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Alkanes (4.1.2)
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Alkanes are saturated hydrocarbon chains with the general formula
Ethane has the formula .
An alkane with carbons will have hydrogens.
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, .
Alkanes contain only single covalent bonds called σ bonds, which can freely rotate.
σ is pronounced ‘sigma’.
All carbon atoms in an alkane form four single covalent bonds. The eight electrons are arranged as four pairs.
The electrons in the bonds around carbon repel each other. They orientate to form a tetrahedral shape, according to electron pair repulsion theory, to minimise repulsion between the electron pairs.
The angle between covalent bonds in an alkane is approximately 109.5°.
The 3D structure of an alkane is typically displayed with two bonds on the plane of the page, (solid lines), one bond receding into the page, (dashed line), and one bond coming out from the plane of the page (bold line).
Alkanes are the least reactive of all organic compounds.
Alkanes are composed of strong covalent bonds, which require a large amount of energy to break.
The similar electronegativity values of hydrogen and carbon means there is very low polarity in the carbon-hydrogen bonds. There are no electron-rich areas to attract electrophiles nor electron-deficient areas to attract nucleophiles.
The lack of molecular polarity in alkanes makes them insoluble in polar solvents, such as water.
The density of alkanes increases with chain length, and is always lower than water. When combined with an aqueous solution, the alkane will always form the upper layer.
Alkane molecules only interact through London forces, (induced dipole–dipole interactions).
The impact of structure on boiling points should only be considered for molecules of the same size.
Longer chain alkanes have higher boiling points since they are larger. They have a larger electron cloud leading to greater London forces requiring more energy to break.
Alkanes have lower boiling points than equivalently sized molecules containing polar groups, such as alcohols, carboxylic acids, and haloalkanes.
Linear alkanes have higher boiling points than their branched isomers. There is greater surface contact between linear molecules, resulting in closer packing and stronger London forces that require more energy to break.
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.
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 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.
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.
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.
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.