Alkanes and Halogenoalkanes (AS)

• A halogenoalkane is a alkane with at least one halogen atom in place of a hydrogen atom.
• Halogens are much more electronegative than carbon, so carbon-halogen bonds are polar.
• The slight positive charge on the carbon makes it prone to attacks from nucleophiles.
• A nucleophile is an electron pair donor.
• OH-, CN- and NH3 are all nucleophiles that can react with halogenoalkanes.
• Halogeneoalkanes react with hydroxides to form alcohols.
• Nitriles are formed by reacting halogenoalkanes with cyanide.
• Reacting halogenoalkanes with ammonia forms amines.
• If you warm a halogenoalkane with hydroxide ions dissolved in ethanol instead of water, an elimination reaction happens, and you end up with an alkene.
• When reacting a halogenoalkane with hydroxide, you need to look at the conditions to see what reaction is occuring.
• Alkanes are saturated hydrocarbons.
• Alkanes have the general formula CnH2n+2.
• Alkanes contain only single bonds due to being saturated.
• Cycloalkanes have a ring of carbon atoms with two hydrogens attached to each carbon.
• Cycloalkanes have the general formula CnH2n, but are still saturated.
• Crude oil goes through a fractionating column to form separate alkanes (fractions).
• Cracking is breaking long chain hydrocarbons down into smaller, more useful hydrocarbons.
• Thermal Cracking:

  • Takes place at high temperatures (up to 1000 degrees) and high pressure (up to 70 atm).
  • Produces a large amount of alkenes.
  • These alkenes are used to produce valuable products like polymers.
• Catalytic Cracking:

  • Uses a zeolite catalyst, at slight pressure and high temperature (about 450 degrees).
  • It mostly produces aromatic hydrocarbons and motor fuels.
  • Cuts cost, because reaction can be done at a low pressure and a lower temperature.
  • Catalyst speeds up the reaction which saves time.
• If you burn alkanes with plenty of oxygen, you get carbon dioxide and water. This is complete combustion.
• When incomplete combustion occurs, you can get carbon monoxide gas instead of, or as well as, carbon dioxide.
• Carbon Monoxide:

  • Is a silent killer.
  • Isn't visible.
  • Binds to haemoglobin molecules in red blood cells so oxygen can't be carried around the body.
  • Can be removed from exhaust gases by catalytic converters on cars.
• Carbon molecules can also be formed by incomplete combustion.
• Burning fossil fuels produces carbon dioxide which is a greenhouse gas.
• Engines don't burn all of the fuel molecules. Some come out as unburnt hydrocarbons.
• Hydrocarbons and nitrogen oxides react in the presence of sunlight to form ground-level ozone which is a major component of smog.
• Ground-level Ozone:

  • Irritates peoples eyes.
  • Aggravates repiratory problems.
  • Causes lung damage.
• Catalytic converters on cars remove unburnt hydrocarbons and oxides of nitrogen from the exhaust.
• Some fossil fuels contain sulfur. When they are burnt, the sulfur reacts to form sulfur dioxide gas.
• If sulfur dioxide gets into the atmosphere, it dissolves in the moisture and is converted into sulfuric acid causing acid rain.
• Acid rain destroys trees and vegetation, as well as corroding buildings and statues and killing fish in lakes.
• Sulfur dioxide can be removed from power station flue gases before it gets into the atmosphere.
• A free radical is a particle with an unpaired electron.
• Free radicals form when a covalent bond splits equally, giving one electron to each atom.
• The unpaired electron makes them very reactive.
• You can show a free radical in a mechanism by putting a dot next to it like this (Cl·)
• Halogens react with alkanes in photochemical reactions - reactions that are started by ultraviolet light.
• Example: Reacting Chlorine with Methane

  Stage 1: Initiation reaction - free radicals are produced.

  1. Sunlight provides enough energy to break the Cl-Cl bond.
  2. The bond splits equally and each atom gets to keep one electron. The atom becomes a highly reactive free radical, Cl·, because of its unpaired electron.
• Example: Reacting Chlorine with Methane

  Stage 2: Propagation reactions - free radicals are used up and created in a chain reaction.

  1. Cl· attacks a methane molecule.
  2. The new methyl free radical, CH³·, can attack another Cl² molecule.
  3. The new Cl· can attack another CH⁴ molecule, and so on, until all the Cl² or CH⁴ molecues are used up.
• Example: Reacting Chlorine with Methane

  Stage 3: Termination reactions - free radicals are mopped up.

  1. If two free radicals join together, they make a stable molecule. The two unpaired eletrons form a covalent bond.
  2. There are many possible termination reactions.