Alkenes & alcohols

• What is an alkene?

  Unsaturated hydrocarbon- can make more bonds with extra atoms in addition reactions. As there are 2 pairs of electrons in C=C double bond, it has a high electron density- makes alkenes reactive.
• What is an electrophile?

  Electron pair acceptors (usually short of electrons).
• What is an electrophilic addition reaction?

  Double bond in alkene opens up & atoms added to carbons. Double bond has many electrons so attacked by electrophiles. C=C bond repels electrons in electrophile, which polarises electrophile's bond. Bond formed & carbocation intermediate formed. Remaining ion acts as nucleophile & forms bond. Saturated compound formed (alkane).
• Explain the test for alkenes (unsaturation):

  Alkene + orange bromine water = colourless. Bromine added across double bond to form colourless dibromoalkane via electrophilic addition.
• How do you form halogenoalkanes?

  Alkenes undergo electrophilic addition with hydrogen halides. If hydrogen halide adds to unsymmetrical alkene, there's 2 possible products. Amount of each product formed depends on how stable carbocation formed is. (Primary is least stable).
• Why are carbocations with more alkyl groups more stable?

  Alkyl groups feed electrons towards the positive charge. (Move some of their negatively charged electrons nearer to it, which stabilises positive charge). More product formed via stable carbocation (major product).
• Addition polymerisation:

  Double bonds in alkenes open up & join together.
• Polymer properties:

  Main carbon chain of polyalkenes usually non-polar so addition polymers unreactive- polyalkenes chemically inert. Monomers within polymer chain have strong covalent bonds. IMF between polymer chains are much weaker. Longer chains with fewer branches have stronger IMF (stronger & more rigid) as they pack closely together.
• Why is PVC hard & brittle?

  Polar covalent bonds between chlorine & carbon- permanent dipole dipole forces between chains.
• What are plasticisers?

  Chemicals added to polymer to modify their properties-makes polymer bendier. Plasticiser molecules get between polymer chains & push them apart- reduces strength of IMF between chains- chains slide around more (more flexible). Plasticised PVC more flexible than rigid PVC- used to make electrical cable insulation, flooring tiles & clothing.
• What is a dehydration reaction?

  Make alkenes by eliminating water from alcohols. Allows produce of alkenes from renewable resources (can produce ethanol by fermentation of glucose from plants). Can produce polymers without needing oil.
• What is the catalyst in a dehydration reaction?


  Concentrated sulfuric/phosphoric acid.
• How do you react & distil to separate the alkene?

  Add conc. H2SO4 & H3PO4 to round-bottomed flask with cyclohexanol. Mix solution by swirling flask & add 2-3 carborundum boiling chips. Connect flask to rest of distillation apparatus (thermometer, condenser & cooled collection flask). Gently heat mixture in flask to around 83.C (bp of cyclohexene) using water bath. Warm gas rises into condenser. Cold water in condenser turns gas back into liquid- collected in flask.
• How do you separate the alkene from impurities?

  Transfer product mixture to separating funnel & add water to dissolve water-soluble impurities & create an aqueous solution. Allow mix to settle into layers. Drain off aqueous layer at bottom, leaving impure cyclohexene.
• Explain purification:

  Drain off impure cyclohexene into round-bottomed flask. Add anhydrous CaCl2 (drying agent- removes water traces) & stopper flask. Let mixture dry for at least 20 mins with occasional swirling. Distil- cyclohexene may still have impurities. Only collect product released when mixture's at around 83.C- this is pure cyclohexene.
• What does hydrating alkenes produce?

  Alcohols. Uses steam & acid catalyst. Ethene hydrated by steam at 300.C & pressure of 60atm. Solid phosphoric (V) acid catalyst. Reaction yield low, however recycle unreacted ethene gas to increase overall yield profitable. Expensive equipment but low labour costs, fast reaction, pure product & ethene is from oil (finite resource).
• Explain fermentation:

  Exothermic process carried out by yeast in anaerobic conditions. Yeast produces enzymes that convert glucose into ethanol & CO2. Temperature (optimum) : 30-40.C. Cheap equipment & renewable resource (glucose), slow rate of reaction, high labour costs & impure product.
• What is a biofuel?

  Fuel made from biological material that's recently died.
• Advantages of biofuel use:

  Renewable energy sources (more sustainable), 'carbon neutral' (release same amount of CO2 when burnt as plant took in when growing).
• Disadvantages of biofuel use:

  Deforestation so crops can grow on land-destroys habitats & trees cut down often burnt (releases CO2). Land could grow food instead. Fertilisers in soil can pollute waterways & some release nitrous oxide (greenhouse gas). Practical issues with switching from fossil fuels to biofuels- most car engines unable to run on fuels with high ethanol concentrations without being modified.
• Why is bioethanol called carbon neutral?

  Photosynthesis: 6CO2 + 6H2O --> C6H12O6 + 6O2 Fermentation: C6H12O6 --> 2C2H5OH + 2CO2 Ethanol burnt: 2C2H5OH + 6O2 --> 4CO2 + 6H20 However, fossil fuels burnt (releases CO2) to power machinery used to make fertilisers & machinery that harvests crops. Refining & transporting bioethanol uses energy. So not actually carbon neutral.
• Uses of alcohols:

  Solvents in cosmetics & aftershaves, fuel, drinks, detergents & inks & intermediate in manufacture of drugs
• How can you oxidise an alcohol?

  Oxidising agent acidified potassium dichromate (VI). Oxidises primary & secondary alcohols. Orange dichromate (VI) ion reduced to green chromium ion.
• How can you oxidise primary alcohols to just aldehydes?

  Gently heat excess alcohol with controlled amount of oxidising agent in distillation apparatus. Aldehyde distilled off immediately (boils at lower temperature than alcohol).
• How can you produce ethanal (apple smelling aldehyde)?

  Gently heat ethanol with potassium dichromate (VI) solution & sulfuric acid.
• What is heating under reflux?

  Can increase temperature of organic reaction to boiling without losing volatile solvents, reactants or products in distillation.
• How can you oxidise primary alcohols to carboxylic acids?

  Vigorously oxidised. Alcohol mixed with excess oxidising agent & heated under reflux. Any vaporised compounds cooled, condense & drip back into reaction mixture- aldehyde stays in reaction mixture & is oxidised.
• How can you oxidise a tertiary alcohol?

  Burn it.