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Alkanes (Topic 6B)
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Alkanes are saturated hydrocarbon chains with the general formula
Ethane has the formula .
An alkane with carbons will have hydrogens.
Alkanes can be branched but will still conform to the same general formula.
Cyclic alkanes, despite similarities in reactivity and properties to alkanes, form their own homologous series, with the same general formula as alkenes, .
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.
Reforming is a catalytic process that converts straight chain alkanes into branched and cyclic forms, as well as aromatics. The process works by breaking the alkane chains, and allowing them to rejoin. Much hydrogen is also produced during the reformation as more unsaturated and cyclic products are formed.
Cyclic, aromatic and branched alkanes burn more efficiently and make better fuels than straight chain alkanes.
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.
All carbon based products of fuel combustion have negative environmental impacts.
Carbon dioxide emissions are linked to the greenhouse effect and global warming, and are present in both complete and incomplete combustion.
Carbon monoxide and particulate carbon are formed in incomplete combustion.
Carbon monoxide is a colourless, odourless, and toxic gas. It binds to haemoglobin in the blood more strongly than oxygen, reducing the capacity of the blood to transport oxygen around the body. Carbon monoxide poisoning is first noticeable through dizziness and can eventually lead to death.
Particulate carbon is linked to global dimming. It can settle on the leaves of plants, reducing photosynthesis, and can be inhaled, causing irritation to the lungs.
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 .
Biofuels are alternative fuels to petroleum-based hydrocarbons. They are from renewable sources, such as crop plants, or from fermentation of plant-based carbohydrates, such as starch.
Ethanol and methanol are fermentation products and biodiesel from vegetable oils is widely produced.
A comparison of biofuels with fossil fuels is shown below.
A free radical or radical is a species with an unpaired electron and is formed by the homolytic fission of covalent bonds. This is where the bond pair of electrons are split evenly so both participating atoms receive one electron only.
The radical is symbolised by a dot .
UV light has sufficient energy content to be able to split chlorine molecules homolytically forming radicals.
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.