Melting Point and Boiling Point
Trends of Alkanes: Teacher Information
C11-5-04
Introduction
Background Information and Applications of Alkanes:
Alkanes, also known as paraffins, are chemical compounds that consist only of the elements carbon and
hydrogen and are thus known as hydrocarbons. In alkanes the atoms are linked together exclusively by
single bonds and so are referred to as saturated hydrocarbons. Alkanes belong to a homologous series
of organic compounds in which the members differ by a –CH 2- (methylene) group. This gives them the
general formula CnH2n+2.
The first four alkanes are methane, ethane, propane, and butane. Their structures are shown below:
All of the alkanes are highly combustible and are valuable as clean fuels, burning to form water and
carbon dioxide in complete combustion. The first four alkanes - methane, ethane, propane and butane
are gases at room temperature, and are used directly as fuels. Below is a propane torch:
Alkanes from pentane up to around C17H36 are liquids at room temperature. Gasoline is a mixture of
alkanes from pentane up to about decane. Kerosene contains alkanes from about n=10 to n=16. Above
n=17 they are solids at room temperature. Alkanes with higher values of n are found in diesel fuel, fuel
oil, petroleum jelly, paraffin wax, motor oils, and for the highest values of n, asphalt. Alkane derivatives
are used in hundreds of products such as plastics, paints, drugs, cosmetics, detergents, insecticides, etc.,
so the fossil fuel resource from which we obtain the alkanes is much too valuable to burn it all as a motor
fuel.
In nature, alkanes exist in many areas:
, Methane is produced in the gut of cows from archaea, which are a group of prokaryotic and single-celled
microorganisms (see below). These cows are responsible for some of the methane gas detected in the
atmosphere.
Archaea
In plants, it is the solid long-chain alkanes that are found; they form a firm layer of wax, the cuticle, over
areas of plants exposed to the air. This protects plants against water loss, while preventing the leaching
of important minerals by the rain. It is also a protection against bacteria, fungi, and harmful insects. In
the picture below, water forms droplets on a thin film of alkane wax on the skin of the apple.
Alkanes are also found in animal products. One example is the shark liver oil, which is approximately
14% pristane (C19H40). Their occurrence is more important in pheromones (chemical messenger
materials), however, on which above all insects are dependent for communication.
In addition, alkanes form a significant portion of the atmospheres of the outer gas planets such as Jupiter,
Saturn, Uranus and Neptune. Titan, a satellite of Saturn, was examined by the Huygens probe, which
indicates that Titan's atmosphere periodically rains liquid methane onto the moon's surface. Also on
Titan, a methane-spewing volcano was spotted and this volcanism is believed to be a significant source of
the methane in the atmosphere. There also appear to be methane/ethane lakes near the north polar
regions of Titan, as discovered by Cassini's radar imaging. Methane and ethane have also been detected
in the tail of the comet Hyakutake. Alkanes have also been detected in meteorites such as carbonaceous
chondrites.
Alkanes have many environmental effects:
If there is not enough oxygen present during combustion, then instead of carbon dioxide, carbon
monoxide, CO, is produced. Carbon monoxide is particularly toxic and absorbed into blood, through
respiration, very easily. For domestic heating systems it is particularly important that enough air can get
to the flame to avoid carbon monoxide being generated in the home. Car engines also require a lot of air
and there is a lot of research going on to make the internal combustion engine more efficient, and so put
out less carbon monoxide. Note also that both alkanes and carbon dioxide are green house gases, i.e.
they trap infra-red radiation inside the Earth's atmosphere, gradually increasing global temperatures
Trends of Alkanes: Teacher Information
C11-5-04
Introduction
Background Information and Applications of Alkanes:
Alkanes, also known as paraffins, are chemical compounds that consist only of the elements carbon and
hydrogen and are thus known as hydrocarbons. In alkanes the atoms are linked together exclusively by
single bonds and so are referred to as saturated hydrocarbons. Alkanes belong to a homologous series
of organic compounds in which the members differ by a –CH 2- (methylene) group. This gives them the
general formula CnH2n+2.
The first four alkanes are methane, ethane, propane, and butane. Their structures are shown below:
All of the alkanes are highly combustible and are valuable as clean fuels, burning to form water and
carbon dioxide in complete combustion. The first four alkanes - methane, ethane, propane and butane
are gases at room temperature, and are used directly as fuels. Below is a propane torch:
Alkanes from pentane up to around C17H36 are liquids at room temperature. Gasoline is a mixture of
alkanes from pentane up to about decane. Kerosene contains alkanes from about n=10 to n=16. Above
n=17 they are solids at room temperature. Alkanes with higher values of n are found in diesel fuel, fuel
oil, petroleum jelly, paraffin wax, motor oils, and for the highest values of n, asphalt. Alkane derivatives
are used in hundreds of products such as plastics, paints, drugs, cosmetics, detergents, insecticides, etc.,
so the fossil fuel resource from which we obtain the alkanes is much too valuable to burn it all as a motor
fuel.
In nature, alkanes exist in many areas:
, Methane is produced in the gut of cows from archaea, which are a group of prokaryotic and single-celled
microorganisms (see below). These cows are responsible for some of the methane gas detected in the
atmosphere.
Archaea
In plants, it is the solid long-chain alkanes that are found; they form a firm layer of wax, the cuticle, over
areas of plants exposed to the air. This protects plants against water loss, while preventing the leaching
of important minerals by the rain. It is also a protection against bacteria, fungi, and harmful insects. In
the picture below, water forms droplets on a thin film of alkane wax on the skin of the apple.
Alkanes are also found in animal products. One example is the shark liver oil, which is approximately
14% pristane (C19H40). Their occurrence is more important in pheromones (chemical messenger
materials), however, on which above all insects are dependent for communication.
In addition, alkanes form a significant portion of the atmospheres of the outer gas planets such as Jupiter,
Saturn, Uranus and Neptune. Titan, a satellite of Saturn, was examined by the Huygens probe, which
indicates that Titan's atmosphere periodically rains liquid methane onto the moon's surface. Also on
Titan, a methane-spewing volcano was spotted and this volcanism is believed to be a significant source of
the methane in the atmosphere. There also appear to be methane/ethane lakes near the north polar
regions of Titan, as discovered by Cassini's radar imaging. Methane and ethane have also been detected
in the tail of the comet Hyakutake. Alkanes have also been detected in meteorites such as carbonaceous
chondrites.
Alkanes have many environmental effects:
If there is not enough oxygen present during combustion, then instead of carbon dioxide, carbon
monoxide, CO, is produced. Carbon monoxide is particularly toxic and absorbed into blood, through
respiration, very easily. For domestic heating systems it is particularly important that enough air can get
to the flame to avoid carbon monoxide being generated in the home. Car engines also require a lot of air
and there is a lot of research going on to make the internal combustion engine more efficient, and so put
out less carbon monoxide. Note also that both alkanes and carbon dioxide are green house gases, i.e.
they trap infra-red radiation inside the Earth's atmosphere, gradually increasing global temperatures
