Reactions of alkanes
Alkanes are generally unreactive compounds due to the
- very small difference in electronegativity between carbon and hydrogen.
- strong carbon-carbon and carbon-hydrogen bonds
Alkane molecules are non-polar, they have neither partial positive charges on any of their carbon atoms to attract nucleophiles nor areas of high electron density to attract electrophiles. They do not react with acids, bases, oxidising agents, and reducing agents.
However, they undergo combustion with oxygen substitution with halogens in the presence of sunlight. Because they are non-polar, alkanes do not react with polar compounds such as water.
Combustion of alkanes
Alkanes are often used as fuels because they burn in air to produce large quantities of heat. For example,
- butane and propane are used as bottled gas in homes for cooking food.
- diesel oil, which is an alkane is used by ships trains and lorries as a fuel.
- octane (petrol) is used as a fuel by cars and motorbikes.
Complete combustion of alkanes
In a plentiful supply of oxygen, an alkane will undergo complete combustion to form carbon dioxide and water.
alkane + oxygen → carbon dioxide + water
octane + oxygen → carbon dioxide + water
2C8H18 + 25O2 → 16CO2 + 18H2O
Incomplete combustion of alkanes
In a limited supply of oxygen, an alkane will undergo incomplete combustion. During incomplete combustion, carbon is partially oxidised to carbon monoxide.
octane + oxygen → carbon monoxide + water
2C8H18 + 17O2 → 16CO + 18H2O
Dangers of carbon monoxide
Incomplete combustion produces carbon monoxide which is a very dangerous gas because it is:
- 200 times more easily absorbed in the haemoglobin of your blood than oxygen.
- invisible and odourless, which means you can’t detect it easily when it is being produced.
Substitution reactions of alkanes
Alkanes undergo substitution reactions with halogens in the presence of sunlight, as follows:
CH4 + Cl2 → CH3Cl + HCl
methane + chlorine → chloromethane + hydrogen chloride
However, the reaction does not take place in darkness.
The reaction between methane and chlorine does not take place in the dark because it needs ultraviolet light from the sun to proceed. Then it proceeds through the following stages:
The reaction between alkanes and halogens, involving initiation, propagation and termination steps, is called free-radical substitution.
The first step in the reaction mechanism of the reaction between methane and chlorine is the homolytic fission of the Cl-Cl bond by ultraviolet light from the Sun. This is called the initiation step in the reaction mechanism.
Cl2 → 2Cl⠂
The Cl-Cl bond breaks and each chlorine atom takes one electron from the pair of bonding electrons in the Cl-Cl bond, forming two Cl⠂ atoms. These Cl⠂ atoms, each with an unpaired electron, are called free radicals.
Free radicals are very reactive to the point that they will attack even the normally unreactive alkanes. In this reaction, a chlorine free radical will attack the methane molecule as follows:
CH4 + Cl⠂ → ⠂CH3 + HCl
In this step, a C-H bond in CH4 breaks homolytically to produce a methyl free radical, ⠂CH3.
The methyl free radical produced above can then attack a chlorine molecule, forming chloromethane and, in the process, generating another chlorine free radical.
⠂CH3 + Cl2 → CH3Cl + Cl⠂
The chlorine free radical can then attack another methane molecule to produce a methyl free radical, repeating the first propagation step again. The methyl free radical the attacks chlorine to produce the chlorine free radical again. The cycle continues on and on.
However, this reaction is not really suitable for laboratory preparation of specific halogenoalkanes, because the result is a mixture of substitution products. For example, in the reaction between methane and chlorine, the products can include dichloromethane, trichloromethane and tetrachloromethane as well as chloromethane.
- dichloromethane results from the propagation steps in which a chlorine free radical attacks the chloromethane produced.
- trichloromethane results from the propagation steps in which a chlorine free radical attacks the dichloromethane produced.
- tetrachloromethane results from the propagation steps in which a chlorine free radical attacks the trichloromethane produced.
For example, dichloromethane is produced as follows:
CH3Cl + Cl⠂ → ⠂CH2Cl + HCl
⠂CH2Cl + Cl2 → CH2Cl2 + Cl⠂
The more chlorine gas in the reaction mixture to start with, the greater the proportions of CH2Cl2, CHCl3 and CCl4 formed as products.
Whenever two free radicals meet they react with each other to produce a single chlorine molecule. This reaction produces no free radicals that can carry on the reaction sequence and therefore the chain reaction stops. Examples of termination steps include:
⠂CH3 + Cl⠂ → CH3Cl
⠂CH3 + ⠂CH3 → C2H6
Summary of free radical substitution
- Initiation step – we start with a molecule and get two free radicals formed.
- Propagation steps – we start with a molecule and a free radical and end up with a different molecule and a different free radical.
- Termination steps – we start with two free radicals and
end up with a molecule and no free radicals.
Cracking of alkanes
As we previously stated, alkanes are produced extracted by the fractional distillation of crude oil. However, from the fractions produced, longer chain fractions are not as useful as shorter chain fractions and therefore have a lower value economically.
To meet the demand for the shorter chain hydrocarbons, many of the longer chain fractions are broken into shorter lengths. The process is called cracking.
Cracking is a very important industrial process because it produces useful products such as:
- shorted chain hydrocarbons, especially petrol.
- alkenes, especially ethene, which is the starting material for the manufacture of polythene.
Thermal cracking involves the heating alkanes to a high temperature, around 1000 K, under high pressures of up to 7000 kPa. Under such harsh conditions the strong carbon-carbon bonds in alkenes break in such a way that one electron from the pair in the covalent bond goes to each carbon atom. This is known as homolytic fission and each product, ending in a carbon atom with an unpaired electron, is called a free radical.
The free radicals then react in a number of ways to form a variety of shorter chain molecules. However, a single alkane molecule does not have enough hydrogen atoms to produce two alkanes. One of the new chains produced must have a C=C double bond, and is therefore an alkene. For example let us use a butane molecule as an example and show how it can be cracked:
CH3CH2CH2CH3 → CH3CH2⠂+ CH3CH2⠂
CH3CH2⠂+ CH3CH2⠂ → CH3CH3 + CH2CH2
The products are ethane (CH3CH3) and ethene (CH2CH2).
Take note that:
- there is no limit to the number of carbon-carbon bonds that may break.
- the chain does not necessarily break in the middle.
- hydrogen may also be produced as a product.
- to avoid too much decomposition (ultimately to carbon and hydrogen) the alkanes are kept in these conditions for a very short time, typically one second.
Catalytic cracking takes place at a lower temperature and lower pressure than thermal cracking, using a zeolite catalyst, consisting of silicon dioxide and aluminium oxide (aluminosilicates).
Zeolites have a honeycomb structure with a very large surface area and are acidic. Catalytic cracking is used mainly to produce motor fuels and the products are mostly branched alkanes, cycloalkanes, and aromatic compounds.