Alkanes and Halogenoalkanes (AS)

Cards (44)

  • 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.