Reactions of alkenes — AQA GCSE Chemistry
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Reactions of alkenes explained
Reactions of alkenes covers 5 specification points, including Alkenes are hydrocarbons with the functional… and It is the generality of reactions of….
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Alkenes are hydrocarbons with the functional group C=C.
Alkenes are a homologous series of unsaturated hydrocarbons whose molecules contain at least one carbon-carbon double bond, C=C. This double bond is the functional group: the reactive part that gives the series its characteristic chemistry. Because of the C=C double bond, alkene molecules contain fewer hydrogen atoms than the maximum possible, meaning they are not saturated with hydrogen. Ethene, C₂H₄, is the simplest alkene; propene, C₃H₆, and butene, C₄H₈, follow. The general formula CₙH₂ₙ applies to non-cyclic alkenes with one double bond. Students should draw the displayed formula of ethene, recognise the C=C in a larger molecule, and distinguish alkenes from the saturated alkanes, which contain only single C–C bonds.
It is the generality of reactions of functional groups that determine the reactions of organic compounds.
Organic compounds are grouped into homologous series, and each series has a characteristic functional group. The functional group is the reactive part of the molecule, so compounds that share a functional group undergo the same type of reaction, even when their carbon chains differ in length. This generality means a student can predict the chemistry of an unfamiliar member of a series from the behaviour of a known member. For example, because ethene decolourises bromine water by addition across C=C, propene and but-1-ene do the same. The carbon chain affects physical properties such as boiling point, but the functional group determines the reactions.
Alkenes react with oxygen in combustion reactions in the same way as other hydrocarbons, but they tend to burn in air with smoky flames because of incomplete combustion.
Like all hydrocarbons, alkenes burn in oxygen, releasing carbon dioxide and water when combustion is complete. The general equation for complete combustion of an alkene is CₙH₂ₙ + 1.5n O₂ → n CO₂ + n H₂O, so ethene gives C₂H₄ + 3O₂ → 2CO₂ + 2H₂O. In a plentiful supply of air, combustion is complete. In a limited supply of air, combustion is incomplete: some carbon is not fully oxidised, producing carbon monoxide, a toxic gas, and solid carbon particles, which appear as soot. These particles glow and give a smoky, yellow flame. Alkenes have a higher carbon-to-hydrogen ratio than the corresponding alkanes, so they tend to burn with a smokier flame.
Alkenes react with hydrogen, water and the halogens, by the addition of atoms across the carbon-carbon double bond so that the double bond becomes a single carbon-carbon bond.
The C=C functional group allows alkenes to undergo addition reactions. Atoms add across the double bond, which opens out to become a single carbon-carbon bond, so the product is saturated. Hydrogen adds in the presence of a nickel catalyst to form an alkane: ethene + hydrogen → ethane. Water adds as steam with a phosphoric acid catalyst to form an alcohol: ethene + steam → ethanol. Halogens add directly: bromine water is decolourised from orange-brown to colourless, which is the test for an alkene, and ethene + bromine → 1,2-dibromoethane. In each case two new single bonds form to the carbons and the C=C is lost.
Students should be able to: describe the reactions and conditions for the addition of hydrogen, water and halogens to alkenes draw fully displayed structural formulae of the first four members of the alkenes and the products of their addition reactions with hydrogen, water, chlorine, bromine and iodine.
Alkenes contain a C=C double bond and undergo addition reactions in which the double bond opens and two new single bonds form. Hydrogen adds with a nickel catalyst, typically around 150 °C, converting ethene to ethane. Steam adds with a phosphoric acid catalyst at about 300 °C and 60 atm, converting ethene to ethanol. Halogens add directly at room temperature: chlorine gives a dichloroalkane, bromine gives a dibromoalkane and iodine gives a diiodoalkane; bromine water decolourises, which tests for a C=C. You must draw fully displayed formulae showing every atom and every bond. The first four alkenes are ethene, propene, butene and pentene, with butene existing as structural isomers. For each addition, draw the alkene, identify the two atoms now bonded to the former double-bond carbons, and complete the product with all C–H and C–X bonds shown.
Your focus
- Define an alkene as a hydrocarbon containing the C=C functional group.
- Draw and interpret the displayed formula of ethene and other simple alkenes.
- Use the general formula CₙH₂ₙ to deduce the molecular formula of a non-cyclic alkene.
Show all 15 objectives
- Explain that the reactions of organic compounds are determined by their functional groups.
- Apply the general reactions of a functional group to predict the behaviour of another member of the same homologous series.
- Distinguish between the role of the functional group and the role of the carbon chain in organic chemistry.
- Write balanced equations for the complete combustion of alkenes.
- Describe the products and observations of incomplete combustion of alkenes.
- Explain why alkenes tend to burn with smoky flames in air.
- Describe the addition reactions of alkenes with hydrogen, water and halogens.
- Explain how the C=C becomes a single C–C bond during addition.
- Use bromine water to test for an alkene and interpret the result.
- Describe the reagents and conditions for the addition of hydrogen, water and halogens to alkenes.
- Draw fully displayed structural formulae of ethene, propene, butene and pentene.
- Draw fully displayed structural formulae of the products formed when alkenes react with hydrogen, water, chlorine, bromine and iodine.
Reactions of alkenes exam tips
Marking Points
- Alkenes are hydrocarbons: their molecules contain carbon and hydrogen only.
- The functional group of an alkene is the carbon-carbon double bond, C=C.
- The C=C makes an alkene unsaturated, because the carbons are bonded to fewer hydrogen atoms than the maximum possible.
- Ethene, C₂H₄, is the simplest alkene and its displayed formula shows a double line between the two carbon atoms.
- The general formula of a non-cyclic alkene with one C=C is CₙH₂ₙ, so ethene is C₂H₄ and propene is C₃H₆.
- Alkanes are saturated and contain only single carbon-carbon bonds, so they are a different homologous series from alkenes.
- A functional group is the reactive part of an organic molecule and defines its homologous series.
- Compounds with the same functional group react in the same general way, so reactions are characteristic of the group rather than the individual compound.
- The length and branching of the carbon chain mainly affect physical properties, not the general reaction type.
- Predictions about an unfamiliar organic compound can be made by identifying its functional group and applying the known reactions of that group.
- For alkenes, the C=C functional group is responsible for addition reactions, so all alkenes undergo addition with hydrogen, water and halogens.
- Alkenes are hydrocarbons and burn in oxygen to release energy, forming carbon dioxide and water when combustion is complete.
- The balanced equation for complete combustion of ethene is C₂H₄ + 3O₂ → 2CO₂ + 2H₂O.
- Incomplete combustion occurs when oxygen supply is limited and produces carbon monoxide and/or carbon (soot) instead of carbon dioxide.
- Carbon monoxide is toxic, and carbon particles cause the smoky, yellow flame seen when alkenes burn in air.
- The high carbon-to-hydrogen ratio of alkenes compared with alkanes helps explain why their flames are smokier.
- Addition reactions of alkenes involve atoms adding across the C=C, which becomes a single C–C bond.
- Hydrogen adds to an alkene in the presence of a nickel catalyst to produce the corresponding alkane.
- Water adds as steam in the presence of a phosphoric acid catalyst to produce an alcohol, for example ethene to ethanol.
- Halogens add across the double bond, for example bromine reacting with ethene to form 1,2-dibromoethane.
- Bromine water changing from orange-brown to colourless is the standard test for an alkene.
- The product of an addition reaction is saturated because it contains only single carbon-carbon bonds.
- State that addition reactions of alkenes involve the C=C double bond opening so that each of the two carbons forms a new single bond.
- Give the correct reagent and condition for each addition: hydrogen with a nickel catalyst; steam with a phosphoric acid catalyst; chlorine, bromine or iodine at room temperature.
- Name the products correctly: hydrogen gives an alkane, water gives an alcohol, and each halogen gives a dihaloalkane, for example ethene plus bromine gives 1,2-dibromoethane.
- Draw fully displayed formulae in which every atom is shown and every bond is drawn as a line, including all C–H bonds and the new C–H or C–X bonds.
- Recognise the first four alkenes as ethene, propene, butene and pentene, and represent butene as straight-chain and branched structural isomers.
- Use the decolourisation of bromine water as evidence for a C=C double bond and link this observation to the addition of bromine.
Examiner Tips
- 💡When asked to identify an alkene, locate the C=C and state that the compound is unsaturated.
- 💡Use the general formula CₙH₂ₙ to deduce a molecular formula, then check that the structure contains one C=C.
- 💡In displayed formulae, draw the double bond as two parallel lines between the carbon atoms and complete the outer shells with hydrogen atoms.
- 💡Name the functional group first, then state the general reaction expected of that group.
- 💡When predicting a reaction, compare the unknown compound with a familiar member of the same homologous series.
- 💡Use phrases such as 'all alkenes' or 'compounds containing C=C' to show that the reaction is general to the functional group.
- 💡Balance combustion equations by counting carbon, then hydrogen, then oxygen atoms.
- 💡Link a smoky flame to incomplete combustion and name the products carbon monoxide and carbon.
- 💡When comparing alkenes with alkanes, refer to the higher proportion of carbon in alkene molecules.
- 💡Show the structural change by drawing the C=C as a single bond in the product and adding the new atoms to the two carbons.
- 💡Name the catalyst and condition for each addition: nickel for hydrogenation, phosphoric acid and steam for hydration.
- 💡Use the bromine water test result as evidence that a compound is unsaturated.
- 💡When asked to draw a displayed formula, count the bonds on each carbon and check that every carbon has four bonds before moving on.
- 💡For each addition reaction, write the word equation first, then convert it into displayed structures so no atom is lost.
- 💡If a question mentions bromine water losing its colour, link the observation to the presence of a C=C double bond and to an addition reaction.
Common Mistakes
- Calling any hydrocarbon with a double bond an alkane; the correct term is alkene, and the C=C must be identified.
- Writing the general formula as CₙH₂ₙ₊₂, which describes alkanes; the correct alkene formula is CₙH₂ₙ.
- Drawing ethene with a single C–C bond; the corrected displayed formula must show C=C.
- Assuming every organic compound reacts differently; the correction is that members of a homologous series share the reactions of their functional group.
- Attributing the reactions of an alkene to its carbon chain length; the correction is that the C=C functional group determines the addition reactions.
- Treating a functional group as a physical label only; the correction is that it is the reactive site where bonds are made or broken.
- Writing that incomplete combustion produces only carbon dioxide and water; the correction is that it also produces carbon monoxide and/or carbon.
- Balancing ethene combustion as C₂H₄ + 2O₂ → 2CO₂ + 2H₂O; the correct balanced equation is C₂H₄ + 3O₂ → 2CO₂ + 2H₂O.
- Stating that carbon monoxide is harmless; the correction is that carbon monoxide is toxic because it reduces the oxygen-carrying capacity of the blood.
- Describing addition as substitution, in which an atom is replaced; the correction is that addition adds atoms across the double bond with no atoms removed.
- Saying the double bond remains after reaction; the correction is that the C=C becomes a single C–C bond.
- Writing that bromine water turns colourless with alkanes; the correction is that alkanes do not decolourise bromine water under these conditions, so the test distinguishes alkenes from alkanes.
- Drawing a condensed formula such as CH₂=CH₂ instead of a fully displayed formula; correct this by drawing every atom and every bond explicitly.
- Adding only one halogen atom to the product; correct this by adding two halogen atoms, one to each carbon of the former double bond.
- Omitting the catalyst or temperature condition when describing hydrogenation or hydration; correct this by stating nickel for hydrogen and phosphoric acid for steam.
- Writing the product of ethene and water as ethane; correct this by writing ethanol, because water adds across the double bond to give an alcohol.