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Alkanes (3.3.2)
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Alkanes are saturated hydrocarbon chains with the general formula
Ethane has the formula .
An alkane with carbons will have hydrogens.
Crude oil is a complex mixture of mainly alkanes that can be separated by fractional distillation.
The mixture is heated and fed into a fractionating column with a temperature gradient; the temperature decreases with height. This separates the mixture into groups of similar sized hydrocarbons with similar boiling points called fractions.
The fractions produced can be used directly, as fuels, lubricants, or bitumen, or processed further.
Certain less useful fractions from fractional distillation of crude oil are modified by industrial processing to increase the supply of in-demand products, such as petrol.
This makes use of the heavier (higher boiling point) fractions, where supply exceeds demand.
Processing involves cracking and/or reforming.
Cracking involves breaking up larger hydrocarbon molecules into smaller, more useful ones. This can be done by direct heating, which breaks carbon-carbon bonds. It can also be done by using catalysts.
Thermal cracking involves heating the petroleum vapour to a high temperature, 700 oC, under a high pressure ~70 MPa.
Thermal cracking produces mainly shorter chain alkanes and also some alkenes.
Catalytic cracking involves passing heated petroleum vapour over aluminium silicate-based zeolite catalyst, at a lower temperature than thermal cracking (450 oC) and a much lower pressure of 20 MPa.
The products of catalytic cracking are also smaller alkanes, but contain a greater proportion of alkenes compared to thermal cracking
Alkanes burn readily in oxygen, producing carbon dioxide and water.
Making bonds releases energy to the environment; it is exothermic.
The high bond enthalpy of the carbon-oxygen bond, in the carbon dioxide product, results in a highly exothermic combustion reaction.
Alkanes are commonly used as fuels.
With excess oxygen gas, alkanes burn to exclusively produce carbon dioxide and water.
This is complete combustion.
To produce a balanced equation for the complete combustion of a hydrocarbon, balance the carbons and hydrogens first and then consider the oxygens.
When there is insufficient oxygen available, the carbon from the alkane is not fully oxidised.
This is incomplete combustion.
Incomplete combustion also results in the formation of carbon monoxide and/or particulate carbon (soot) alongside carbon dioxide and water.
The ratio or carbon based products will depend on the amount of oxygen available.
During the practical combustion of alkane fuels, a variety of products are produced.
Alongside complete combustion products, other pollutants can be found within the exhaust fumes.
Additional pollutants including carbon monoxide, oxides of nitrogen and sulfur, carbon particulates, and unburned hydrocarbons.
Combustion products are largely removed from vehicle exhaust emissions by passing the hot gases through a catalytic converter. This is a ceramic honeycomb-like material coated with catalytic metals, such as platinum, palladium, and rhodium.
The catalytic converter is situated in the exhaust train just after the engine.
There is a combination of oxidation and reduction reactions to convert the waste gases containing , , and unburnt hydrocarbons into less harmful products such as , , and .
Combustion of hydrocarbons that contain sulfur compounds as impurities, leads to sulfur dioxide formation.
Sulfur dioxide causes air pollution.
is linked to the formation of acid rain. It is very soluble and forms an acidic solution in rainwater.
Sulfur dioxide can be removed from flue gases by scrubbing. Scrubbing involves passing the waste gases through a suspension of bases, such as a limestone slurry made from and (lime).
The sulfur dioxide dissolves in the suspension and is then converted to non-toxic calcium sulfite and calcium sulfate salts.
This is flue gas desulfurisation.
Alkanes undergo free radical substitution reactions with chlorine or bromine in the presence of UV radiation.
The radical substitution of an alkane with a halogen involves the homolytic fission of bonds and proceeds via three types of reaction;
- Initiation – the formation of radicals from a non radical, in this case the halogen,
- Propagation – formation of a radical and non radical from a radical and non radical
- Termination – combination of two radicals to form a non radical product.
The table below shows how to write mechanisms for free radical substitution of alkanes.
Methane reacts with chlorine in the presence of UV light to form a mixture of chlorinated alkanes.
The products formed depend on the concentration of the reactants present.
If excess chlorine is used with methane as the limiting reagent, then further substitution reactions can occur, forming di, tri, and tetrachloro products.
An excess of methane and limiting reagent of chlorine ensures that the monosubstituted chloromethane is the major product.
Radical substitution has limited use in organic synthesis as a mixture of products are formed.
Haloalkanes formed during free radical substitution are susceptible to repeated radical attack when the halogen is in excess. This leads to multiple substitutions .
During radical substitution carbon-halogen bonds can form in multiple positions on the carbon chain. This leads to a range of structural isomers.