Alkanes Booklet

• Alkanes

  Saturated hydrocarbons containing only carbon-carbon single bonds
• Fractional distillation of crude oil

  1. Crude oil is a mixture consisting mainly of alkane hydrocarbons that can be separated by fractional distillation

  2. Fractional distillation is the method of separating the mixture into 'fractions' a group of compounds with similar boiling points

  3. The vapours are hottest at the bottom and cool as they rise, condensing at their melting point where the fraction collects and is piped off

  4. The shorter chain molecules are collected towards the top of the tower

  
• Cracking

  Breaking C–C bonds in alkanes to break longer chain hydrocarbon molecules into more useful shorter chain molecules which are in high demand

  
  

  ("To meet the demand for refinery gas, naphtha and petroleum, longer chain molecules in the less useful fractions are broken down into more useful shorter chain hydrocarbons")
• When molecules are cracked, the total number of atoms does not change, therefore sum of all the carbon and hydrogen atoms in the products adds up to the number of carbon and hydrogen atoms in the starting molecule
• Thermal Cracking

  
  

  1. High temperature (1000-1200K)
  2. High pressure (up to 70 atm)
  3. Short chain alkanes with a high percentage of alkenes
• Catalytic cracking

  • Slight pressure (1-2 atm)
  • High temperature (800-1000K)
  • Zeolite catalyst (honeycomb structure made from Al and SiO2, increasing the rate of reaction)
  • Branched hydrocarbons (motor fuels) and aromatic hydrocarbons
  • Lower temperature for catalytic cracking but still high, reflecting how hard it is to break carbon-carbon bonds
• Complete combustion of alkanes

  Alkanes burn in a plentiful supply of oxygen to produce carbon dioxide and water
• Incomplete combustion of alkanes

  In a limited supply of air, alkanes will burn to form water and carbon monoxide and in very limited oxygen will form carbon (soot)
• Combustion of alkanes and other organic compounds can be complete or incomplete
• Complete combustion of alkanes

  Forms carbon dioxide which is known to contribute to the greenhouse effect
• UV energy from the sun warms the surface of the earth which radiates IR back into the atmosphere. Molecules such as carbon dioxide, water and methane (eg from farming) absorb IR radiation as it makes their bonds rotate and vibrate. This traps the energy in the atmosphere causing it to warm up. Some warming is essential (without it the surface of the earth would be 60C lower than it is). Excessive warming, known as climate change, has huge environmental impacts
• Incomplete combustion of alkanes forms more harmful products. Carbon monoxide is a toxic gas which in confined spaces can cause asphyxiation and death. Soot contributes to particulate matter in the atmosphere which can cause/worsen asthma and cause global dimming
• Catalytic converter

  A ceramic honeycomb structure coated in platinum and rhodium (making it expensive) with a high surface area. As the gases pass over the catalyst they are held in place and their bonds weakened enabling them to react with each other more readily
• Flue gas desulfurisation

  1. Passes the gases through a wet slurry of CaO or CaCO3 which produces CaSO3

  2. CaO and CaCO3 are bases which neutralise the acidic SO2

  3. Converting this to CaSO4 using an oxidising agent, forms the useful building material gypsum (CaSO4)
• Sulfur dioxide can be removed from flue gases in power stations using calcium oxide or calcium carbonate in a process known as flue gas desulfurisation
• Formation of NOx in internal combustion engines

  In the extreme temperatures and pressures, nitrogen and oxygen combine to form nitrogen oxide and nitrogen dioxide
• NOx and SO2 cause acid rain and photochemical smog
• Free radical substitution mechanism

  • Involves initiation, propagation and termination steps
  • Initiation: Cl2 → Cl• + Cl• (homolytic fission)
  • Propagation: Cl• + CH4 → HCl + •CH3, •CH3 + Cl2 → CH3Cl + Cl•
  • Termination: Cl• + Cl• → Cl2, Cl• + •CH3 → CH3Cl, •CH3 + •CH3 → C2H6
• Further substitution can occur during the propagation steps, leading to a mixture of products which require separation by fractional distillation due to their different boiling points
• Halogenoalkanes

  Much more reactive than alkanes, have many uses including as refrigerants, solvents and in pharmaceuticals
• The use of some halogenoalkanes has been restricted due to the effect of chlorofluorocarbons (CFCs) on the atmosphere
• Ozone depletion by halogenoalkanes

  1. Chlorine atoms are formed in the upper atmosphere when ultraviolet radiation causes C–Cl bonds in chlorofluorocarbons (CFCs) to break

  2. Chlorine atoms catalyse the decomposition of ozone and contribute to the hole in the ozone layer
• Ozone, formed naturally in the stratosphere in the upper atmosphere, is beneficial because it absorbs ultraviolet radiation. In the absence of ozone, ultraviolet light of wavelengths 200nm to 300nm can reach the Earth, causing skin cancer, cataracts in eyes, damage to plant tissue and reduce the plankton population in the oceans
• Over the past decades, scientists have observed a steady decrease in ozone in the stratosphere, due to a photochemical chain reaction by halogen free radicals from halogenoalkanes used as solvents, propellants and refrigerants
• Free radical substitution reactions in the upper atmosphere

  1. Initiation: CF3Cl → Cl• + •CF3

  2. Propagation chain reaction: Cl• + O3 → ClO• + O2, ClO• + O3 → Cl• + 2O2

  3. Overall: 2O3 → 3O2
• A single chlorine radical can catalyse the decomposition of 100,000 molecules of ozone
• Chemists have now developed alternative chlorine-free compounds. Molecules that have fluorine as the only halogen cause no damage and do not deplete the ozone layer
• Fractional Distillation: Industrially

  1. Oil is pre-heated

  2. Then passed into column

  3. The fractions condense at different heights

  4. The temperature of column decreases upwards

  5. The separation depends on boiling point

  6. Boiling point depends on size of molecules

  7. The larger the molecule the larger the van der waals forces

  8. Similar molecules (size, bp, mass) condense together

  9. Small molecules condense at the top at lower temperatures

  10. Big molecules condense at the bottom at higher temperatures
• Fractional Distillation

  • This is a physical process
  • Involves the splitting of weak van der waals forces between molecules
• Vacuum Distillation Unit

  1. Heavy residues from the fractionating column are distilled again under a vacuum

  2. Lowering the pressure over a liquid will lower its boiling point
• Vacuum Distillation

  Allows heavier fractions to be further separated without high temperatures which could break them down
• Petroleum

  Mixture consisting mainly of alkane hydrocarbons
• Fractional Distillation: In the Laboratory

  1. Heat the flask

  2. Vapours of all the components in the mixture are produced

  3. Vapours pass up the fractionating column

  4. The vapour of the substance with the lower boiling point reaches the top first

  5. The thermometer should be at or below the boiling point of the most volatile substance

  6. The vapours with higher boiling points condense back into the flask

  7. Only the most volatile vapour passes into the condenser

  8. The condenser cools the vapours and condenses to a liquid and is collected
• Fractional Distillation

  Used to separate liquids with similar boiling points
• Branched and cyclic hydrocarbons burn more cleanly and are used to give fuels a higher octane number
• Carbon (soot) can cause global dimming - reflection of the sun's light
• Flue Gas Desulfurisation

  1. Gases pass through a scrubber containing basic calcium oxide which reacts with the acidic sulfur dioxide in a neutralisation reaction

  2. The calcium sulfite which is formed can be used to make calcium sulfate for plasterboard
• Pollutants from Combustion

  • Nitrogen oxides
  • Carbon monoxide
  • Carbon dioxide
  • Unburnt hydrocarbons
  • Soot
• Nitrogen Oxides

  • Formed when N2 in the air reacts at the high temperatures and spark in the engine
  • NO is toxic and can form acidic gas NO2
  • NO2 is toxic and acidic and forms acid rain
• Synthesis of Haloalkanes: Reaction of Alkanes with Bromine/Chlorine in UV Light

  1. In the presence of UV light alkanes react with chlorine to form a mixture of products with the halogens substituting hydrogen atoms

  2. The mechanism is a free radical substitution