Alkanes, alkenes, addition reactions and ethanol production
Revise the key ideas
Alkanes and alkenes
Hydrocarbons contain only carbon and hydrogen. An alkane is saturated: it has only single carbon–carbon bonds and no carbon–carbon double bond; each carbon makes four covalent bonds.
The first four alkanes are methane CH₄, ethane C₂H₆, propane C₃H₈ and butane C₄H₁₀. For open-chain alkanes the general formula is CₙH₂ₙ₊₂.All carbon and hydrogen atoms and single bonds are shown.
Displayed formulae show every atom and every covalent bond. Condensed formulae such as CH₃–CH₂–CH₃ group hydrogen atoms; they are useful but are not a complete displayed formula.All hydrogen atoms and bonds are shown; double bonds count as two bonds.
Alkenes are unsaturated hydrocarbons containing the functional group C=C. Open-chain alkenes with one double bond have formula CₙH₂ₙ; ethene is C₂H₄, propene C₃H₆ and butene C₄H₈.
But-1-ene has CH₂=CH–CH₂–CH₃; but-2-ene has CH₃–CH=CH–CH₃. Their molecular formulae are the same but the double bond occupies a different position.Both have C₄H₈, with the double bond in a different position.
In a homologous series, members have the same functional group and similar chemical reactions, and successive members differ by CH₂. Physical properties such as boiling point change gradually with chain length.
Check that each carbon atom forms four bonds: this is carbon’s valency. Count a double bond as two when checking the total. An alkane must not contain a C=C bond.
Bromine addition and combustion
Alkenes undergo addition reactions: the carbon–carbon double bond opens and atoms add across it. Ethene reacts with bromine to form 1,2-dibromoethane: CH₂=CH₂ + Br₂ → CH₂Br–CH₂Br.One bromine atom attaches to each of the two carbons formerly linked by the double bond.
Bromine water is orange-brown. Shaking it with an alkene changes it to colourless as bromine is used in addition; an alkane does not decolourise it under the normal test conditions without ultraviolet light.
Use small samples and teacher-approved dilute bromine water, wearing eye protection. A colour change supports unsaturation, but identify the unknown in context rather than assuming every decolourising substance must be an alkene.
Complete combustion of both alkanes and alkenes requires sufficient oxygen and produces carbon dioxide and water. Balance carbon first, then hydrogen, then oxygen: CH₄ + 2O₂ → CO₂ + 2H₂O.
Incomplete combustion can produce carbon monoxide and/or carbon soot as well as water. Carbon monoxide is toxic; a smoky flame indicates incomplete combustion, not an absence of carbon.
For a homologous series, use the same functional-group reaction to predict products of another member. An addition product keeps all the atoms of the two reacting molecules, unlike substitution or condensation.
Producing ethanol by fermentation
Yeast enzymes convert dissolved sugars to ethanol and carbon dioxide under anaerobic conditions: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂. The sugar feedstock can come from renewable crops.Distillation concentrates the ethanol after fermentation; it does not cause fermentation.
Provide an aqueous sugar solution and a suitable warm temperature, commonly around 30 °C, with oxygen excluded as far as practical. Low temperature slows enzyme reactions; high temperature can denature enzymes and kill yeast.
An airlock allows carbon dioxide to escape while reducing air entry. Do not seal a fermenting vessel so tightly that gas pressure can build without a safe outlet.
Fermentation makes a dilute ethanol solution containing water and other substances. Ethanol becomes toxic to yeast at high concentrations, limiting continued production.
Fractional distillation concentrates ethanol because ethanol and water have different boiling behaviours. Ethanol boils near 78 °C at atmospheric pressure; collect the ethanol-rich fraction using suitable apparatus, with no naked flame near flammable ethanol.
Ordinary fractional distillation concentrates ethanol but cannot make it completely pure. At a particular composition, ethanol and water boil together without further separation: this mixture is called an azeotrope. For GCSE, describe the product as a concentrated ethanol solution.
Compare production methods using renewability, land use, rate, energy for separation, purity and costs. Renewable feedstock alone does not make every stage environmentally harmless.
Revise hydrocarbons with this narrated video. Use the player controls to pause, seek, adjust the volume or mute. Turn English captions on or off using the captions menu.
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