Alkanes & halogenoalkanes

• Thermal cracking:

  High temperature & pressure (up to 1000.C & 70 atm). Produces a lot of alkenes used to make polymers, e.g, poly(ethene).
• Catalytic cracking:

  Uses zeolite catalyst, slight pressure, high temperature (500.C), mostly produces aromatic hydrocarbons & alkanes needed to produce motor fuels. Catalyst cuts costs- reaction done at low pressure & lower temperature. Speeds reaction rate, saving time.
• Fractional distillation:

  Crude oil vaporised & goes in bottom of fractionating column & rises up through trays. Largest hydrocarbons don't vaporise at all as their bp too high-run to bottom & form gooey residue. As crude oil vapour goes up column, it gets cooler, creating temperature gradient. Each fraction condenses at different temperature due to their bp.
• What are the 3 main pollutants from vehicle exhausts?

  Nitrogen oxides, unburnt hydrocarbons & carbon monoxide.
• What are nitrogen oxides?

  Series of toxic & poisonous molecules which have general formula NOx. Nitrogen monoxide formed when high pressure & temperature in car engines cause nitrogen & oxygen atoms from air to react together. Can react further to produce nitrogen dioxide. If this gets into atmosphere, nitric acid produced.
• What are unburnt hydrocarbons?

  Engines don't burn all fuel molecules-some come out as unburnt hydrocarbons. These react with nitrogen oxides in presence of sunlight to form ground-level ozone (O3)-major component of smog. Irritates eyes, aggravates respiratory problems & causes lung damage.
• What is sulfur dioxide?

  Fossil fuels sometimes contain sulfur-when burnt in e.g, car engines & power stations, sulfur reacts to form SO2. Dissolves in moisture in atmosphere & converted into sulfuric acid-causes acid rain. Destroys trees & vegetation, corrodes buildings & kills fish in lakes.
• How can sulfur dioxide be removed from flue gases?

  Powdered calcium carbonate (limestone) or calcium oxide is mixed with water to make an alkaline slurry. When flue gases mix with alkaline slurry, acidic sulfur dioxide gas reacts with calcium compounds to form a harmless salt (calcium sulfite).
• What do greenhouse gases do?

  Absorb infared energy (heat) & emit some back to Earth, keeping it warm (greenhouse effect). Burning fossil fuels produces CO2.
• What happens in nucleophilic substitution?

  Nucleophile attacks polar molecule & replaces functional group.
• What is a nucleophile?

  Electron-pair donor.
• Halogenoalkane + OH- makes...

  Alcohols. Use warm aqueous sodium or potassium hydroxide.
• How do you make nitriles from halogenoalkanes?

  Warm halogenoalkane with ethanolic potassium cyanide.
• How do you make amines from halogenoalkanes?

  Warm halogenoalkane with excess ethanolic ammonia in sealed tube. (2 steps)
• What happens when you warm halogenoalkane with OH- dissolved in ethanol instead of water?

  Elimination reaction- results in alkene. OH- acts as a base & takes proton to make water.
• What is a halogenoalkane?

  Alkane with at least 1 halogen atom in place of hydrogen atom.
• What is a photochemical reaction?

  Reaction started by UV light. Halogens react with alkanes like this to form halogenoalkanes. A hydrogen atom is substituted by chlorine or bromine (free radical substitution reaction).
• What is a free radical?

  Particle with an unpaired electron. Form when covalent bond splits equally, giving 1 electron to each species. Very reactive.
• What happens in the initiation step?

  Free radicals produced- sunlight provides energy to break Cl-Cl bonds (photodissociation). Bond splits equally & 2 highly reactive free radicals formed.
• What happens in the propagation step?

  Free radicals used up & created in chain reaction. Cl free radical attacks methane. New methyl free radical attacks another chlorine molecule. New Cl free radical attacks another methane- continues until all Cl2 or methane used up.
• What happens in the termination step?

  Free radicals mopped up. If 2 free radicals join they form stable molecule (2 unpaired electrons form covalent bond). Terminates chain reaction.
• What are chlorofluorocarbons (CFCs) :

  Halogenoalkane molecules where all hydrogens have been replaced by chlorine & fluorine atoms.
• What is ozone (O3) ?


  Acts as chemical sunscreen in upper atmosphere- absorbs UV from sun, stopping it reaching us. UV can cause sunburn or skin cancer. Ozone formed when oxygen broken down into 2 free radicals by UV. Free radical reacts with oxygen to form O3.
• How is the ozone layer being destroyed by CFCs ?

  Cl free radicals formed when C-Cl bonds in CFCs brokendown by UV. These free radicals are catalysts- react with ozone to form intermediate (ClO free radical) & oxygen. Cl free radical regenerated & attacks more ozone.
• Name some nucleophiles that will react with halogenoalkanes:

  Cyanide ion, ammonia & hydroxide ion.
• Why are halogen-carbon bonds polar?

  Halogens generally more electronegative than carbon. Halogen atom withdraws electron density from carbon atom. Leaves carbon atom with partial positive charge & halogen atom with partial negative charge. Carbon atom doesn't have enough electrons so can be attacked by nucleophile.
• What are the products of complete combustion?

  Carbon dioxide & water. Alkanes burnt in plenty of oxygen.
• What are the products of incomplete combustion?

  Hydrocarbon burnt in insufficient amount of oxygen. Produces particulate carbon (soot) & carbon monoxide gas instead of or as well as carbon dioxide.
• Why is carbon monoxide gas poisonous?

  CO molecules bind to same sites on haemoglobin molecules in red blood cell as oxygen molecules. Oxygen can't be carried around body now. CO can be removed from exhaust gases by catalytic converters on cars.
• What are the issues with soot?

  Can cause breathing problems & can build up in engines meaning they don't work properly.
• How is sulfur dioxide gas produced?

  Fossil fuels can contain sulfur. When burnt (e.g, car engines & power stations) sulfur reacts to form sulfur dioxide gas- gets into atmosphere & dissolves in the moisture & converted into sulfuric acid-causes acid rain. (When nitrogen dioxide gets into atmosphere nitric acid is produced).
• What does acid rain do?

  Destroys trees & vegetations & corrodes buildings & statues & kills fish in lakes.
• How can SO2 be removed from power station flue gases?

  Powdered calcium carbonate (limestone) or calcium oxide is mixed with water to make an alkaline slurry. When flue gases mix with alkaline slurry , acidic sulfur dioxide gas reacts with the calcium compounds to form a harmless salt (calcium sulfate).
• Explain greenhouse gases:

  Burning fossil fuels produces CO2 (greenhouse gas)- absorbs infared energy (heat). Emit some of energy they absorb back towards Earth, keeping it warm (greenhouse effect).
• What are nitrogen oxides?

  Series of toxic & poisonous molecules which have general formula NoX. Nitrogen monoxide produced when high pressure & temperature in car engine cause nitrogen & oxygen atoms from air to react together- can react further to produce nitrogen dioxide.
• What are unburnt hydrocarbons?


  Engines don't burn all fuel molecules- some come out as unburnt hydrocarbons. These react with nitrogen oxides in presence of sunlight to form ground-level ozone (O3)-major component of smog. Irritates eyes, aggravates respiratory problems & causes lung damage.
• What are catalytic converters?

  Remove pollutants from car exhausts by reactions. Have large surface areas.
• Thermal cracking:

  Up to 1000.C & 70 atm (high temperature & pressure). Produces a lot of alkenes used to make polymers e.g, poly(ethene).
• Catalytic cracking:


  Uses zeolite catalyst & slight pressure. High temperature (500.C). Mostly produces aromatic hydrocarbons & alkanes needed to produce motor fuels. Catalyst cuts costs - reaction done at low pressure & lower temperature speeds reaction rate , saving time.
• Explain fractional distillation :


  Crude oil vaporised at 350.C-vaporised crude oil goes in bottom of fractionating column & rises up through trays. Largest hydrocarbons don't vaporise at all as their bp too high-run to bottom & form gooey residue. As crude oil vapour goes up fractionating column, it gets cooler, creating a temperature gradient. Bp of alkanes increase as molecules get bigger so each fraction condenses at different temperature. Fractions drawn off at different levels in column. Hydrocarbons with lowest bp don't condense- drawn off as gases at top of column.