3.2 Alkanes

Compounds of carbon and hydrogen with only single bonds between them

They are more useful as fuels

• Thermal Cracking • Catalytic Cracking

• requires high temperatures (up to 1000 C) • and high pressure (up to 70 atmospheres) and produces alkanes and a lot of alkenes Remember that cracking is an endothermic reaction.

• uses a lower temperature (around 450 C) • and slight pressure in the presence of a catalyst such as a zeolite or aluminium oxide to produce mainly aromatic hydrocarbons Remember that cracking is an endothermic reaction.

Excess (plenty of) oxygen

• Carbon Monoxide • Carbon (Soot)

In a car engine, high temperatures and pressures are reached causing the oxidation of nitrogen to take place:

Acid rain can cause corrosion of buildings/statues, endangers plant and aquatic life (as lakes and rivers become too acidic) as well as directly damaging human health

To reduce the amount of pollutants released in car exhaust fumes

Platinum

• The main way to reduce sulfur dioxide emissions is to treat the waste gases from coal fired power stations • The waste gases are passed into a scrubbing chamber which sprays a wet slurry of calcium oxide and calcium carbonate into the gases • This process is also known as sulfur scrubbing or flue gas desulfurisation

Any species with a free, unpaired electron

Alkanes can undergo free-radical substitution in which a hydrogen atom gets substituted by a halogen (chlorine/bromine)

Alkanes are very unreactive

• Initiation • Propagation • Termination

• In the initiation step the Cl-Cl or Br-Br is broken by energy from the UV light • Each atom takes one electron from the covalent bond • This produces two radicals in a homolytic fission reaction

• This refers to the progression (growing) of the substitution reaction in a chain reaction • Free radicals are very reactive and will attack the unreactive alkanes • A C-H bond breaks homolytically • Remember: Homolytic fissions is where each atom gets one electron from the covalent bond • An alkyl free radical is produced

• … This can attack another chlorine / bromine molecule to form a halogenoalkane and regenerate the chlorine / bromine free radical • The regenerated chlorine / bromine free radical can then repeat the cycle

A mixture of substitution products are formed

• This is when the chain reaction terminates (stops) due to two free radicals reacting together and forming a single unreactive molecule • Two radicals → one product • Multiple products are possible, dependent on the radicals involved • For example, in the single substitution of ethane with chlorine: • Exam Tip - *If you are asked to give an equation for the termination step of a free radical reaction / mechanism, you should not give the equation reforming the original halogen as this is often marked as “ignore” on mark schemes *

This will result in the formation of a precipitate The rate of formation of these precipitates can also be used to determine the reactivity of the halogenoalkanes