one mark for cooling water entering the condenser at the lower end and leaving at the upper end
- one mark for collecting only the fraction that distils over the expected boiling range
- one mark for naming a drying agent such as anhydrous calcium chloride and its purpose, to remove water
- one mark for releasing the pressure in a separating funnel when carbon dioxide is produced
Examiner Tips
- 💡Name the solvent, not just the reagent: "KOH in ethanol" and "KOH(aq)" are what distinguish the two answers.
- 💡Label which hydrogen you are removing; from 2-bromopropane it must come from a CH₃ group.
- 💡If a question says "concurrent", it expects both products, so give both and say which conditions favour which.
- 💡Before answering, mark every carbon next to the hydroxyl carbon that carries a hydrogen; each gives a product.
- 💡Check for E-Z isomerism in each alkene you produce, since it is often worth a separate mark.
- 💡Say the acid is a catalyst if asked why it does not appear in the equation.
- 💡Set the route out as three labelled arrows with reagents and conditions above each.
- 💡Separate the source of the carbon from the properties of the polymer; only the source changes.
- 💡Where the command word is discuss, give one advantage and one disadvantage.
- 💡Draw the carbocation as a separate intermediate with the charge clearly on one carbon.
- 💡Where a C–H bond breaks, show both the bond and the hydrogen even in a skeletal structure.
- 💡Finish with the H⁺ leaving, since regeneration of the catalyst is often a separate mark.
- 💡Learn the thermometer position and the water flow direction as the two marks almost always available.
- 💡If sodium carbonate is used to wash out acid, explain the pressure build-up as carbon dioxide gas.
- 💡Quote the purpose of each reagent in the work-up, not just its name.
Common Mistakes
- assuming two different reagents are used, when the same potassium hydroxide does both jobs
- naming the alkene route "nucleophilic elimination", which contradicts the base role of the hydroxide
- drawing the hydroxide arrow to the carbon instead of to the hydrogen in the elimination
- writing that changing the solvent stops one reaction entirely rather than changing the proportions
- writing the acid as a reactant that is consumed in the overall equation
- leaving water out of the products
- giving only one alkene from butan-2-ol, missing but-1-ene or one of the E-Z pair
- calling the reaction substitution because the hydroxyl group has gone
- claiming the polymer made this way is biodegradable, when it is the same compound as poly(ethene) from oil
- calling the whole process carbon neutral with no qualification
- confusing ethanol with ethene in the equations for the two stages
- saying crude oil is not used at all, ignoring the fuel used to run the process
- drawing the final arrow from the C–H bond to the positive carbon instead of into the C–C bond
- using a skeletal formula without drawing the C–H bond and its hydrogen, which AQA does not credit
- removing a hydrogen from the carbon that carried the hydroxyl group
- showing OH⁻ leaving directly, rather than protonating the oxygen first so water leaves
- placing the thermometer bulb in the liquid, so it records the flask temperature rather than the vapour
- running the condenser water in at the top, so the jacket never fills with water
- heating a flammable organic mixture with a naked flame
- sealing the apparatus completely, leaving no vent for expanding vapour